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Management of Major Peanut (Arachis hypogaea L.) Diseases Using Bahiagrass (Paspalum notatum fluegge) Rotation

Permanent Link: http://ufdc.ufl.edu/UFE0021449/00001

Material Information

Title: Management of Major Peanut (Arachis hypogaea L.) Diseases Using Bahiagrass (Paspalum notatum fluegge) Rotation
Physical Description: 1 online resource (117 p.)
Language: english
Creator: Tsigbey, Francis Kodjo
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: bahiagrass, diseases, management, peanut, rotation, tswv
Plant Pathology -- Dissertations, Academic -- UF
Genre: Plant Pathology thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Suppression of peanut early leaf spot (ELS) (Cercospora arachidicola S. Hori,), and late leaf spot (LLS) Cercosporidium personatum (Berk. & M.A. Curtis) Deighton was accomplished in a cotton (C)-bahiagrass (B)-bahiagrass (B)-peanut (P) CBBP rotation when compared to a conventional peanut (P)-cotton (C)-cotton (C)-peanut (P) PCCP rotation in north Florida during 2003-2006. The CBBP rotation significantly (P < 0.05) reduced ELS and LLS in peanut in all years as compared to the PCCP rotation. Final ELS and LLS severities (Florida 1-10 scale) in the PCCP rotation were 7, 8, 7, and 8; whereas those for the CBBP were 5, 6, 6, and 6 during 2003, 2004, 2005, and 2006, respectively. The apparent infection rates (r) were significantly (P < 0.05) higher in the PCCP than the CBBP rotation in all years. During 2003, peanut in the CBBP rotation had significantly (P < 0.05) higher incidence of rust (Puccinia arachidis Spegg) than those in the PCCP rotation. Pod yields were significantly (P < 0.05) lower (2,229; 2,297; 1,703; and 3,278 kg/ha) for the PCCP rotation than for the CBBP rotation (2,935; 3,053; 2,250; and 4,504 kg/ha) in 2003, 2004, 2005, and 2006, respectively, when not sprayed with fungicide. Tomato Spotted Wilt (TSW) incidence and severity were lower in the CBBP than the PCCP peanut in all years. Incidence of TSW on peanut ranged 6-16, 24-36, 21-37, 18-25% in 2003, 2004, 2005, and 2006, respectively, in a CBBP rotation, whereas the incidence ranged 15-24, 28-73, 28-77, 39-53% in 2003, 2004, 2005, and 2006, respectively, in a PCCP. Peanut seedlings suffered more thrips feeding damage (100%) under the PCCP rotation as compared to 45% under the CBBP rotation. Thrips (Frankliniella spp.).population on peanut seedlings were similarly higher on the PCCP than the CBBP rotation in 2005. Reduction in the incidence of southern stem rot (SSR) Sclerotium rolfsii Sacc. was achieved in CBBP rotation when compared to a PCCP rotation in north Florida during 2003. Peanut grown in the CBBP rotation had significantly (P < 0.05) lower SSR incidence. Field soils amended with leaves and roots of bahiagrass reduced survival of sclerotia of S. rolfsii in the range of 75-100%. Incorporation of plant parts encouraged colonization of sclerotia by Trichoderma spp. and bacteria. Amendments with higher proportions of leaf material reduced sclerotia survival the most and encouraged the growth of antagonists. Bahiagrass rotations reduced soil populations of Meloidogyne spp., Rotylenchulus spp., and Helictylenchulus spp., in exception of Criconemoides spp. Bahiagrass root exudates actively attracted juveniles of M. arenaria to root zones in water agar. Field soils incorporated with bahiagrass residues significantly (P < 0.05) suppressed egg production of M. arenaria when compared to non-amended soils. Amendments with higher proportions of leave were more effective in suppressing egg production.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Francis Kodjo Tsigbey.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Marois, James J.
Local: Co-adviser: Datnoff, Lawrence E.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021449:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021449/00001

Material Information

Title: Management of Major Peanut (Arachis hypogaea L.) Diseases Using Bahiagrass (Paspalum notatum fluegge) Rotation
Physical Description: 1 online resource (117 p.)
Language: english
Creator: Tsigbey, Francis Kodjo
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: bahiagrass, diseases, management, peanut, rotation, tswv
Plant Pathology -- Dissertations, Academic -- UF
Genre: Plant Pathology thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Suppression of peanut early leaf spot (ELS) (Cercospora arachidicola S. Hori,), and late leaf spot (LLS) Cercosporidium personatum (Berk. & M.A. Curtis) Deighton was accomplished in a cotton (C)-bahiagrass (B)-bahiagrass (B)-peanut (P) CBBP rotation when compared to a conventional peanut (P)-cotton (C)-cotton (C)-peanut (P) PCCP rotation in north Florida during 2003-2006. The CBBP rotation significantly (P < 0.05) reduced ELS and LLS in peanut in all years as compared to the PCCP rotation. Final ELS and LLS severities (Florida 1-10 scale) in the PCCP rotation were 7, 8, 7, and 8; whereas those for the CBBP were 5, 6, 6, and 6 during 2003, 2004, 2005, and 2006, respectively. The apparent infection rates (r) were significantly (P < 0.05) higher in the PCCP than the CBBP rotation in all years. During 2003, peanut in the CBBP rotation had significantly (P < 0.05) higher incidence of rust (Puccinia arachidis Spegg) than those in the PCCP rotation. Pod yields were significantly (P < 0.05) lower (2,229; 2,297; 1,703; and 3,278 kg/ha) for the PCCP rotation than for the CBBP rotation (2,935; 3,053; 2,250; and 4,504 kg/ha) in 2003, 2004, 2005, and 2006, respectively, when not sprayed with fungicide. Tomato Spotted Wilt (TSW) incidence and severity were lower in the CBBP than the PCCP peanut in all years. Incidence of TSW on peanut ranged 6-16, 24-36, 21-37, 18-25% in 2003, 2004, 2005, and 2006, respectively, in a CBBP rotation, whereas the incidence ranged 15-24, 28-73, 28-77, 39-53% in 2003, 2004, 2005, and 2006, respectively, in a PCCP. Peanut seedlings suffered more thrips feeding damage (100%) under the PCCP rotation as compared to 45% under the CBBP rotation. Thrips (Frankliniella spp.).population on peanut seedlings were similarly higher on the PCCP than the CBBP rotation in 2005. Reduction in the incidence of southern stem rot (SSR) Sclerotium rolfsii Sacc. was achieved in CBBP rotation when compared to a PCCP rotation in north Florida during 2003. Peanut grown in the CBBP rotation had significantly (P < 0.05) lower SSR incidence. Field soils amended with leaves and roots of bahiagrass reduced survival of sclerotia of S. rolfsii in the range of 75-100%. Incorporation of plant parts encouraged colonization of sclerotia by Trichoderma spp. and bacteria. Amendments with higher proportions of leaf material reduced sclerotia survival the most and encouraged the growth of antagonists. Bahiagrass rotations reduced soil populations of Meloidogyne spp., Rotylenchulus spp., and Helictylenchulus spp., in exception of Criconemoides spp. Bahiagrass root exudates actively attracted juveniles of M. arenaria to root zones in water agar. Field soils incorporated with bahiagrass residues significantly (P < 0.05) suppressed egg production of M. arenaria when compared to non-amended soils. Amendments with higher proportions of leave were more effective in suppressing egg production.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Francis Kodjo Tsigbey.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Marois, James J.
Local: Co-adviser: Datnoff, Lawrence E.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021449:00001


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MANAGEMENT OF MAJOR PEANUT ( Arachis hypogaea L.) DISEASES USING BAHIAGRASS ( Paspalum notatum Fluegge) ROTATION BY FRANCIS KODJO TSIGBEY A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORI DA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007 1

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2007 Francis Kodjo Tsigbey 2

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3 To my children and entire family

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ACKNOWLEDGMENTS I am highly indebted to Dr. Jim Marois for th e opportunity to work with him and for his invaluable mentoring and patience. I thank the re st of my supervisory committee, Dr. Lawrence Datnoff, Dr. David Wright, Dr. Je ffery Jones, and Dr. Jimmy Rich for their support, insight, and constructive critique of this work. My thanks go to all the staff at the Extension Agronomy section, NFREC Quincy for their time and assistance in conducting this research. I thank the staff of Dept. of Plant Pathology, Univ. of Florida especially Gail Harris for taking time to sort out my complex paper work during my study. My gratitude goes to all my friends Dr. and Mrs Clottey, Enoch Osekre, Jennifer McGriff, Dr. Tawainga Katsvairo, Ma ry Arhinful, Ernest Ankrah, Dr. Daniel Mailhot, Dr Susan Bambo, Loraine Gibson, C ynthia Holloway and all others for their encouragement throughout all these years. To my family Grace Dikro, Peace Amoako, Rev. Dr. Kofi Asimpi, Linda Dzah, Kwaku Bansah, Mr. Aigbovi obsia and many others, I say thank you for standing with me and supporting me in unimaginable ways. I thank my siblings (Amy Acolatse, Dora Boso, and Emmanuel Ts igbey among others) for their prayers and sacrifices. I also thank my bel oved parents Bertha Afare and the late Tefe Tsigbe. I am proud to be their son and I thank them for the suffering th ey endured in bringing me up. I also thank my children (Akpedonu Kodzo Tsigbey Jr., Edem Ts igbey, and Mawuena Tsigbey). Nothing I can do could pay for their sacrifice, for they have pa id the price for my pride. It has been a long journey, and I thank all whose name I am not ab le to mention here. I thank them for allowing God to use them in bringing me this far. Finally, I thank the Lord God Almighty for His guidance, wisdom and protection. For unto G od belong the glory and honor of my life. 4

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TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF FIGURES.........................................................................................................................9 ABSTRACT...................................................................................................................................11 CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW..............................................................13 Importance of Peanut in the United States (US) Economy.............................................13 Overview of Peanut Diseases and their Management.....................................................14 Peanut Diseases Caused by Nematodes..........................................................................19 Impact of Crop Rotations on Diseases............................................................................19 Cover Crops and Crop Disease Management..................................................................20 Influence of Organic Amendments on Plant Diseases....................................................20 Role of Root Exudates in Rhizosphere Interactions........................................................22 Allelopathic Properties of Bahiagrass.............................................................................23 2 EFFECT OF BAHIAGRASS ( Paspalum notatum fluegge ) AND CONVENTIONAL ROTATION ON EA RLY LEAF SPOT ( Cercospora arachidicola s. hori ), LATE LEAF SPOT (C ercosporidium personatum (Berk. & M.A. Curtis) Deighton ), AND RUST (P uccinia arachidis Speg.) DISEASES ON PEA NUT IN NORTH FLORIDA.........25 Introduction................................................................................................................... ..........25 Importance of Peanut Diseases and their Management...................................................25 Epidemiology of ELS and LLS.......................................................................................26 Peanut Rust ( Puccinia arachidis Speg.)..........................................................................27 Management of Peanut Leaf Spots..................................................................................28 Materials and Methods...........................................................................................................30 Rotations and Field Practices..........................................................................................30 Field Practices in 2003 and 2004.....................................................................................31 Field Practices in 2005 and 2006.....................................................................................32 Disease Assessments.......................................................................................................33 Pod Yield and Grade.......................................................................................................34 Statistical Analysis..........................................................................................................3 4 Results.....................................................................................................................................35 Rust ( Puccinia arachidis)................................................................................................37 Influence of Rotations on P eanut Pod Yield and Quality................................................37 Discussion...............................................................................................................................37 5

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3 SUPPRESSION OF TOMATO SPOTTED WILT (TSW) OF PEANUT IN A BAHIAGRASS ( Paspalum notatum Fluegge) ROTATION..................................................49 Introduction................................................................................................................... ..........49 Materials and Methods...........................................................................................................52 Rotation and Cultural Practices.......................................................................................52 Field Practices in 2003 and 2004.....................................................................................52 Field Practices in 2005 and 2006.....................................................................................53 Tomato Spotted Wilt Assessment...................................................................................53 Thrips Infestation Studies................................................................................................54 Statistical Analysis..........................................................................................................5 5 Results.....................................................................................................................................55 Monitoring of Thrips Activity.........................................................................................57 Pod Yield and Grade.......................................................................................................59 Discussion...............................................................................................................................59 4 EFFECT OF ROTATIONS ON SOUTHERN STEM ROT (SSR) (S clerotium rolfsii sacc) AND SURVIVAL OF SCLEROTIA IN FIELD SOIL AMENDED WITH BAHIAGRASS CUTTINGS UNDER GREENHOUSE CONDITIONS..............................70 Introduction................................................................................................................... ..........70 Material and methods.............................................................................................................73 Field Studies....................................................................................................................73 Isolation and Maintenan ce of Micro-organisms..............................................................74 Soil Treatment.................................................................................................................74 Determination of Sclerotia Survival in Soils...................................................................75 Data Analyses..................................................................................................................75 Results.....................................................................................................................................76 Field Results....................................................................................................................76 Greenhouse Results.........................................................................................................76 Discussion...............................................................................................................................77 5 EFFECT OF BAHIAGRASS ON NEMATODE POPULATION, REPRODUCTION AND MOVEMENT IN THE FIELD, GREENHOUSE AND LABORATORY CONDITIONS..................................................................................................................... ...81 Introduction................................................................................................................... ..........81 Nematodes Diseases of Peanut in So utheastern US and their Management...................81 Organic Amendments and Nematode Suppression.........................................................82 Role of Root Exudates in Rhizosphere Interactions........................................................84 Materials and Methods...........................................................................................................86 Rotation and Cultural Practices.......................................................................................86 Effect of Bahiagrass Cuttings on Populations of M. arenaria Inter-Planted with Tomato......................................................................................................................... 87 Nematode Juvenile Movement on Agar-g rown Bahiagrass and Tomato Seedlings.......88 Data Analyses..................................................................................................................89 6

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Results.....................................................................................................................................89 Discussion...............................................................................................................................91 6 SUMMARY AND CONCLUSIONS.....................................................................................98 LIST OF REFERENCES.............................................................................................................103 BIOGRAPHICAL SKETCH.......................................................................................................117 7

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8 LIST OF TABLES Table page 2-1 Effect of rotations on final severity (Flori da 1-10 scale), appare nt infection rate (r) and SAUDPC on peanut in Quincy, FL during 2003-2006...............................................47 2-2 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on peanut pod yield in Quincy, FL during 2003-2004 under a nofungicide and fungicide regimes................48 2-3 Influence of bahiagrass (CBBP) and c onventional (PCCP) rotations, and fungicide treatments on peanut grade and damaged kernels in Quincy, FL......................................48 5-1 Effect of rotations on soil nematode populations during 2003-2006.................................94 5-2 Effect of rotations on soil nematode populations across 2003-2006 ................................95 5-3 Influence of bahiagrass residues on devel opment of tomato plants infected with M. arenaria ..............................................................................................................................96 5-4 Effect of bahiagrass and tomato root s on the behavior of juveniles (J2) of M. arenaria on water agar.......................................................................................................97

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LIST OF FIGURES Figure page 2-1 Effect of bahiagrass (CBBP) and conve ntional (PCCP) rotation on of leaf spot severity progress on Georgi a Green peanut during 2003...................................................42 2-2 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on leaf spot severity progress on Georgia Gree n peanut during 2004................................................................42 2-3 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on leaf spot severity progress on AP3 peanut during 2005.........................................................................43 2-4 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on leaf spot severity progress on AP3 peanut during 2006.................................................................................43 2-5 Effect of bahiagrass (CBBP) and conve ntional (PCCP) rotation on the incidence of peanut rust during 2003.....................................................................................................44 2-6 Total monthly rainfall in Quincy, FL during 2003-2006...................................................44 2-7 Variation in mean monthly relative humidity in Quincy, FL during 2003-2006...............45 2-8 Variation in average atmosphe ric temperature in Quincy, FL...........................................45 2-9 Linearized transformation of Cercospora leaf spot severity..............................................46 3-1 Effect of rotations on the acro ss year incidence of TSW on peanut..................................64 3-2 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on progression of TSW incidence on Georgia Green peanut during 2003.....................................................64 3-3 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on progression of TSW incidence on Georgia Green peanut during 2004.....................................................65 3-4 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on progression of TSW incidence on AP3 peanut during 2005......................................................................65 3-5 Effect of different cropping sequences on thrips population on AP3 peanut seedlings during 2005........................................................................................................................66 3-6 Effects of rotations on thrips feeding damage on peanut seedlings in Quincy FL during 2005........................................................................................................................66 3-7 Effect of different cropping sequenc es on progression of TSW incidence on AP3 peanut during 2005............................................................................................................67 3-8 Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on progression of TSW incidence on Georgia Green peanut during 2006.....................................................67 9

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10 3-9 Thrips population on peanut seedlings, oa t seed, and bahiagrass inflorescence in Quincy, FL during early May 2006...................................................................................68 3-10 Relationship of early thrips population on peanut seedlings on feeding damage and the final TSW incidence on peanut....................................................................................68 4-1 Effect of bahiagrass (CBBP) and conve ntional (PCCP) rotation on the incidence of southern stem rot (SSR) in Quincy, FL during 2003.........................................................79 4-2 Experiment 1: Effect of bahiagrass amendments on survival of sclerotia of Sclerotium rolfsii ................................................................................................................79 4-3 Experiment 2: Effect of bahiagrass amendments on survival of sclerotia of Sclerotium rolfsii ................................................................................................................80 4-4 Experiment 3: Effect of bahiagrass amendments on survival of sclerotia of Sclerotium rolfsii ................................................................................................................80

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy MANAGEMENT OF MAJOR PEANUT ( Arachis hypogaea L.) DISEASES USING BAHIAGRASS ( Paspalum notatum Fluegge) ROTATION By Francis K. Tsigbey December 2007 Chair: James J. Marois Cochair: Lawrence E. Datnoff Major: Plant Pathology Suppression of peanut ea rly leaf spot (ELS) ( Cercospora arachidicola S. Hori,), and late leaf spot (LLS) [ Cercosporidium personatum (Berk. & M.A. Curtis) Deighton was accomplished in a cotton (C)-bahiagrass (B)-bah iagrass (B)-peanut (P) [CBBP] rotation when compared to a conventional peanut (P)-cotton (C )-cotton (C)-peanut (P) [PCCP] rotation in north Florida during 2003-2006. The CBBP rotation significantly (P 0.05) reduced ELS and LLS in peanut in all years as compared to the PCCP rotation. Final ELS and LLS severities (Florida 1-10 scale) in the PCCP rotation were 7, 8, 7, and 8; whereas t hose for the CBBP were 5, 6, 6, and 6 during 2003, 2004, 2005, and 2006, respectively. The apparent inf ection rates (r) were significantly (P 0.05) higher in the PCCP than the CBBP rotation in all years. During 2003, peanut in the CBBP rotation had significantly (P 0.05) higher incidence of rust ( Puccinia arachidis Spegg) than those in the PCCP rotation. Pod yields were significantly (P 0.05) lower (2,229; 2,297; 1,703; and 3,278 kg/ha) for the PCCP rotation than for the CBBP rotation (2,935; 3,053; 2,250; and 4,504 kg/ha) in 2003, 2004, 2005, and 2006, respectively, when not sprayed with fungicide. Tomato Spotted Wilt (TSW) incidence and sever ity were lower in the CBBP than the PCCP peanut in all years. Incidence of TSW on peanut ranged 6-16, 24-36, 21-37, 18-25% in 2003, 11

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12 2004, 2005, and 2006, respectively, in a CBBP rotati on, whereas the incidence ranged 15-24, 2873, 28-77, 39-53% in 2003, 2004, 2005, and 2006, respectively, in a PCCP. Peanut seedlings suffered more thrips feeding damage (100%) unde r the PCCP rotation as compared to 45% under the CBBP rotation. Thrips ( Frankliniella spp.).population on peanut seedlings were similarly higher on the PCCP than the CBBP rotation in 2005. Reduction in the incidence of southern stem rot (SSR) [ Sclerotium rolfsii Sacc.] was achieved in CBBP rotation when compared to a PCCP rotation in north Florida during 2003. Peanut gr own in the CBBP rotation had significantly (P 0.05) lower SSR incidence. Field soils amended with leaves and roots of bahiagrass reduced survival of sclerotia of S. rolfsii in the range of 75-100%. Incorpor ation of plant parts encouraged colonization of sclerotia by Trichoderma spp. and bacteria. Amendments with higher proportions of leaf material reduced sclero tia survival the most and encour aged the growth of antagonists. Bahiagrass rotations reduced soil populations of Meloidogyne spp., Rotylenchulus spp., and Helictylenchulus spp., in exception of Criconemoides spp. Bahiagrass root exudates actively attracted juveniles of M. arenaria to root zones in water agar. Field soils incorporated with bahiagrass residues significantly (P 0.05) suppressed egg production of M. arenaria when compared to non-amended soils. Amendments with higher proportions of leave were more effective in suppressing egg production.

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CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW Importance of Peanut in the United States (US) Economy The cultivated peanut ( Arachis hypogaea L.) is an annual self-pollinating, herbaceous legume native to South America (Hammons, 1973). As a geotropic plant it produces pods (fruits) in the soil. After flower fertilization 4-6 weeks after plantin g, carpophores (pegs or pointed needle-like structures) develop a nd grow into the soil to form th e pod seed. Peanut is one of the most important legume crops in the US economy. The southeastern US is the largest peanut production region, with Florida producing approximately 8.5% of the total crop during 2006 (USDA, 2006). The US is a major exporter as well as consumer of peanuts. The use of peanut in confectionery is a widespread practice and constitutes 24% of total production. A greater proportion of peanut produced worldwide is pressed for oil and meal. It a ccounts for one-sixth of the worlds supply of vegetabl e oil (Garciacasellas, 2004). According to Fletcher (2002), counties in th e southeastern US obtain 50-70% of their agricultural income from peanut s, and the crop serves as an integral component of their agriculture. Profit of any agricultural commodity is a function of yield, pr ice, quality and cost, thus farmers must pay particular attention to these factors. Howe ver, farmers have least control over price, and hence, the only way to achieve pr ofitability is to reduce costs. Approximately 3050% of input costs in peanut production are allocated to managi ng weeds, insects, and disease (Garciacasellas, 2004). The quest by southeastern farm ers is to find ways to reduce input costs to boost profit. One approach will be the use of crop rotation with compatible crops to reduce disease impact. 13

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Overview of Peanut Diseases and their Management Two major leaf spot diseases, ea rly leaf spot (ELS) caused by Cercospora arachidicola S. Hori (teleomorph: Mycosphaerella arachidis Deighton) and late leaf spot (LLS) Phaeoisariopsis personata (Berk. & M.A. Curtis) Arx (also referred to as Cercosporidium personatum (Berk. & M.A. Curtis) Deighton (teleomorph: Mycosphaerella berkeleyi Jenk.)), are the most devastating diseases of peanut in the s outheastern US. Both ELS and LLS are prevalent in all peanut producing regions of the world and result in yi eld reductions in the range of 50-70%, and may cause complete defoliation in the absence of management practices (Nutter and Shokes, 1995). Both ELS and LLS are initiated during prol onged periods of high leaf wetness. The primary inocula for these diseases are from stroma tic tissue that are resident in soils previously grown to peanut as well as the formation of conidia from volunteer peanuts (Jackson and Bell, 1969). Infections of C. arachidicola affect all above ground parts (lea ves, petioles, and stems) of the peanut plant resulting in the formation of brow n necrotic lesions which may be either discrete or coalesce to form larger lesions. Primary inf ections usually occur on the adaxial leaf surfaces of lower leaves on the stem (Smith and Littrel, 1980). Extensive research on the epidemiology of ELS and LLS has been conducted by several rese archers over the years (Poter and Wright, 1991; Nutter and Shokes, 1995). The incubation and latent periods of ELS and LLS vary in different peanut varieties, and this is influenced by enviro nmental conditions but, can be as short as 9 to 18 days (Shew et al., 1988). Secondary spread of both ELS and LLS by conidia is weather dependent, with wind, rain splash, and insect s as the major disper sal agents (Shokes and Culbreath, 1997). Management of leaf spot diseases of peanut has been accomplished through the combination of crop rotation, burying of crop residue with moldboard plow, and multiple applications of fungicides (Nu tter and Shokes, 1995; Smith and Littrell, 1980; Kucharek, 1999). 14

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Brenneman et al. (1995) reported in creased peanut yield under bahiag rass rotations as well as the reduction of ELS and LLS, and noted that this was dependent upon the number of years between rotations with the perennial grass. The freque ncy of cropping peanut and rotation crop selection had a significant impact on peanut yield although yield did no t always increase as the time interval between peanut crops was lengthened (Hag an et al., 2003). Peanut yields in bahiagrasspeanut (Brenneman et al., 1995; Johnson et al., 1999), corn-peanut (John son et al., 1999), and cotton-peanut (Johnson et al., 1999 ; Rodriguez-Kabana et al., 1991) cropping patterns were consistently higher than those observed in plots maintained in a peanut monoculture. Southern stem rot (SSR) caused by Sclerotium rolfsii Sacc., teleomorph, Atelia rolfsii (Curzi) Tu & Kimbrough, can be a devastating pathogen on p eanut (Backman and Brenneman, 1984). Stems, pegs, and pods of the peanut pl ant are susceptible to the pathogen, and all commercially grown cultivars are susceptible. Sclerotium rolfsii attacks most plant species and is difficult to control due to th e production of sclerotia that can survive in soil under varied conditions for several years (Backman and Brenne man, 1984). Signs of SSR on peanut consist of the presence of white fluffy and cottony mycelia of S. rolfsii (but most often or iginating from the soil line on the stem of affected plant parts). Management of SSR usi ng fungicides was difficult prior to the development of Folicur (tebuconazole, Bayer Crop Science) and later Moncut (flutolanil, Bayer Crop Science) (Brenneman et al., 1995). Aycock (1966) and Umaerus (1992) described the difficulty in managing stem rot due to the presence of numerous hosts and ability of the fungus to survive as sclerotia and dry mycelia on debris Crop rotation using grasses has been found to suppress SSR (Flowers, 1976; Mint on et al., 1991). The actual mechanism of SSR reduction in a bahiagrass rotation has not been studied and may not possibly be solely due to the non-host status of bahiagrass to S. rolfsii Sclerotium. rolfsii grew and produced sclerotia on 15

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agar-grown bahiagrass seedlings that were co-inoculated with S. rolfsii (Tsigbey, unpublished). Other factors such as the enhancement of microbial population antagonistic to S. rolfsii propagules as well as gaseous products from the decomposition of bahiagrass may contribute to population decrease of S. rolfsii. Timper et al. (2001) reported lower incidence of stem rot on peanut grown after two years of bahiagrass than in continuous peanut or two years cotton, or corn before peanut in plots that were spra yed with fungicide. In the previous study, yield increase of 22% was reported for plots grown to peanut after two years of bahiagrass than two years of corn (12%), but it was not clear whether those plots received either aldicarb or aldicarb and flutolanil combined. Tomato spotted wilt (TSW) of peanut is caused by thrips-vectored tomato spotted wilt virus (TSWV), a Tospovirus that belongs to the family Bunyaviridae, and causes severe problems in many of the worlds cropping system s (Moyer, 1999). Symptoms of TSW on peanut vary and could be dictated by cultivar, but ch aracteristically include concentric ringspots, chlorotic leaflets, stun ting of plants, misshapen pegs, pods, and kernels, reddish discoloration, and cracking of the seed coats (Costa, 1941; Culbreath et al. 1992). Tobacco thrips [ Frankliniella fusca Hinds (Sakimura)] and we stern flower thrips [ F. occidentalis (Pergande)] are confirmed vectors of peanut TSWV, and these insects are present in the southeaste rn US (Todd et al., 1993; Todd et al., 1995). TSWV is acquired and transmitte d by both larvae and adults (Wijkamp et al., 1993) through feeding. TSW is now a seri ous and complex disease of peanut (Arachis hypogaea L.) and is common across the peanut growing regions in southeastern US including Alabama, Florida, Georgia, and North Ca rolina (Culbreath et al., 1997). The impact of TSW on peanut production in southeastern US has been devastati ng since its first appear ance in 1971 (Culbreath et al., 2003). Less than two decades after its entr y, TSW of peanut destroyed 50% of the peanut 16

PAGE 17

crop in southern Texas in 1985 with near 100% lo ss in some fields (Black et al., 1987), while losses to peanut due to TSW were around $40 million in Georgia in 1997 (Culbreath et al., 1999). Volunteer peanuts and weeds in fields serve as virus reservoirs and hosts to thrips which aids the persistence of the virus in fields (Chamberlin et al., 1992; Chamberlin et al., 1993). Several factors including crops in rotation with peanut, cultivar susceptibility, pesticide, soil type, tillage methods, and time of planting affect TSW incidence and severity. However, the role of soil type and rotation crops on the survival of thrips and their impact on TSW have not been thoroughly studied. Information on the contribution of soil to thrips infestation is scant y, however Barbour et al (1994) found fewer thrips emerging from soils than those collected on open-sticky cards in North Carolina and concluded that soils from pean ut fields were not a major source of thrips. Timper et al. (2001) did not find any significant difference in injury due to thrips feeding as a result of rotation, though the comb ined treatment of aldicarb and flutolanil significantly reduced thrips feeding damage in comparison to control plots. Management of peanut TSW poses tremendous challenges due to the multi-factors involved in disease incidence and severity. Since TSW is vectored by insects, the first approach was to control the thrips vector s, but that has been found to be inconclusive (Todd et al., 1996). Chemical control of thrips has not been effective in managing TSW on peanut (Mitchelle et al., 1991; Todd et al., 1996), possibly due to the mode of virus transmission and vector mobility. Modified thrips feeding behavior has been reported for F. fusca when Admire (imidacloprid, Bayer Crop Science) was applied on tomato (C haisuekel and Riley, 2001). Black et al. (1993) reported the difficulty in quantifying the effect of insecticide treatments on the incidence and 17

PAGE 18

severity of TSW on peanut due to interplot inte rference by thrips from non-insecticide treated plots as well as insects from other locations. In-furrow application of phorate suppressed TSW epidemics on peanut, and Culbreath et al. (2003) reported that in-furrow application of thimet 20-G (phorate, Amvac Chemical Corp) reduced TS W in 63 out of 93 tests over a 3 year period, though the mechanism of suppression did not correlate with thrips control. Gallo-Meagher et al. (2001) reported that the mechanism of TSW cont rol in phorate treated pe anuts appeared to be due to activation of defense genes. However, the application of acibenzolar-S-methyl, a known defense gene activator, failed to show any signi ficant suppression of TSW on peanut (Culbreath et al., 2003). Winter and spring insecticide spray app lication of Furadan (carbofuran, FMC Agr. Chem) reduced initial thrips population but ga ve no consistent TSW reduction (Todd et al., 1996). Application of Classic (chl orimuron ethyl, DuPont) was reported to increase the incidence of TSW on peanut (Prostko et al., 2002). In an extensive review of the epidemiology and management of TSW on peanut, Cu lbreath et al. (2003) proposed the integration of chemical, genetic, and cultural practices involving planti ng date, manipulation of plant population, tillage practices, row pattern as well as in-furrow insecticide applicat ion among other options in the management of TSW on peanut Genetic resistance holds promise to manage TSW, but incorporating resistance to all economic diseases of peanut with acceptabl e yield and quality is a major challenge. No-till and minimum tillage systems for peanut have become an economic option for peanut cultivation in the southeas tern US. Use of minimum tillage in peanut has been reported to reduce the impact of TSW and leaf spots compared to a conventional till (Baldwin et al., 2001; Johnson et al., 2001; Monfort, 2002). Cantowine et al. (2006) reporte d the interacti on effect of cultivar and tillage method on the suppression of ELS and TSW. 18

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Peanut Diseases Caused by Nematodes Plant parasitic nematodes are damaging to pea nut causing an estimated loss of 12% or US$ 1.03 billion annually (Sasser and Fr eckman, 1987). Several nematode species attack peanut but the most prevalent include Meloidogyne spp., Pratylenchus brachyurus, and Belonailamus longicaudatus (Shama, 1985). Sasser (1977) de signated three species of Meloidogyne that are damaging to peanuts: M. arenaria M. javanica and M. hapla. Among these M. arenaria (the most dominant species on peanut in the US) and M. javanica occur in warm and hot regions of the world while M. hapla occurs only in cooler regions (D ickson and Waele, 2005). Nematodes are widespread and destructive pests on peanut and could be described as the hidden enemy since their damage is often imperceptible to farmers. Damaging nematodes are not evenly distributed across a field and scat tered patches with damage can range in size from a few meters to several hectares. Impact of Crop Rotations on Diseases Crop rotations have been used for centuries in the management of cr op diseases and most often are more effective in the manageme nt of soil-borne disease (Sullivan, 2004). The exact modes of disease suppression during rotation is not fully understood, though it ranges from the reduction of pathogen inoculum during periods when non-host crops are cultivated, soil microbial population and diversity changes that favor non-pathogenic organisms to increasing the number of pathogen antagonists. The efficiency of crop rotation in the management of plant diseases is influenced by the interacting factors of various co mponents, ranging from environmental, edaphic (soil texture, structure, infiltration, fertility etc), type of rotation crop, the target pathogen, as well as the time interval between rotations (Summer, 1982). One major principle in crop rotation for pl ant disease management is the planting of a poor host plant for pathogen reproduction for more than one year (D ickson, 1956). It is the complexity of these 19

PAGE 20

interacting factors that make it difficult to attribute the actual mechanisms involved in disease suppression under any rotation system. Katsvairo et al. (2007) reported th at bahiagrass rotation in a traditional southeastern US agricultural sy stem improved cotton root development, biomass, and increased earthworm densities in the soil. Cover Crops and Crop Disease Management Green-manuring and cover cropping are somewh at similar with the exception that cover crops are often killed before in corporation into the soil. In a bahiagrass rotation scheme, the grass is killed either late fall or early sp ring and the most common tillage system currently practiced is strip-till. To a large extent, both improve soil characteristics, and many times control soil pests. The mechanisms of soil pest suppre ssion are also identical. Whether cover crops are incorporated as green manure or as killed rotation crops, the pro cess of decomposition releases a large array of active compounds that tremendously impact the diversity of soil microbial populations. Organic amendments bring about significant changes in soil physical and biological properties during the process of decomposition which affects the survival and multiplication of both pathogen inocula and antagoni sts (Bulluck and Ristaino, 2002). Influence of Organic Amendments on Plant Diseases Cover crops alter both the dive rsity and abundance of pathogen and pest antagonists, thus aiding in their reduction. Severa l mechanisms are proposed to be responsible for the reduction of plant-parasitic nematodes by cover crops including: 1) cover crops act as non-host or poor hosts (Rodriguez-Kabana et al., 1992, 1994); 2) producti on of allellochemicals that are toxic or inhibitory (Haroon and Smart, 1983; Gommens and Baker, 1988; Halbrent, 1996); 3) cover crops provide a niche for antagonistic flora and fauna (L inford et al., 1937; Evans et al., 1988; Caswell at al., 1990; Kloepper et al., 1991); a nd 4) cover crops may act to tr ap nematodes (Gallaher et al., 1991; Gardner and Caswell-Chen, 1994; Lamondia, 1996). Cover crops can also influence soil 20

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nematode populations by their failure to repr oduce in non-hosts, and in some cases, produce fewer eggs (Rich and Rahi, 1995, McSorley, 1999). Organic amendments and other naturally occurring compounds can effectively suppress a number of plant-parasitic nematodes. Chavarria-Carvajal et al. (2001) ev aluated pine bark, velvetbean, kudzu (Peuraria montana va r. lolobata) paper waste and benzaldehyde for control of plant-parasitic ne matodes and found that most amendments alone or in combination with benzaldehyde reduced damage from nematodes. Wang (2000) suggested that the reduction in R. reniformis population in a pineappleCrotalaria juncea intercropping cycle might be due to the enha nced activities of bacterivorous nematode population as well as nematode-trapping fungi. Ti mper et al. (2001) observed a cropping system effect on M. arenaria and its antagonist Pasteuria penetrans where populations of P. penetrans endospores were higher in a continuous peanut than a system of bahiagrass for two years followed by peanut. The ability of bahiagrass to support P. penetrans or other antagonistic organisms has not been well studied. The influence of plant residue incorporation on the survival of pathogens and reproduction of nematodes has been well studied in greenhous e experiments (Haroon and Smart, 1983; Akhtar and Malik, 2000; Widmer and Abawi, 2000), though th e successful applicati on of such studies to large scale field research has been lim ited. Such studies notwithstanding, allow the understanding of the probable modes of disease suppression of organic residues in the field. Most often, such studies were designed to study the impact of amendments on soil-borne diseases but could as well be us ed to study the effect of such systems on foliar diseases. Organic amendments could release gases during decomposition that affect both foliar and soil-borne pathogens (Sayre et al., 1964; Hollis and Rodriguez-Kabana, 1966; Elmiligy and Norton, 1973). Incorporation of organi c amendments affect survival of soil-borne pathogens such as S. rolfsii, 21

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and Sclerotinia minor. Ferguson and Shew (2001) reported th at although stem rot incidence of peanut was not affected by applic ation of wheat straw mulch and the final inoculum density of S. rolfsii was highest in those plots. Straw application reduced Sclerotinia blight in some years of their study and was found to be dependent on the initial inoculum density. All agricultural soils have a variety of micro-organisms that can s uppress plant pathogens w ith mycorrhizal fungi found among them. Role of Root Exudates in Rhizosphere Interactions The term allelopathy was coined by Molisch in 1937 and was later adopted by Rice (1984) to include both harmful and benefi cial interactions between all t ypes of plants and interactions involving microorganisms (Alam et al., 1980). Effect s of allelopathy in plants are manifested under several conditions including incorporating crop residues as gr een manure, as well as mulch stubble, replanting problems, autotoxicity, and ro tation using different pl ants (Alam, 1980). Rice (1984) reported that plants influence each other by means of exudates, and similarly, reports by Buchholtz (1971), Fisher et al. (1978), Bhowmi k and Doll (1980, 1983) suggested that leachates from residues incorporated into the growing medi um or residues in natural undisturbed condition may also play a role. Allelopathic substances escape into the environment by volatilization, exudation from roots, or from decay of plant materi als, and it appears that all plant parts possess allelopathic properties (Alam et al., 1980). Beneficial allelopath ic interaction effects were reported between crops and weeds to the extent that the presence of one type of weed resulted in the suppression of another kind of weed (Alteiri and Doll, 1978). Bais et al. (2006) reported that some of the most complex chemical, physical, and biological interactions experienced by terres trial plants occur between roots and their surrounding environment of soil. Several workers (K neer et al., 1999; Bais et al., 2003; Bais et al., 2002) reported the importance of root exudates in biological processes including root-root, 22

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root-insect, and root-microbe in teractions. The rhizosphere is al so considered a dynamic front for interactions between roots and beneficial soil microbes, invert ebrates, and root systems of competitors (Hirsch et al., 2003). Chemical signali ng is believed to occur between plant roots and other soil organisms and is often based on root-derived chemicals (Bais et al., 2006). Chemical components of root exudates may deter one organism while attracting others with varied consequences to the plant and its nei ghbors. Morris et al. ( 1998) reported that the secretion of isoflavones by soybean roots attract both a mutualist ( Bradyrhizobium japonicum ) and a pathogen (Phytophthora sojae ). Root exudates may play direct roles as phytotoxins in mediating chemical interference as seen in alle lopathic relationships, an d are critical to the development of associations between some parasitic plants and their hosts as well as playing indirect roles in resource competition by altering soil chemistry (Bais et al., 2006). Allelopathic Properties of Bahiagrass Allelopathy has been demonstrat ed in bahiagrass. Fisher and Adrian (1981) observed that as the percentage of ground covered with bahiag rass increased, the height of a 3-year-old pine decreased markedly, and further demonstrated th at both living and decaying bahiagrass residue are allelopathic to pine. Wateri ng pine seedlings with root leach ates of bahiagrass also reduced root, shoot and total dry weight of pine seedlings, which clearly de monstrated allelopathic effect. An allelopathic effect of bahiagrass on peanut have not been directly demonstrated. Despite the previous observations, vigorous growth and higher yi eld of have been reported when peanut is grown after bahiagrass in comparison to a conven tional cotton-cotton-peanut rotation (Hagan et al., 2003; Katsvairo et al., 2006). It has been known for centuries that crop rotation is generally beneficial to crop production, a nd suggestions are that these benefits are due to improved nutrition, decreased disease leve ls and improved soil structur e (Abawi and Widmer, 2000). 23

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24 Johnson and Pfleger (1992) indicated that the be neficial effects of a rotation may be due to the population dynamics of VAM fungi It is therefore highly probabl e that the presence of VAM fungi in bahiagrass may be accounting for the higher yields and lower diseases in a bahiagrasspeanut rotation system. Bahiagrass exudates have been reported to enhance mycelial and spore production in VAM fungi (Ishii et al., 1996; Cruz et al., 2003). Higher hyphal as well as high root infection and spore numbers of G. margarita were found in a bahiagrass and millet compartment than in an adjacent papaya compartm ent when each were separated from a central compartment in which Gigaspora margarita spores were placed (C ruz et al., 2003). Methanol extraction of bahiagrass and millet roots eluates showed a similar result as above (Cruz et al., 2003), an indication that bahiagrass produ ced compounds that stimulate VAM fungi development. Bahiagrass root ex tracts stimulated the growth of Gigaspora ramisporophora in axenic culture (Ishii et al., 1997), and the substances later id entified as flavonoids: eupalitin and two other unidentified co mpounds (Ishii et al., 1997).

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CHAPTER 2 EFFECT OF BAHIAGRASS ( Paspalum notatum fluegge ) AND CONVENTIONAL ROTATION ON EA RLY LEAF SPOT ( Cercospora arachidicola s. hori ), LATE LEAF SPOT (C ercosporidium personatum (BERK. & M.A. CURTIS) Deighton ), AND RUST (Puccinia arachidis Speg.) DISEASES ON PEANUT IN NORTH FLORIDA Introduction Importance of Peanut Diseases and their Management All parts of the peanut plant are susceptible to insect pests and diseases (Jackson and Bell, 1969). The three major leaf spot diseases are early leaf spot (ELS) caused by Cercospora arachidicola S. Hori (teleomorph: Mycosphaerella arachidis Deighton) and late leaf spot (LLS) caused by Phaeoisariopsis personata (Berk. & M.A. Curtis) Ar x, also referred to as Cercosporidium personatum (Berk. & M.A. Curtis) Deighton, (teleomorph: Mycosphaerella berkeleyi Jenk.), and rust ( Puccinia arachidis) (Jackson and Bell, 1969). ELS and LLS are prevalent in all peanut producing regions of the world and can result in complete defoliation and yield reductions of 50 to 70% in the absence of management practices (Nutter and Shokes, 1995). ELS infections result in defoliation, loss of integrity of the peg, poor pod filling, as well as fewer pods per plant that result in yield loss. Seed quality is al so affected since photosynthetic ability of the plant is compromised. Aerts and Nesheim (2001) found that yield losses to ELS and LLS may vary from near zero to as much as 91% in Florida, and on a statewide basis, annual yield losses attributed to peanut l eaf spot vary from 5 to 40%. Their assessment is consistent with reports of the diseases in othe r parts of the world where peanut is grown (Waliyar et al., 2000; Tsigbey et al., 2003). Garciacasella s (2004) reported that currently 25 to 30% of the input costs of producing peanuts are allocated to managing the major insect pests and diseases, and a greater proportion of these budgets are allocate d to managing leaf spot diseases. In view of the extensive input requirement s in leaf spot management, producers will require sound science in making decisions in order to minimize cost. Management of leaf spot 25

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diseases poses challenges to farmers due to the influence of weather on disease onset, severity, and dissemination. The fungi that cause ELS and LLS are persistent in most locations but are especially troublesome during wet and humid years. Frequent rainfall or irrigation during the growing season exacerbate the difficulty in managing these diseases. Epidemiology of ELS and LLS Both ELS and LLS are initiated during prol onged periods of high leaf wetness. The primary inocula for these diseases are from stroma tic tissue that are resident in soils previously grown to peanut as well as the formation of conidia from volunteer peanuts (Jackson and Bell, 1969). Infections of C. arachidicola affect all above ground parts (lea ves, petioles, and stems) of the peanut plant resulting in the formation of brow n necrotic lesions which may be either discrete or coalesce to form larger lesions. Primary inf ections usually occur on the adaxial leaf surfaces of lower leaves on the stem (Smith and Littrel, 1980). Extensive research on the epidemiology of ELS and LLS has been conducted by several rese archers over the years (Poter and Wright, 1991; Nutter and Shokes, 1995). The incubation and latent periods of ELS and LLS vary in different peanut varieties, and this is influenced by enviro nmental conditions but, can be as short as 9 to 18 days (Shew et al., 1988). Secondary spread of both ELS and LLS by conidia is weather dependent, with wind, rain splash, and insect s as the major disper sal agents (Shokes and Culbreath, 1997). Besides weather factors, the epidemics of ELS and LLS are influenced by host resistance, leaf spot management practices, tillage, and cropping system. Monfort et al. (2004) reported the suppression of ELS in strip-till plots compared to those in conventionally-t illed plots. Similarly, Cantonwine et al. (2006) charac terized the suppression of ELS of peanut under strip-till and conventional tillage. It was established in the pr evious study that the reduc tion of ELS in peanut under strip-till is due to the reduc tion in initial inoculum in the form of stroma which in turn 26

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delays the epidemics (Cantonwine et al., 2006). An average of 8 to 11 days of delay in ELS onset was reported by Cantonwine et al. (2006) under a strip-till system, whic h can significantly prolong the timing of the first fungicide appli cation without compromisi ng yield. They further reported that the epidemic rate (r) was significant ly lower for the strip-ti ll than the conventional tillage system. Peanut Rust ( Puccinia arachidis Speg.) Peanut rust is caused by Puccinia arachidis Speg., and affects all above ground parts of the plant, (Kucharek, 1979). The disease is charact erized by rusty pustules particularly on the undersides of peanut leaves with associated chlorotic symptoms on th e corresponding adaxial surfaces. Mixed infections of rust, ELS, and LLS are a common occurrence in peanut fields and complicate their individual impact on yield reduction. While all of them manifest as leaf spots, a characteristic distinguishing feature of peanut rust is the observa tion that rust affected peanut leaves still remain attached to the stem and gi ve the field a burnt appearance, whereas leaves attacked by both ELS and LLS are shed onto the ground. Though still attached, rusted leaves lose their photosynthetic ability and serve as a drain to the plant. Peanut rust appears as minute leaf spots or flecks that are visible from both sides of the leaf. As the number of infections increase and become older, leaves become chlorotic. The fungus produces uredos pores within uredial pustules found primarily on the leaves of the hos t. Uredospores readily become airborne and serve to disseminate the fungus Under appropriate conditions of temperature and moisture, uredospores germinate, penetrate, and infect the host within hours and a new crop of uredospores matures within 10 days (Bromfield and Kenne th, 1969). Infections may also develop on stems and leaf petioles. Kucharek (2000) reported that occurrence of peanut ru st does not follow any predictable manner in fields in Florida and often only becomes noticeable in August. Since rust occurs late in the cropping season, its contri bution to yield reduction could be minimal. 27

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Management of Peanut Leaf Spots Management of leaf spot diseases of peanut has been accomplished through the combination of crop rotation, burying of crop residue with moldboard plow, and multiple applications of fungicides (Nu tter and Shokes, 1995; Smith and L ittrell, 1980; Kucharek, 1999). However, increased production co sts, suppressed crop prices, hi gh energy costs, and a reduced labor force is making peanut cultivation increasi ngly risky. To be competitive, farmers need to be more innovative and adopt sustainable producti on methods in order to reduce production costs and preserve soil productivity. One such approa ch is crop rotation by alternating peanut and cotton, which is a traditional pr oduction pattern in the southeaste rn United States. Peanut in rotations with corn and bahiagrass are also a viable option, particularly in the management of soil-borne fungi and nematodes (Flowers, 1976; Brenneman et al., 1995; Johnson et al., 1999; Timper et al., 2001), although Hagan et al. (2003) did not find any significant differences in the populations of root-knot ne matode juveniles when peanut was planted after bahiagrass, corn or cotton. Rotations of peanut with perennial grasses such as bahiagrass have been extensively researched in the management of soil-borne dise ases, with other advantages including soil health maintenance (Beaty and Tan, 1972; Katsvairo et al., 2007) and improved yields. Although the impact of a bahiagrass-peanut rotation on nematode population reduction is well documented, (Timper et al., 2001; Dickson and Hewlett,1989; No rden et al., 1980; Rodriguez et al., 1988), the same cannot be said for other major peanut diseases such as leaf spots an d TSW. In most studies, where peanut leaf spot was reportedly reduced by bahiagrass, the crop was also sprayed with a fungicide (Hagan, 2003; Br enneman et al., 1995). Continuous cropping often resu lts in yield reduction and the build up of pathogenic organisms in soils, whereas crop yields are genera lly higher in rotation with other crops (Garrett, 28

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1944). Crop rotations with perennial grasses increased yields thr ough improved soil nutrition and structure and reduction of plan t pathogens (Katsvairo et al., 2006). The primary pathogens reduced by crop rotations are thos e infecting the roots and stem s, though soil nutrition may play a part in influencing crops reac tion to other diseases. Efficient crop rotation systems to manage diseases must consist of at least one non-host plant species of the target pathogen(s). This is important in order to reduce pathogens that rely on one single plant species for proliferation. The rate of decline and length of rotation necessary for effective suppression depend on the longevity of the pathogen survival stage and the choice of crop(s) in the rotation cycle. Disease management involves practices that reduce the initial leve ls of inoculum and include selecting appropriate pl anting materials, destruction of crop residues (elimination of living plants that carry pathogens), and crop rota tion. The efficiency of crop rotation systems in the management of plant diseases is influenced by the interacting factors of various components, ranging from environmental, edaphi c (soil texture, structure, inf iltration, fertility etc), type of rotation crop, and the target pat hogen as well as the time interv al between rota tions (Summer, 1982). Several workers (Johnson et al., 1999; Rodri guez-Kabana et al., 19 94; Rodriguez-Kabana et al., 1991) have reported yield increases in peanut under one or more rotation cropping system. Brenneman et al. (1995) reported increased peanut yield in ba hiagrass rotations as well as the reduction of ELS and LLS, and noted that this was dependent upon the number of years between rotations with the perennial grass. Th e frequency of cropping pe anut and rotation crop selection had a significant impact on peanut yiel d although yield did not al ways increase as the time interval between peanut crops was length ened (Hagan et al., 2003). Peanut yields in bahiagrass-peanut (Brenneman et al., 1995; Johns on et al., 1999), corn-peanut (Johnson et al., 1999), and cotton-peanut (Johnson et al., 1999; Rodr iguez-Kabana et al., 1991) cropping patterns 29

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were consistently higher than those observed in plots maintained in a peanut monoculture. The use of perennial crops in rotation with peanut has not been a common practice, possibly due to the fact that many farmers may not receive dire ct economic benefits from the grass between rotations (Brenneman et al., 1995). Of the crops ev aluated in a one year rotation with peanut, consistent yield gains were obtained in both y ears with velvet bean and winter rye/summer fallow and in two of three years with corn, when compared with the yield recorded for continuous peanuts (Flanders et al., 2005). Hagan et al. (2003) reported a study in Ge orgia where it was found that ELS and LLS diseases on peanut were more seve re in short term rotations. This was contrary to the report of Bowen et al. (1996), who did not find any impact due to cropping patter n on the level of leaf spot-induced defoliation on peanut in fields in Alab ama. Thus, the effect of rotations on leaf spot diseases has been inconsistent. Such inconsistencies might be due to the fact that in most of these studies leaf spot was co ntrolled using different fungicides an d spray regimes. Consequently, the regimes as well as fungicide types and cultiv ar differences would have introduced these variations across locations. In or der to fully quantify the impact of rotations on leaf spot diseases, non-fungicide treated control plots need to be assessed for disease severity as well. The objective of this study was to determine the suppressive e ffect of bahiagrass rotations on peanut leaf spots in a bahiagra ss-peanut rotation cropping systems. Materials and Methods Rotations and Field Practices The study was conducted on a Dothan sandy loam (fine loamy siliceous, thermic Plinthic Kandiudult) at the North Florida Research and Education Center Quincy, FL from 2003 to 2006. Rotation plots were established in the year 2000 and consisted of a bahiagrass rotation with peanut and a conventional rotation for peanut typical in the south eastern US. The cropping 30

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sequence for the bahiagrass rotation involved the growing of cotton in the first year and then followed by bahiagrass for two consecutive years and in the fourth year the plots were planted to peanut (CBBP). The conventional rotation consiste d of growing peanut in the first year with cotton in the two subsequent years followed by pea nut in the fourth year (PCCP). Each plot in the rotation cycle was split into irrigated and nonirrigated sub plots. The irrigation sub-plots were then split again, one of which was treated with fungicide and the other was not treated when peanut was planted in the rotation year. Irrigated plots received scheduled amounts of water over the four years as needed according to standard extension recommendations for peanut production in Florida (Smajstrla et al., 2006). General weed manage ment practices in all years (2003-2006) were done in accordance with the Florida Cooperative Extension Services recommendations for peanut (Aerts and Nesh eim, 2001; Whitty, 2002; Whitty and Chambliss, 2002). Except for 2003 in which two fungicide treatme nts were carried out dur ing the earlier part of the season 30 days after planting (DAP), the same split sections of the plots under fungicide treatments remained treated or not treated (zero spray) for 2004, 2005, and 2006. Each (rotation* irrigation*fungicide) sub-sub-plot consisted of ten rows measuring 22.8 m long by 9.2 m wide (10 peanut rows). Field Practices in 2003 and 2004 The bahiagrass cover crop was killed in December of 2003 and 2004 by applying glyphosate (Roundup WeatherMAX; Monsanto, Kansas City, MO). The winter oats ( Avena sativa L.) cv. Florida 501 cover cr op that was planted at 125 kg/ha, and killed 124 DAP in 2003 and 97 DAP in 2004 by broadcast spraying with gl yphosate. Seedbeds were prepared in both years by strip-tilling with a KMC (Kelly Mfg. Corporation, Tifton, GA). Georgia Green peanut cultivar was planted on 7 May 2003 and 10 May in 2004 with a Monosem pneumatic planter 31

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(ATI, Inc. Lenexa, KS) at 6 seeds per 31 cm of row on 91-cm row spacing. Phorate (Thimet 20-G; Micro Flo Company LLC. Me mphis, TN) at 5.6 kg/ha was a pplied in furrow at planting. Leaf spot management during 2003 and 2004 involved the alternating broadcast applications of chlorothalonil (Bravo Weatherstik 720 F; Syngenta, Crop Protection, Inc., Greensboro, NC) at 1.26 kg a.i/ha, tebuconazole ( Folicur 3.6 F; Bayer CropScience Research Triangle Park, NC) at 0.23 kg a.i./ha, and pyr aclostrobin ( Headlin e; BASF Corporation, Research Triangle Park, NC) at 224 g a.i. /ha. During 2003, plots were sprayed with chlorothalonil on 36, 78, and 121 DAP; tebucona zole on 51, 96, and 134 DAP; pyraclostrobin on 64, and 106 DAP. However, at 106 DAP manco zeb (Dithane M-45; Dow AgroSciences, Indianapolis, IN) was tank-mixed with Headline. All fungicides were applied using tractor (John Deer Model 6415, Johns Tractor Company, Jay FL .) mounted boom sprayers (nozzle size 1103). The tractor was driven at C-range at 1,600 RPM and the pressure was 30 PSI, that delivered 47 gals water/ha. The same tracto r specifications were used in all years. During 2004, leaf spots were controlled by alternating ap plications of chlorothalonil at 1.26 kg a.i./ha and tebuconazole at 0.23 kg a.i./ha tank-mixed with Induce as a spreader at 41, 55, 69, 73, 87, 101, and 115 DAP, respectively. Field Practices in 2005 and 2006 Bahiagrass cover crop was killed in fall of 2004 and 2005 for the coming years cropping season. Oats (cv. Chapman, Maynand Douglas Fa rms, Cottondale, FL.) were planted at 125 kg/ha on 26 November 2004 as winter cover cr op for the 2005 cropping season, and cv. Florida 501 (Maynand Douglas Farms, Cottondale, FL.) was planted on 10 December 2005 at 125 kg/ha for the 2006 season. Cover crops were managed according to recommended practices in both years, and killed 123 DAP in 2005 and 120 DAP in 2006. The seedbed was prepared by doubleripping 10 rows using a KMC (Kelly Mfg. Cor poration, Tifton GA) 4 row ripper in April of 32

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both years, and subsequently planted to p eanut cv. AP3 on 13 May 2005 and 17 May 2006 with a Monosem pneumatic planter (ATI, Inc. Lenexa, KS) Twin Row Planter at three seeds per 30 cm of row with simultaneous application of phorate (Thimet) at 6 kg/ha into furrows. Leaf spot management in 2005 involved the alternate sprays of chlorothalonil (Bravo Ultrex 82.5 DWG; Syngenta, Cr op Protection, Inc., Greensboro, NC) at 32, 75, and 116 DAP at 1.26 kg a.i/ha, tebuconazole (Folicur 3.6 F, Bayer CropScience) at 0.23 kg a.i./ha at 45, and 88 DAP; and pyraclostrobin (Headline) at 0.22 kg a. i./ha at 61 and 103 DAP. All fungicides were applied using tractor (John D eer Model 6415, Johns Tractor Co mpany, Jay FL.) mounted boom sprayers (nozzle size 1103). The tractor was dr iven at C-range at 1,600 RPM and the pressure was 30 PSI, that delivered 178 L water/ha. The same tractor specifications were used in all years. Leaf spot management in 2006 was done by alternating sprays of chlorothalonil, tebuconazole, and pyraclostrobin in a resistance manageme nt strategy on 44, 57, 71, 85, 99, 113, and 127 DAP. Disease Assessments ELS and LLS were assessed in all four years when leaf spots first appeared until harvest using the Florida 1 -10 scale [where 1 = no leaf spot; 2 = very few spots on leaves with none on upper canopy leaves; 3 = few lesions on the leav es, very few on upper canopy; 4 = some lesions with more on the upper canopy, 5 % defoliation; 5 = lesions noticeable on upper canopy, 20% defoliation; 6 = lesions numerous and very evident on upper canopy, 50 % defoliation; 7 = lesions numerous on upper canopy, 75 % defoliation; 8 = upper canopy covered with lesions, 90 % defoliation; 9 = very few leaves remaining a nd those covered with lesions, 98 % defoliation; and 10 = plants completely defoliated and killed by leaf spot (Chiteka et, al. 1988)]. Twenty plants were randomly scored in all plots. Disease assessments were conducted 32, 46, 61, 75, 100, 137 days after planting (DAP) in 2003; 40, 63, 91,104, 124, 132, and 140 DAP in 2004; 34, 45, 52, 64, 96, 12, and 134 DAP in 2005; and 34, 51, 66, 85, 100, 123, 145 DAP in 2006. 33

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Disease severity data were analyzed separately for each year for the non-fungicide sprayed plots, and the standardadized area unde r the disease progress curve (S AUDPC) was computed (Shaner and Finey, 1977). Disease assessments were converted into propo rtions [y = (Florida rating 1) / 9], and transformed using the linearizing tr ansformation for the Gomperzt = [-In(-In y)], and logistic = [In(y/(1-y))] models. Transformed data were linea rly regressed on time and with the first date of each year of assessment set as the beginning of the epidemic. The logistic model, which consistently had the highest R2 value, was selected for the epidemics, and the slope of the linearly regressed transformed data was used as the rate parameter (r) to estimate the epidemic rate for each year and rotation. Effects of rotation on SAUDPC a nd r were determined for each rotation and year separately. Southern stem rot incidence was assessed only in 2003 by examining twenty plants fo r signs of the pathogen, S. rolfsii, and similarly for peanut rust ( Puccinia arachidis ). Pod Yield and Grade Peanuts were inverted 144 DAP and picked 24 h later, then dried at 115 oC for 24 hours and brought to 10% moisture befo re weight determination. A 500 g sample of harvested pods per plot was removed and analyzed for commercial grading according to Federal Inspection Services methods (USDA, 2002). Statistical Analysis All the data were analyzed using Statistica l Analyses System (version 8.0, SAS Institute Inc Cary, NC) GLM approach. Data for all years were first analyzed as a sp lit-split experiment to investigate interaction of main a nd subplots. In the absence of a c onsistent interact ion effect, the data was subjected to analyses of vari ance (ANOVA) as a randomized block design. 34

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Means from the rotations were compared using the least significance difference (LSD) test to determine the differences (P 0.05) in severity measured by the SAUDPC, yield, and quality between the PCCP and CBBP rota tion for each year separately. Results Two years of a bahiagrass rotation (CBBP) si gnificantly reduced severity of ELS and LLS when compared to the conventional (PCCP) system in all years. The increase in disease severity over time was best described by the logistic model for each plot rotation in all years; R2 = 0.92 and 0.91 for the PCCP and CBBP rotations, re spectively. In 2003, ELS began appearing on plants 32 DAP (Fig. 2.1) and gradually progressed over time. Estimates of the apparent infection rate of epidemics (r) computed from the slope of the linearized logistic model was comparable for both rotations but were slightly higher fo r the PCCP (0.024) than for the CBBP (0.019) rotation. Leaf spot epidemics measured by the st andardized area under the disease progress curve (SAUDPC) were not significant for either rotation (Table 2.1). Initial infections on peanuts were caused by ELS but LLS become the dominant leaf disease 90 DAP. LLS was the predominant disease until harvest throughout the four years of the study. Since distinctions were not made when rating ELS and LLS, the mean severity was a combined score for both diseases and hereafter, referred to as leaf spots. There was no significant difference in severity rating between the CBBP and PCCP peanuts at earlier dates of disease assessment, but thereafter was consistently significant (P 0.05) until harvest (Fig. 2.1). Similarly, the proportion of plants showing higher ratings was higher in the PCCP rotation than in the CBBP rotation resulting in a higher proportion (P 0.05) of disease throughout 2003. Similar to the observations in 2003, leaf spot in 2004 star ted significantly earlier (P 0.05) for the PCCP rotation compared to the CBBP ro tation (Fig. 2.2). Except at 132 DAP, severity ratings were higher (P 0.05) for the PCCP rotation than for the CBBP until harvest. Estimates 35

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of the apparent infection rate of epidemics (r) co mputed from the slope of the linearized logistic model were higher (P 0.05) in 2004 than 2003 and compar able for the PCCP and the CBBP rotation (Table 2.1). Leaf spot epidemics (SAUDPC) were not significantly different between the two rotations (Table 2.1). However, there was a significantly higher leaf spot development for the PCCP rotation for the individual assessmen t dates in comparison to the CBBP rotation. The increase in disease severity over time was best described by the logistic model (logistic = [In(y/(1-y))]) for each rotation in 2005; R2 = 0.98 and 0.90 for the PCCP an CBBP rotations, respectively (Fig. 2.3). Estimates of the apparent infection rate of epidemics (r) computed from the slope of the linearized logistic model were almost the same as were observed in the rotations in 2004. L eaf spot epidemics measured by the standardized area under the disease progress curve (SAUDPC) were not significantly different between the two rotations (Table 2.1) although higher for the PCCP rotation. As in the previous years, disease onset was ev ident as early as one month after planting in some years, but incidence was lower in both rotations (Fig. 2.3). There was no significant difference (P 0.05) between rotations at 34 and 45 DAP. At 54 DAP, leaf spot severity increased sharply for the PCCP rotation while the CBBP peanut remained relatively disease-free. There was a corresponding higher (P 0.05) disease proportion on the PCCP rotation peanut than those in the CBBP rotation. In the PCCP ro tation, leaf spot reached 6.0 and 7.0 rating while in the CBBP rotation, leaf spot reached 5.0 a nd 6.0 ratings at 123 and 134 DAP, respectively. Leaf spot progression on AP3 peanut during 2006 was best described by the logistic model for each rotation in 2006, R2 = 0.90 and 0.88 for the PCCP and CBBP rotations, respectively (Fig. 2.4). Estimates of the appare nt infection rate of epidemics (r) computed from the slope of the linearized logistic model was comparable an d similar for both rotations but slightly higher 36

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(0.044) for the PCCP than (0.040) for the CBBP rotation. Leaf spot epidemics measured by SAUDPC were not significantly di fferent between the rotations (T able 2.1) although they were higher for the PCCP rotation. Leaf spot sever ity remained significantly higher in the PCCP rotation than the CBBP ro tation throughout the season. Rust ( Puccinia arachidis ) Rust was virtually absent in 2004, 2005, and 2006. In 2003, rotations significantly (P 0.05) affected the incidence of rust, and th e incidence was found to be more pronounced on the CBBP rotation than on PCCP rotation (Fig. 2.5). Influence of Rotations on Peanut Pod Yield and Quality Significant differences (P 0.05) were found between the yields from the bahiagrass and conventional rotations in both fungicide sprayed and non-sprayed plots in all four years (Table 2.2). In 2003 and 2004 when c.v. Georgia Green wa s planted, peanut yields were higher (P 0.05) in the bahiagrass rotation whether sprayed or not sprayed than in the conventional rotation with corresponding fungicide spray regime. Similarly, significantly higher yiel ds were recorded in the bahi agrass rotations than in the conventional rotations in 2005 and 2006 with si milar fungicide spray regimes (Table 2.2). Peanuts grown in the bahiagrass rotation produced consistently greater pod yields (6-35 %) over the four year period. CBBP rotation increased peanut yield 24, 25, 24, and 33 % in 2003, 2004, 2005, and 2006, respectively, over thos e in the PCCP rotation when both received no fungicide sprays. Peanut grade was significantly (P 0.05) improved under CBBP than PCCP rotation, and the same was observed for percent damaged kernels (Table 2.3). Discussion In this study, epidemics of peanut leaf spot were suppre ssed by two years of bahiagrass rotation (CBBP) when compared to a convent ional cotton-peanut (P CCP) rotation. Under a no 37

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fungicide spray regime, cultivati ng peanut after two years of ba hiagrass significantly reduced the severity of leaf spot diseases by delaying dise ase onset when compared to the PCCP system. Though leaf spot disease suppression in a bahiag rass rotation has been extensively reported (Brenneman et al., 1995; Hagan et al., 2003), such studies involved bahiagrass plots that were either burned before planting peanut (Brenneman et al., 1995) or the peanuts were sprayed with fungicide (Hagan et al., 2003). Such treatment s made it difficult to estimate the actual contribution of bahiagrass rotation to leaf spot suppression since there were confounding effects due to the fungicide spray or bur ning. It appeared th at the initial two f ungicide sprays during 2003 resulted in a lower disease se verity although it was higher for the PCCP rotation than in the CBBP. Consequently lower final leaf spot severity ratings for both rotations were recorded in 2003 compared to 2004. The fluctuations in disease severity in 2004 and other years could be attributed to weather variations that we re experienced (Fig. 2.6, 2.7, and 2.8). Whereas temperature variations followed si milar patterns in all four year s, those of total rainfall and relative humidity varied greatly bot h across years and within years. The epidemic rate parameter (r) calculated from the logistic transformation was similar for both rotations however; linearization of the actua l Florida severity ratings produced a s lightly higher rate parameter in all years in the PCCP than the CBBP rotation. Fluctua tions in leaf spot severity as a result of environmental conditions could have lowered the epidemic rate in the logistic model. The influence of rotation on leaf spot severity was most noticeable in 2006 when disease severity was high and the CBBP rotations still had moderate di sease. Nearly 60% defoliation occurred in the CBBP in 2006 compared to nearly 90% for the PCCP rotation. Rotations have been reported to manage peanut leaf spot (Kucharek, 1975), but the mechanism of disease suppression in a rotation is difficult to identify. A general conclusion is 38

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that planting an incompatible crop in the rotati on cycle will help break th e pathogen cycle (Curl, 1963; Flowers, 1976; Brenneman et al., 1995). However, other mechanisms such as the contribution of the crops in the cycle in modifica tion of soil properties and their benefits to plant health by means of proper nutrition must be co nsidered. Bahiagrass has an extensive deep rooting system that breaks through hardpan la yers in the soil, and thus, improves water infiltration. Katsvairo et al. (2007) demonstrated that cotton plants in a bahiagrass rotated plots had improved root biomass. Other mechanisms mi ght include allelopathic properties of crops, rhizosphere interactions of microbial population and root ex udates that could enhance mycorrhizal associations in soils (Harsh et al., 2006). Since the rate of disease increase was comparab le for both rotations in all four years, the impact of the rotations on leaf spot severity was mainly due to the delayed onset in the CBBP rotation based on the disease progress curve. Labo ratory studies (data not presented) indicated that a greater proportion (83%) of inoculated detached leaflets ta ken from peanut growing in the PCCP rotation on the field showed symptoms of ELS compared to 33% recorded on those from the CBBP rotation four weeks post-inoculation. It is likely leaf spot epidemics could follow similar trends as was observed in field diseas e progression over the f our years of the study. Pod yield and quality were si gnificantly greater in the CBBP peanut than in the PCCP rotation in all years. CBBP rotations with no f ungicide had 26 and 25% increase in pod yield when GA Green was the cultivar planted in single row pattern in 2003 and 2004, respectively. An increase of 32 and 28% was recorded, respecti vely, during the same period of time when the plots were both treated with fungicide. This indi cates that whether fungici de treated or not, the CBBP rotation resulted in a higher peanut yield than in the PCCP rotation. These yields were comparable to those of the AP3 variety that was grown in 2005 and 2006. When not treated with 39

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fungicide, the AP3 variety in the CBBP rotation had a 24 and 33% yield increase over the PCCP rotation during 2005 and 2006, respectively. However, under a fungicide trea ted regime, the AP3 in the CBBP had a 6% increase in yield over the PCCP rotation in both years. The lower percentage increase in yield of the fungicide-treated plots fo r the rotations suggests that fungicide sprays in a twin row system benef ited both rotations. Thus, an AP3 variety under the CBBP rotation which was not sprayed had a bett er chance of producing higher yield increases than when sprayed compared to the PCCP rotati on. Peanuts treated with f ungicides in bahiagrass (CBBP) rotation gave 11 and 29% yield increase in the same rotation over an unsprayed plot, whereas 3 and 26% yield increase was reco rded for the PCCP rotation in 2003 and 2004, respectively. During 2005 and 2006, sprayed pea nut in the CBBP rotation had 26 and 8% increase in pod yield above those in unsprayed pl ots in the respective years compared to that of 41 and 22% in the PCCP rotation. Fungicide application to peanut in the CBBP rotation, thus, appeared not to be as beneficial as it was in the PCCP rotation as consequence of th e lower disease severity in the CBBP than the PCCP rotation. Increased yield in peanut under a bahiagrass rotation had been reported by several workers (Brenneman et al., 1995; Hagan et al., 2003; Ka tsvairo et al., 2007), though there has been no measurement of yield increase in a no-fungicide sprayed regime. The results of this research suggest a possible benefici al effect of reduced fungicide sprays on a bahiagrass rotation that could result in subsequent cost reduction. Such st udies will be necessary in order to further encourage farmers to adopt the sod rotation in peanut production. There ha ve been no reports on the influence of varying fungici de sprays on peanut under a bahi agrass rotation in comparison to that on the conventional (cottoncotton-peanut) rotation. Though the pe rcentage increase in yield 40

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realized under the same rotation whether spra yed or not sprayed had not been consistent throughout the four years of the studies, it does suggest that fewer number of fungicide sprays on the CBBP rotation may be possible. Garciacasella s (2004) reported that about 30 to 50% of the farmers cost in peanut production is used on pe st management; hence any system that will help them to reduce such costs might be helpful. 41

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1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.032466175100137 Days after plantingLeaf spot severity (Florida 1-10 scale ) CBBP PCCP Figure 2-1. Effect of bahiagrass (CBBP) a nd conventional (PCCP) rotation on progression of leaf spot severity measured using the Flor ida 1-10 scale, over time on Georgia Green peanut during 2003. Treatment means for a minimum of 4 replications and the standard error bars are shown for each a ssessment date. Plots received only two fungicide sprays during early stages of growth. Yearly rotation sequences were bahiagrass (B), cotton (C), and peanut (P). 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 406391104124132140 Days after plantingLeaf spot severity (Florida 1-10 scale ) CBBP PCCP Figure 2-2. Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on disease progress leaf spot severity measured using the Flor ida 1-10 scale, over time on Georgia Green peanut during 2004. Treatment means for a minimum of 4 replications and the standard error bars are shown for each assessment date. None of the plots were sprayed with fungicide. Yearly rotation se quences were bahiagrass (B), cotton (C), and peanut (P). 42

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1 2 3 4 5 6 7 83445526496123134Days after plantingLeaf spot severity (Florida 1-10 scale ) CBBP PCCP Figure 2-3. Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on disease progress leaf spot severity measured using the Flor ida 1-10 scale, over time on AP3 peanut during 2005. Treatment means for 6 replicat ions and the standard error bars are shown for each assessment date. None of the plots were sprayed with fungicide. Yearly rotation sequences were bahiagra ss (B), cotton (C), and peanut (P). 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 34516685100123145Days after plantingLeaf spot severity (Florida 1-10 scale ) CBBP PCCP Figure 2-4. Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on disease progress leaf spot severity measured using the Flor ida 1-10 scale, over time on AP3 peanut during 2006. Treatment means for a minimum of 6 replications and the standard error bars are shown for each assessment date. None of the plots were sprayed with fungicide. Yearly rotation seque nces were bahiagrass (B), co tton (C), and peanut (P). 43

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0 10 20 30 40 50 60 70 80 90 100 46 61 75 100 Days after plantingRust incidence (%) CBBP PCCP Figure 2-5. Effect of bahiagrass (CBBP) and conventional (PCCP) rotati on on the incidence of peanut rust during 2003. Incidence represen ts the percentage of 20 plants showing pathogen signs. Data represents means fo r a minimum of 4 replications and the standard error bars are shown for each assessment date. None of the plots were sprayed with fungicide. Yearly rotation se quences were bahiagrass (B), cotton (C), and peanut (P). 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 240.0 260.0 280.0 300.0March Apr i l M ay Jun e July Aug Sep t O c to be r NovTotal rainfall (ml) 2003 2004 2005 2006 Figure 2-6. Total monthly rainfall in Quincy, FL during 2003-2006. Source: http://fawn.ifas.ufl.edu/ 44

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50 55 60 65 70 75 80 85 90Mar ch A pril May June July Aug S e pt October NovRelative humidity (%) 2003 2004 2005 2006 Figure 2-7. Variation in mean monthly relative humidity in Quincy, FL during 2003-2006. Source: http://fawn.ifas.ufl.edu/ 10 12 14 16 18 20 22 24 26 28March April M a y Jun e July Au g Sept October N o vAverage temperature (C) 2003 2004 2005 2006 Figure 2-8. Variation in average atmosphe ric temperature in Quincy, FL. Source: http://fawn.ifas.ufl.edu/ 45

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Logistic Model 2003 y = 0.0183x 2.606 R2 = 0.9006 y = 0.0256x 2.8335 R2 = 0.9362 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 0 50 100 150 PCCP CBBP Linear (CBBP) Linear (PCCP) Gompertz Model 2003 y = 0.0152x 1.2985 R2 = 0.9667 y = 0.0097x 1.0916 R2 = 0.938 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0 50 100 150 PCCP CBBP Linear (PCCP) Linear (CBBP) Logistic Model 2004 y = 0.0397x 5.8527 R2 = 0.9095 y = 0.0387x 5.0215 R2 = 0.9355 -5 -4 -3 -2 -1 0 1 2 0 50 100 150 PCCP CBBP Linear (CBBP) Linear (PCCP) Gompertz Model 2004 y = 0.02x 2.2134 R2 = 0.86 y = 0.0168x 2.2557 R2 = 0.8317 -2 -1.5 -1 -0.5 0 0.5 1 1.5 0 50 100 150 PCCP CBBP Linear (PCCP) Linear (CBBP) Logistic Model 2005 y = 0.0451x 5.4233 R2 = 0.974 y = 0.052x 6.8139 R2 = 0.9671 -6 -5 -4 -3 -2 -1 0 1 0 50 100 150 PCCP CBBP Linear (PCCP) Linear (CBBP) Gompertz Model 2005 y = 0.0215x 2.5138 R2 = 0.9322 y = 0.0221x 2.2553 R2 = 0.9718 -2 -1.5 -1 -0.5 0 0.5 1 0 50 100 150 PCCP CBBP Linear (CBBP) Linear (PCCP) Logistic Model 2006 y = 0.0433x 4.5647 R2 = 0.902 y = 0.0377x 4.7027 R2 = 0.8825 -4 -3 -2 -1 0 1 2 0 50 100 150 200 PCCP CBBP Linear (PCCP) Linear (CBBP) Gompertz Model 2006 y = 0.0181x 1.9225 R2 = 0.9177 y = 0.0246x 2.0922 R2 = 0.9456 -1.5 -1 -0.5 0 0.5 1 1.5 2 0 50 100 150 200 PCCP CBBP Linear (CBBP) Linear (CBBP) Linear (PCCP) Figure 2-9. Linearized transforma tion of Cercospora leaf spot seve rity (Florida 1-10 scale) data on peanut using the Logistic = ([In(y/(1-y))]) and Gomp ertz = ([-In(-In y)]) model transformations. Disease assessments were c onverted into proportions [y = (Florida rating 1) / 9] before transformation. 46

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Table 2-1. Effect of rotations on final severity (Florida 1-10 scal e), apparent infection rate (r) and SAUDPC on peanut in Quincy during 2003-2006 Year, Variety Rotationa Final severity ratingb rc SAUDPCd 2003, Georgia Green CBBP 5 0.019 72.3 PCCP 7 0.024 92.7 LSD (P 0.05) 1 56.7 NSe 2004, Georgia Green CBBP 6 0.039 35.8 PCCP 8 0.04 52.6 LSD (P 0.05) 0.6 21.0 NS 2005, AP3 CBBP 6 0.047 38.8 PCCP 7 0.05 52.6 LSD (P 0.05) 0.08 56.6 NS 2006, AP3 CBBP 6 0.04 70.4 PCCP 8 0.044 92.5 LSD (P 0.05) 1 70.5 NS a Yearly rotation sequences were bahiag rass (B), cotton (C), and peanut (P). b Severity represents the proportion of twenty plants assessed. c Epidemic rate determined from the slope of the linearized disease progress curve. d Standardized area under th e disease progress curve throughout the assessment period. e NS = non significance (P 0.05). 47

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Table 2-2. Effect of bahiagrass (CBBP) and conventional (PCCP) rotation on peanut pod yield in Quincy, FL during 2003-2004 under no-fungicide and fungicide regimes Rotationa Fungicide Variety 2003 2004 2005 2006 CBBP no GA Green 2,935 3,053 b PCCP no GA Green 2,229 2,297 CBBP no AP3 2,250 4,504 PCCP no AP3 1,703 3,278 LSD (P 0.05) 321.1 536.6 550.7 805 CBBP yes GA Green 3,299 4,302 PCCP yes GA Green 2,160 3,114 CBBP yes AP3 3,048 4,866 PCCP yes AP3 2,865 4,216 LSD (P 0.05) 348.1 470.2 717.1 NS)c 222 a B = bahiagrass; C = cotton; P = peanut. b Variety not planted in that year. c NS = non significance (P 0.05). Table 2-3. Influence of bahiagrass (CBBP) a nd conventional (PCCP) rotations, and fungicide treatments on peanut grade and damaged kernels in Quincy FL Damaged kernels (%) Grade (SMK + SS)b Rotationa Fungicide Variety 2004 2005 2006 2004 2005 2006 PCCP no GA Green 1.0 -c 85.2 CBBP no GA Green 0.2 87.5 PCCP no AP3 0.9 NSd 83 NS CBBP no AP3 0.0 NS 87.4 NS LSD (P 0.05) 0.04 2.0 PCCP yes GA Green 0.8 85.4 CBBP yes GA Green 0.3 87.4 PCCP yes AP3 0.5 NS 84 NS CBBP yes AP3 0.3 NS 86 NS LSD (P 0.05) 0.5 3.2 a B = bahiagrass; C = cotton; P = peanut represents the yearly rotation of the crop. b Grade equals the percent sound mature kernels plus sound splits (SMK + SS). c Variety not planted in that year. d No significant difference (P 0.05). 48

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CHAPTER 3 SUPPRESSION OF TOMATO SPOTTED WILT (TSW) OF PEANUT IN A BAHIAGRASS ( Paspalum notatum Fluegge) ROTATION Introduction Tomato spotted wilt virus (TSWV), causal agent of Tomato Spotted Wilt (TSW) is a tospovirus in the Bunyaviridae family. TSW is one of the major peanut diseases in the southeastern US. TSW of peanut is difficult to manage for various reasons including: 1) insect (thrips) transmitted, 2) effective chemical cont rol options were lacking, 3) there is limited availability of plant resistance, and 4) there are increased costs of peanut production with decreasing commodity prices. Tobacco thrips [ Frankliniella fusca Hinds (Sakimura)] and western flower thrips F. occidentalis (Pergande) are confirmed vectors of peanut TSW, and these insects are prevalent in the southeastern US (Todd et al., 1993; Todd et al., 1995). A prevalent peanut cropping system in southe astern US consists of 2 years of cotton followed by peanut with a winter small grain (w heat, oats) cover crop (S holar et al., 1995). Less than two decades after its arrival, TSW of peanut destroyed 50% of the pe anut crop in southern Texas in 1985, with some fields nearing 100% loss (Black et al ., 1986). In Georgia, losses to peanut due to TSW were around $40 million in 1997 (Culbreath et al., 19 99). Symptoms of TSW on peanut vary and could be influenced by cultiv ar, but characteristically include concentric ringspots, chlorotic leaflets, stunting of plants, misshapen pe gs, pods, and kernels, reddish discoloration, and cracking of the seed coats (Costa, 1941; Cu lbreath et al., 1992). TSWV is acquired by larvae and transmitted by both the la rvae and adult thrips through feeding (Wijkamp et al., 1993). The presence of volunteer peanuts in fields as well as many weed species serves as virus and thrips reservoirs which aids the persistence of the virus within fields (Chamberlin et al., 1992; Chamberlin et al., 1993). 49

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Management of peanut TSW poses tremendous challenges due to the multiple factors involved in disease incidence and severity. Since TSW is vectored by insects, the first approach was to control the thrips vectors but that has be en found to be inconclusive. Chemical control of thrips has not effectively manage TSW on peanut as reported by Mitchell e (1991) and Todd et al. (1996), possibly due to the mode of virus transm ission and vector mobility. Increased thrips feeding has been reported on the for F. fusca when imidacloprid (Culbbreath et al., 2003) was applied on tomato (Chaisuekel and Riley, 2001). In-furrow application of phorate has been reported to suppress TSW epidemics on peanut. Cu lbreath et al. (2003) re ported on results from a 3-year Georgia-Florida statewide insecticide tests in which in-furro w application of phorate reduced TSW in 63 out of 93 tests, though the reduc tion of disease did not correlate with thrips control. The current recommendation of in-furrow phorate applications does not appear to effectively reduce TSW. However, Ames (2007) reported that spraying fo liar insecticides in addition to the phorate application could successfully manage TSW. It is difficult to quantify the effect of insecticide treatments on the incidence and severity of TSW on p eanut due to interplot interference by thrips from non-inse cticide treated plots as well as insects from adjacent cotton plots (Black et al., 1993). Combin ed treatment of aldicarb and flutolanil or aldicarb alone significantly reduced thrips feeding damage (Timper et al., 2001). No-till and minimum tillage systems for peanut have become an economic option for peanut cultivation in the southeastern US. Use of minimum tillage in p eanut has been reported to reduce the impact of TSW and early leaf spot (ELS) ( Cercospora arachidicola S. Hori,), late leaf spot (LLS) [ Cercosporidium personatum (Berk. & M.A. Curtis) Deighton], and rust ( Puccinia arachidis Spegg) as compared to conventional tillage (Baldwin et al., 2001; Johnson et al., 2001; Monfort, 2002). Cantowine et al. (2006), reported the interaction of cultivar and tillage 50

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method on the suppression of both leaf spots a nd TSW. Tillage systems have a significant influence on thrips populations as well as feeding injury with less of both occurring in a strip-till and no-till system (Brown et al., 1996; Campbe ll, 1986; Campbell et al., 1985). However, the roles of soil type and rotation crops on the survival of thrips and their impact on TSW have not been thoroughly studied. Barbour et al. (1994), however, found fewe r thrips emerging from soils than those collected on open-sticky cards in No rth Carolina, and conclu ded that soils from peanut fields were not a major source of thri ps. In studying the impact of cropping systems on stem rot and nematode antagonists to Meloidogyne arenaria Timper et al. (2001) did not find any significant difference in injury due to thrips feeding as a result of rotation. In an extensive review on the epidemio logy and management of TSW on peanut, Culbreath et al. (2003) proposed th e integration of chemical, geneti c, and cultural practices that incorporates; planting date, mani pulation of plant population, tillage practices, row pattern as well as in-furrow insecticide application among other options in the management of TSW on peanut. The recommendation to manipulate plant population resulted in the adoption of the twinrow planting system to enhance early canopy closure (Culbreath et al., 2003). The advantages of using perennial grasse s such as bahiagrass in peanut disease management has been well documented for leaf diseases ( Brenneman et al., 1995; Timper et al., 2001). However, there is little information on the influence of perennial grasses in the management of TSW. The objectives of this research were to: 1) to assess the potential impact of bahiagrass rotation in peanut on TSW epidemics and 2) investigate possible mechanisms of TSW suppression. 51

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Materials and Methods Rotation and Cultural Practices Experiments were conducted at the University of Florida, North Florida Research and Education Center in Quincy, Fl orida during 2003-2006. Rotation plot s were first established in year 2000 and consisted of a bahiagrass (cv. Pens acola, Florida Seed Company) rotation with peanut and a conventional cotton-peanut rotation for peanut. Except for 2005 where some plots were in one year bahiagrass rotation (PCBP), a nd two years of consecutive peanut (CCPP) in order to synchronize other rota tions, the cropping sequence for the bahiagrass rotation involved the growing of cotton in the fi rst year and then followed by ba hiagrass for two consecutive years and in the fourth year the plots were plante d to peanut for one y ear (CBBP), whereas the conventional rotation consisted of growing peanut in the first year with cotton in the two subsequent years followed by peanut in the fourth year (PCCP). Each plot in the rotation cycle was split into an irrigated and nonirrigated section, and these were further split into fungicide spray and non-sprayed sections to produce a split-split plot. Irrigated plots received scheduled applications of water over the four years as and when needed according to standard extension recomm endations for peanut production in Florida (Smajstrla et al., 2006). Weed and other crop management practices were done based on the Florida Cooperative extension Services recommendations (Aerts and Nesheim, 2001; 2002; Whitty, 2002; Whitty and Chambliss, 2002). Each (rotation* irrigation*fungicide) sub-sub-plot consisted of ten rows measuring 22.8m in length by 9.2 m (10 peanut rows). Field Practices in 2003 and 2004 The bahiagrass cover crop was killed in December of 2003 and 2004 by spraying recommended herbicides. A winter cover crop of oats ( Avena sativa L.) cv. Florida 501 was planted at 125 kg/ha, and wa s killed 124 DAP in 2003 and 97 DAP in 2004 by broadcast 52

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spraying glyphosate (Roundup Weat herMAX; Monsanto, Kansas City, MO). Seedbeds were prepared in both years by strip-tilling w ith a KMC (Kelly Mfg. Corporation, Tifton, GA). Georgia Green peanut cultivar was plante d on 7 May 2003 and 10 May 2004, with a Monosem pneumatic planter (ATI, Inc. Lenexa, KS) at 6 seeds per 31 cm of row and 91-cm row spacing. Phorate (Thimet 20-G; Micro Flo Company LLC. Memphis, TN) at 5.6 kg/ha was applied in furrow at planting to manage thrips. Field Practices in 2005 and 2006 The bahiagrass cover crop was killed in th e fall of 2004 and 2005 for the next cropping season. Oats (cv. Chapman) were planted at 125 kg/ha on 26 November 2004 as a winter cover crop for the 2005 cropping season, and cv. Flor ida 501 was planted in December 2005 at 125 kg/ha for the 2006 season. Cover crops were managed according to recommended practices in both years, and killed 123 DAP in 2005 and 120 DAP in 2006. For subsequent peanut planting, seedbed was prepared by double-ripping 10 rows using a KMC (Kelly Mfg. Corporation, Tifton GA) 4 row ripper in April of both years, and planted to peanut cv. AP3 on 13 May 2005, and 17 May 2006 with a Monosem pneumatic planter (ATI Inc. Lenexa, KS) Twin Row Planter at 3 seeds per 30 cm of row with simultaneous application of phora te (Thimet) at 6 kg/ha into furrows. The cv. Georgia Green was planted in 2003 and 2004 in a single row pattern, whereas in 2005 and 2006 the cv. AP3 was planted in a twin-row pattern. Tomato Spotted Wilt Assessment TSW disease assessment was done on 32, 46, 61, 75, 100, and 137 DAP in 2003; 40, 63, 91, and 104 DAP in 2004; 34, 45, 52, 64, 96, 123, and 134 DAP in 2005; and 34, 51, 66, 85, and 100 DAP in 2006. Peanut plants were assessed by examining twenty plants within two rows at each time of assessment, and different rows were assessed at each point in time. Plants were examined at 2 m intervals within rows for TSW symptoms on leaves and sc ored using a modified 53

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scale of 0-3: where 0 = no visible symptoms; 1= presence of TSW symptoms on at least one leaf on the plant; 2 = symptoms on majo rity of leaves with moderate stunting of plant; and 3 = severe stunting of plant, and associated death. This me thod of assessment was chosen since measured TSW progression over time. TSW in cidence on any date of assessment was determined as the number of peanut plants showi ng visible symptoms on any plant pa rt out of the twenty plants assessed on each plot and rotation, expressed as a percentage. TSW severity index was then computed from severity ra tings; [Severity Index = { (Ratings for 20 plants)/20} 100], and was used to compute the Standard Area Under the Disease Progress Curve (SAUDPC) over the period of assessment. Thrips Infestation Studies In addition to the traditional rotation plots being st udied (CBBP and PCCP), an attempt to synchronize the rotation cycles resulted in some plots having two years of continuous peanut after two consecutive years of cotton (CCPP), whereas others had p eanut after one year bahiagrass rotation (PCBP). Advantage was taken of these scenarios to investigate their impact on thrips population, feeding damage on peanut seedlings; and subsequent TSW epidemics. During 2005, thrips feeding injury as well as popu lations on peanut seedling were assessed by placing 10 peanut seedlings from the different ro tations into jars cont aining 50% ethanol. The mean number of thrips per plan t was computed for seedlings at 14 and 45 DAP for each rotation. A simple regression analysis was conducted by i nvestigating the relationship between feeding damage, population, and final TSW incidence. Based on 2005 results and before planting in peanut in May 2006, volunteer peanut seedlings (25 plants) in adjacent cotton plots, oa t cover crop (25 plants), and 20 volunteer peanut plants in both killed and green bahiagrass plots were assessed for thrips infestation, and their possible contribution to subsequent TSW epidem ic were analyzed. Samples of 20 bahiagrass 54

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inflorescences and leaves (20 main stems), 20 winter oat panicles, a nd 50 developing seeds of the oat cover crop were also examined for thrips infestation during this period. Both adult and larvae were counted together with no regard to species, though most of the adults were found to be F. fusca Statistical Analysis TSW incidence and severity data were analyzed using Statistical Analyses System (version 8.0, SAS Institute Inc Cary, NC) GLM. Data for all years were first analyzed as a split-split experiment to investigate inter action of main and subplots. In the absence of a consistent interaction effect, the data was subjected to analyses of variance (ANOVA) as a randomized block design, and the data were pooled for all su b-plots and analyzed with rotation as the only factor. Means from the rotations were compared using the least signif icance difference (LSD) test to determine the differences (P 0.05) in TSW incidence and severity as measured by SAUDPC between the PCCP and CBBP rotation within each year. Results Tomato spotted wilt (TSW) epidemics in the ex perimental fields was variable each year. However, it remained consistently higher in the PCCP rotated peanut than the CBBP peanut in all four years irrespective of which variety wa s grown (Fig. 3.1). TSW incidence across years (2003-2006) revealed that the rotations significantly (P 0.05) affected the incidence (Fig. 3.1) and severity of TSW of peanut in all years. Peanut in the CBB P rotation had consistently lower incidence and severity throughout a ll four years. Since there was no consistent significant effect of irrigation and fungicide treatment on TSW inci dence and severity, the data were pooled for each rotation and analyzed further. In 2003, significant differences (P 0.05) between the rotations were observed for both incidence and severity 32 DAP, with the peanut in the PCCP rotation having 39% incidence as 55

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opposed to 22% in the CBBP rotation (Fig. 3.2). Between 32 DAP and 64 DAP, a decrease in the incidence of TSW was observed for both rotations. However, there was a significant difference in the severity, which was higher for the PCCP p eanuts plants. Incidence of TSW of peanut in the PCCP rotation was consistently higher (20, 26, and 25%) compared to that in the CBBP rotation (5, 8,and 6%) at 61, 75, and 100 DAP, respect ively, for the rotations. TSW severity was similarly higher in the PCCP rotation than in the CBBP rotation at all these times. There was no significant difference in TSW incidence between the rotations 137 DAP, though overall severity was higher for the PCCP than on the CBBP rotati on as depicted by significant SAUDPC (Table 3.1). In 2004, incidence of TSW 40 DAP wa s significantly different (P 0.05) between the two rotations with incidence of 38 and 24 % in the PCCP and CBBP rotations, respectively (Fig. 3.3). Subsequent assessments had 45, 70, and 72% incidence for the PCCP and 26, 32, and 32% incidence for the CBBP rotation at 63, 91, and 124 DAP, respectively. Differences in severity are represented by the greater SAUDPC (103.7) for the PCCP, compared to 44.5 for the CBBP rotations (Table 3.1). In 2005, the rotations were plan ted with AP3 peanut variety in a twin-row planting pattern, an approach that is recommended to suppress TS W epidemics. TSW incidence and severity was consistently higher and si gnificantly different (P 0.05) at each time of assessment on peanut in the PCCP than the CBBP rotation (Fig. 3.4). TS W incidence sharply increased for the PCCP rotation within two months after the first assessm ent, with only a slig ht increase for the CBBP rotation within the same time period. TSW incidence was 29, 39, 66, 67, 72, 59, and 59% in PCCP rotation and 21, 23, 31, 33, 33, 30, and 31% in the CBBP rotation on the days of 56

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assessment (Fig. 3.4). SAUDPC fo r TSW was significantly greater on the PCCP (121) than on the CBBP (60) rotation (Table 3.1). Progression of TSW incidence on AP3 peanut in the rotations during 2006 (Fig. 3.8) showed incidence and severity of TSW was significantly higher (P 0.05) in the PCCP rotation than in the CBBP rotation. This tr end had a strong correlation with the number of thrips found on volunteer peanuts in the plots and on the cover crop prior to planting peanut (Fig. 3.9). Subsequently, peanut in the PCCP rotation ha d 40, 45, 51, 53, and 53% incidence compared to 22, 18, 22, 23, and 23% for peanut in the CBBP ro tation when assessed at 34, 51, 66, 85, and 100 DAP, respectively. Thrips feeding damage was va riable in 2006, and it was evident that there was greater damage on peanut in the PCCP than the CBBP rotation as was observed in previous years. Monitoring of Thrips Activity In addition to the traditiona l rotation plots being studied (CBBP and PCCP) in 2005, some plots had two years of continuous peanut after two consecutive years of cotton (CCPP), whereas others had peanut after one year bahiagrass rotation (PCBP). Monito ring of thrips revealed that double cropping of peanut (CCPP) had the highest number of thrips per plant (42) compared to PCCP (22), CBBP (6), and PCBP (4) (Fig. 3.5). Pl ots exhibiting greater th rips feeding damage also had higher TSW incidence (Figs. 3.6 and 3.7), respectively. There was a significant effect (P 0.05) for the number of thrips per seedling and damage on the final incidence of TSW, R2 = 87.9 (Fig. 3.10). Overall, differences in the feeding damage correlated with the number of th rips per plant, (r = 0.60, Pears on correlation). Similarly there was a stronger correlation, r = 0.94, between the number of thrips per seedling and the final TSW incidence. The number of damaged seedlings corr elated (r = 0.84) with the final TSW incidence on plots. A simple regression analysis of da maged seedlings on number of thrips in 2005 57

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revealed that as thrips population increased, damage also incr eased (r = 0.60) (Fig. 3.10). There was a stronger association (r = 0.84) between number of plants with feeding damage and final TSW incidence. This was evident in the field, where an average 13 out of 20 plants showed damage for the CCPP rotation and had a corres pondingly higher final TSW incidence (61%), while rotation with a single year of bahiagrass ha d the least number of damaged plants (5 plants) and also the lowest final TSW in cidence (23%) (Figs. 3.6, 3.7). The two main rotations tested for all four years (PCCP and CBBP) had similar relati onships. PCCP rotated peanut had 19 damaged plants with a final TSW incidence of 54% compared to a CBBP with 9 damaged plants, and a final TSW incidence of 31% (Figs. 3.6 and 3.7). The number of thrips per peanut plant had a significant impact on the final incidence of TSW with a correlation coefficient of r = 0.94. This relationship was evident in 2005, wh ere the CCPP rotation with 42 th rips per plant had 61% final TSW incidence, the PCBP had 3 thrips pe r plant with a correspondingly low final TSW incidence (23%), a value not different from the CBBP of six thrips per plant and a final TSW incidence of 31% (Figs. 3.5 and 3.7). The PCCP rotation mimicked what was found on the CCPP plots with 22 thrips per plant and a fina l TSW incidence of 54% (Figs. 3.5 and 3.7). During 2006, thrips populations in winter oats, bahiagrass leaves and inflorescences and volunteer peanut were different on these plants and plant parts. Volunteer peanut seedlings emerging from plots previously planted to th e winter oat cover crop had a higher thrips population, 45 thrips per seedling (Fi g. 3.9) than plots not previously planted to winter oats, and were associated with severe feeding damage. Vol unteer peanuts in nearby bahiagrass plots that were either killed or green had 21 and 7 thrips per seedling, respectively. Bahiagrass inflorescences harbored 4 thrips per head on aver age compared to 18 thrips per panicle, and six thrips per seed of winter oat (Fig. 3.9). Inciden ce and severity of TSW on peanut in plots planted 58

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to winter oats, and being repres entative of what prevailed on th e PCCP rotation had both initial (40%) and a final (53%) TSW incidence (Fi g. 3.8) during the 2006 season. TSW incidence on the CBBP rotation that was not planted to oats pr eviously had both lower initial (22%) and final (23%) incidence and was significantly different from the PCCP rotation (P 0.05) (Fig. 3.8). Pod Yield and Grade Significant differences (P 0.05) were found between the yields from the bahiagrass (CBBP) and conventional (PCCP) rotations in all f our years (Table 2.2) in Chapter II of this dissertation. Discussion Tomato spotted wilt (TSW) incidence and seve rity on peanut was significantly suppressed by two years of bahiagrass rotation (CBBP) compared to th e conventional (PCCP) rotation system over the course of four years (20032006). Incidence and seve rity of TSW varied between years but was consistently higher fo r the PCCP rotation than on the CBBP rotation. TSW was particularly severe in 2004 and 2005 for the PCCP rotated peanut but remained significantly less in the CBBP peanut s in those years. The lowest incidence and disease severity in both rotations was recorded in 2003. The disease was suppressed in the CBBP rotation throughout 2003-2006 (12-32%) compared to the P CCP rotation (21-72%), with the highest severity in 2004 for both rotations. Except for 2003, when TSW incidence was high 32 DAP and suddenly dropped at 46 DAP, incidence in all othe r years increased more rapidly in the PCCP rotation compared to the CBBP rotation. The sudden decrease in 2003 May was due to the death of highly infected plants. Tomato spotted wilt on peanut is transmitte d by thrips hence their population dynamics on peanut play a primary role in disease inciden ce and severity (Culbrea th et al., 1999). Based on thrips population and damage data in 2005 and 2006, it appears that the in itial thrips population 59

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even at peanut emergence could be one of th e most important factors in determining the incidence and severity of TSW over time on the crop. In this study, thrips damage as a result of feeding had a significant correla tion (r = 0.60) with insect popul ation. The number of plants damaged was highly correlated (r = 0.84) with the final inciden ce and severity of TSW. The high correlation coefficient (r = 0.94) observed be tween number of thrips per seedling and the final TSW incidence is consistent with the ge neral assumptions of the influence of thrips population on TSW incidence (Culbreath et al., 1999) This research sugges ts that th e initial thrips population in the field ev en before seedling emergence could significantly affect TSW incidence. These data are supported by the obser vation that when the number of thrips per seedling in the CBBP rotation was low in bot h 2005 and 2006 there was a correspondingly lower final spotted wilt incidence. Similarly, higher popul ations of thrips in the PCCP rotation resulted in high TSW incidence. It has been hypothesized th at thrips move from afar to infest newly planted peanut plants. However, the contribution of resident thrips population even before seedling emergence has not been fully considered. The results from these trials suggest that the contribution of resident thrips before seedling emergence could be significantly contributing to TSW incidence and/or severity. The role of rotations and the crops in the cycle on TSW epidemics has been little studied. Brenneman et al. (1995) reported th e advantages of the bahiagrass rotation on stem rot, limb rot, and leaf spot diseases on peanut but did not report on the influence of the rotation on TSW. The role of the winter cover crop (winter oats) in this research was found to be a significant contributor to early thri ps infestation on the PCCP rotation th at resulted in higher TSW incidence and severity. These experiments suggest that p eanut after an oat cove r crop, a conventional practice had higher incidence and se verity of TSW. The high number of thrips in oat seed is a 60

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significant contributing factor to the initial thrips population on peanut. Feeding of thrips on peanut was observed just as the hypocotyls broke the soil surfa ce. Intense larval feeding and damage from thrips could predispose peanut seed lings to subsequent viral infection and render them infectious. In 2005, peanut in a PCCP plot adjacent to that of a plot in one y ear bahiagrass rotation (PCBP) had lower thrips numbers per plant, and less feeding damage as well as low TSW incidence. Behaviors such as th e above could be due to the fact that; 1) the oat cover crop might have been a good reproductive host to thrips wi th all developmental stages, thus once peanuts were planted the thrips had clos e proximity to a suitable food source and therefore did not move further, 2) bahiagrass might have not been a good host as evidenced in the low number of thrips recorded and thus did not support thrips reproduction when comp ared to oats, 3) decomposing bahiagrass residue may have been releasing some volatile compounds that could serve to repel thrips from such plots. It does a ppear in this research that bahiag rass is not as suitable host to thrips, as evidenced by the lowe r number of thrips recorded on both leaves and inflorescence compared to the winter oat. The contribution of volunteer peanuts in adjacent plots to TSW epidemics have been suggested bu t not quantified. The role of vol unteer peanuts in TSW severity could be aggravated when there is already an exis ting reproductive host such as oats in the field. Although oats had not been previously reported as a host of TSWV, this plant could harbor the virus since there are numerous asymptomatic hosts of TSWV. Crop rotations have not been considered as a viable management practice for TSW on peanut. This research has established a consiste nt pattern over a period of four years on the advantages of a bahiagrass rotation in si gnificantly reducing TSW epidemics on peanut compared to a conventional PCCP system. A ny attempt to assess the influence of TSW 61

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suppression on yield can be best done in peanut plots under no fungicide spray regimes that exclude the effects of leaf spots and other soil-borne pathogens. In these trials the margin of yield increase in 2003 and 2004 were compar able, and again that of 2005 and 2006 (data presented in Chapter II f this disse rtation), it was inferre d that at least some of the yield increases could have been due to TSW suppression in the CBBP system. The higher percentage yield under the CBBP system over that in the PCCP sy stem could be attributed to the lower TSW severity as evidenced by the lowe r SAUDPC in all four years. During 2005 and 2006 when the AP3 variety was planted in a tw in-row pattern, the percentage increase in yield be tween the PCCP and CBBP rotations were lower than in 2003 and 2004 when GA Green variety was planted in a single -row pattern. This trend suggested that the and also the twin-row pattern did reduce the im pact of yield loss due to TSW confirming the recommendations of Culbreath et al. (2003). The mechanism empl oyed by the twin-row system in affecting TSW epidemics was reported to be po ssibly due to visual in terference of migrating thrips in host recognition (Culbr eath et al., 2003). Since the plot s studied in these experiments were all strip tilled, the reduction in TSW incidence and severity may be attributed to an inherent ability of bahiagrass rotation to suppress the disease. The low percentage increase in yield between the peanut in the PCCP and CBBP that we re planted in a twin -row pattern in 2005 and 2006 could be attributed to plant compensation, in which case seve rely stunted plants in the rotation were smothered by other healthy plants t hus reducing the impact of TSW severity in the PCCP plots. Other yield quali ties such other kernels (data not presented), which was significantly higher in the PCCP rotations could be tter reflect the severity of spotted wilt for the PCCP rotation than the actual harvestable and gradable pods, since TSW infection affect pod filling (Culbreath et al., 1992). 62

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While irrigation did not have a ny consistent significant impact on TSW epidemics in this experiment, results suggest a possible impact of irrigation on TSW epidemics especially at the early stages of growth. The recommendation of planting peanuts in early to mid-May to manage TSW could be due to the normally dry April mont h that predisposes peanut to severe thrips feeding. Severe thrips feeding observed in this tr ial in early May resulted in significantly higher TSW epidemics and suggest that ir rigation at the early stages of peanut may play a role in reducing TSW. In conclusion, planting peanut after two years of bahiagrass in bahiagrass consistently reduced peanut TSW epidemics and improve d yield during 2003-2006. Bahiagrass rotation reduced the number of thrips per peanut seedling, nu mber of damaged peanut seedlings and TSW incidence and severity. Based on the results of this re search the following are suggest ed, 1) investigate the actual mechanisms of thrips population reduction on ba hiagrass rotated to pe anut, 2) study a one-year rotation system for bahiagrass in the management of TSW 3) and determine the critical time that thrips populations peak influence TSW epidemics. 63

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0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 2003 2004 2005 2006 YearTSW incidence (%) CBBP PCCB Figure 3-1. Effect of rotations on the across year incidence of TSW on peanut. The standard error bars are displayed in the chart and represent 4-7 assessment times within a cropping cycle. B = bahiagra ss, P = peanut, C = cotton. 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 32466175100137 Days after plantingTSW incidence (%) CBBP PCCP Figure 3-2. Effect of bahiagrass (CBBP) a nd conventional (PCCP) rotation on progression of TSW incidence on Georgia Green peanut duri ng. Treatment means of 20 plants for a minimum of 4 replications and the standard error bars are shown for each assessment date. 64

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0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 40 63 91 124 Days after plantingTSW incidence (%) CBBP PCCP Figure 3-3. Effect of bahiagrass (CBBP) a nd conventional (PCCP) rotation on progression of TSW incidence on Georgia Green peanut dur ing 2004. Treatment mean s of 20 plants for a minimum of 4 replications and the standard error bars are shown for each assessment date. 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 3445526496123134 Days after plantingTSW incidence (%) CBBP PCCP Figure 3-4. Effect of bahiagrass (CBBP) a nd conventional (PCCP) rotation on progression of TSW incidence on AP3 peanut during 2005. Treatment means of 20 plants for 6 replications and the standard error ba rs are shown for each assessment date. 65

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0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0PCCP CBBP CCPP PCBPRotationThrips per plant PCCP CBBP CCPP PCBP Figure 3-5. Effect of different cropping sequences on thrips popul ation on AP3 peanut seedlings during 2005. Treatment means of 20 plants for 6 replications. Cropping sequences are represented by: B = bahiagra ss, C = cotton, P = peanut. 0.0 5.0 10.0 15.0 20.0 25.0 PCBPCBBPPCCPCCPPRotationDamaged plants Figure 3-6. Effects of rotations on thrips feeding damage on p eanut seedlings in Quincy FL during 2005. Points represent average number of plants damaged out of 20 in plots with different cropping seque nces represented. Points represent mean number of plants assessed 14 and 54 DAP. Croppi ng sequences are represented by: B = bahiagrass, C = cotton, P = peanut. 66

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0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 3445526496123134 Days after plantingTSW incidence (%) CBBP CCPP PCBP PCCP Figure 3-7. Effect of differ ent cropping sequences on progression of TSW incidence on AP3 peanut during 2005 in Quincy, FL. Treatment means of 20 plants for 6 replications. Cropping sequences are represented by: B = bahiagrass, C = cotton, P = peanut. 0.0 10.0 20.0 30.0 40.0 50.0 60.0 34516685100 Days after plantingTSW incidence (%) CBBP PCCP Figure 3-8. Effect of bahiagrass (CBBP) a nd conventional (PCCP) rotation on progression of TSW incidence on Georgia Green peanut during 2006 in Quincy, FL. Treatment means of 20 plants for minimum of 6 repl ications and the standard error bars are shown for each assessment date. 67

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0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0B B P C BCCP CP B B ( Kil l e d) PCCB(Gr e en) O a t se e d Oat hea d Ba h iag r ass headNumber of thrips (per plan t Figure 3-9. Thrips population on peanut seedlings, oat seed, a nd bahiagrass inflorescence in Quincy, FL during early May 2006, with standa rd error bars displayed within bars. B = bahiagrass, P = peanut, C = cotton; repres enting cropping sequences in those plots. Data on BBPC was from volunteer peanuts in adjacent cotton plots; BCCP was volunteer peanuts on plots to be planted to peanut in the summer of 2006; CPBB volunteer peanut in killed bahiagrass plots to be planted to peanut. y = 0.9814x + 24.274 R2 = 0.8791 y = 1.0846x + 37.199 R2 = 0.3635 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 0.010.020.030.040.050.0 Thrips per plantTSW incidence and feeding damage (%) Mean damage Incidence Linear (Incidence) Linear (Mean dama g e ) Figure 3-10. Relationship of early thrips popula tion on peanut seedlings on feeding damage and the final TSW incidence on peanut. Points represent average number of thrips per plant in plots with different cropping se quences as represented in Figure 3.4 above (PCBP, CBBP, PCCP, and CCPP). B = bahiagrass, P = peanut, C = cotton. 68

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Table 3-1. Effect of rotations on final TSW incidence and SAUDPC, on peanut in Quincy during 2003-2006. Year Variety/Rotationa Final TSW incidence (%)b SAUDPCc 2003 Georgia Green CBBP 16.9 10.7 PCCP 21.3 28.5 LSD (P < 0.05) 12.3 4.4 2004 Georgia Green CBBP 31.7 44.5 PCCP 71.9 103.7 LSD (P < 0.05) 13.7 43.7 2005 Georgia Green CBBP 30.8 59.6 PCCP 59.2 121.1 LSD (P < 0.05) 18.6 29.2 2006 Georgia Green CBBP 22.5 33.6 PCCP 53.1 90.1 LSD (P < 0.05) 7.1 30.4 a B = bahiagrass; C = cotton; P = peanut. Each letter represents the sequence of cultivating that crop in a year. b Incidence represents th e proportion of twenty plants assessed for TSW symptoms on a scale of 0-3: where 0 = no visi ble symptoms; 1= presen ce of TSW symptoms on at least one leaf on the plant; 2 = symptoms on majority of leav es with moderate stunting of plant; and 3 = severe stunting of plant, and associated death. c Standardized area under the disease progress curve thr oughout the assessment period. 69

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CHAPTER 4 EFFECT OF ROTATIONS ON SOUTHERN STEM ROT (SSR) (S clerotium rolfsii sacc ) AND SURVIVAL OF SCLEROTIA IN FIELD SOIL AMENDED WITH BAHIAGRASS CUTTINGS UNDER GREENHOUSE CONDITIONS. Introduction Soil-borne pathogens cause yield losses in most plants, but their management is complex because most develop resistant structures that ar e not easily destroyed and the soil ecology is not well understood (Jackson and Bell, 1969). The soil-borne fungus Sclerotium rolfsii Sacc., [teleomorph, Atelia rolfsii (Curzi) Tu & Kimbrough] causes southern stem rot (SSR) (Jackson and Bell, 1969). It is both deva stating and difficult to manage. Although no worldwide estimates of host genera have been published, Fichtner (2 007) reported that over 270 genera have been reported as hosts in the US alone Far et al. (1989) reported that susceptible agricultural hosts include corn ( Zea mays), wheat ( Triticum vulgare ), soybean ( Glycine max ), as well as numerous horticultural crops. Losses in peanut yield due to SSR are 10% in the southeastern US (Melouk and Backman, 1995). Stem, pegs, and pods of the pea nut plant are susceptible to the pathogen. In addition, all commercially grown cultivars are suscep tible. Signs of SSR on peanut consist of the presence of white fluffy and cottony mycelia of S. rolfsii on affected parts, but most often originating from the soil line on the stem. Bre nneman et al. (1995) reported that, until recently, available fungicides were not ve ry effective and thus complicat ed the management of this disease. Sclerotia serve as the principal overwinteri ng structure and primary inoculum and persist in soil freely and in association with plant debris (Jackson and Bell, 1969). Infection is promoted by dense planting, high soil moisture and freque nt irrigation (Aycock, 1966; Sconyers et al., 2005). Sclerotium rolfsii does not produce spores; hence dissemination is through movement of infested soil and plant materials. While debris on the soil surface supports survival of sclerotia, 70

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such environments also harbor an tagonistic organisms that degrade S. rolfsii by biocontrol agents (Smith, 1972). Addition of stubble from rotational or cover crops suppress disease by enhancing production of decomposition products by antagon istic microbial populations that inhibit pathogens and promote host development (Hoitink and Boehm, 1999). A variety of methods are availabl e for managing diseases caused by S. rolfsii including fungicide applications, solarizat ion, use of antagonistic micr oorganisms, deep plowing, crop rotation, and incorporation of organic and inor ganic residues (Punja, 1985). However, Aycock (1966) and Umaerus (1992) describe d how difficult it is to manage stem rot due to the presence of numerous hosts and ability to surviv e as sclerotia and dr y mycelia on debris. Brenneman et al. (1995) reported a 10-fold increase in SSR in cidence during the four years of continuous peanut rotation as compared to peanut grown after bahiagrass. Timper et al. (2001) reported lower incidence of stem rot on peanut grown after two years of bahiagrass than in continuous peanut, two years cotton, or corn before peanut in plots that were sprayed with fungicide. The actual mechanism of SSR reduction in a bahiagrass rotation has not been studied and may not possibly be due to the non-host status of bahiagrass to S. rolfsii as S. rolfsii grew and produced sclerotia on agar-grown bahiagrass seedlings that were co-inoculated with S. rolfsii (Tsigbey, unpublished). Other factors such as the enhancement of microbial population antagonistic to S. rolfsii propagules as well as gaseous products from the decomposition of bahiagrass may contribute to population decrease of S. rolfsii. Burying sclerotia 5 to 6 cm beneath the soil reduced survival of sclerotia, which supports th e program of clean tillage in disease management (Smith et al., 1989). Howe ver, with the current trend towards minimum tillage, the challenge of maintain ing surface debris and its impact on sclerotia survival gives a new dimension to disease management. 71

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Though crop rotation using grasses has been found to suppress stem rot of peanut (Flowers, 1976; Minton et al., 1991), the introduction of the third generation fungicides such as tebuconazole and flutolanil increased the prospect s of stem rot management (Brenneman et al., 1991; Hagan et al., 2003). These f ungicides are also effective agai nst other diseases of peanut, though the continuous extensive use of these chem icals could result in pathogen resistance development. Organic amendments have been wi dely studied in the management of soil-borne pathogens. Chemical composition of organic additiv es determines the efficacy and the type of microorganisms that develop during degrad ation. Decomposing microorganisms have antagonistic activities and these ca n be exploited as a practical bi ocontrol tool to manage plant parasitic nematodes. Stirling (1991) repor ted that antagonistic interactions among microorganisms determine the diversity of organi sms that inhabit the rh izosphere. Addition of organic matter to soil has been reported to stimulate microbial populations of bacteria and fungi that might be antagonistic to nematodes a nd other plant pathogens (Morgan-Jones and Rodriguez-Kabana, 1987). Many researchers have reported the occurrence of volatile compounds with nematicidal and fungicidal pr operties (Bauske et al., 1994; Soler-Serratosa et al., 1996). Camllo et al. (1992) and Soler-Serratosa et al. (1993) reported that these decomposition products stimulated the development of populations of f ungi and bacteria that are antagonistic to soilborne pathogens; while Chavarria-Carrayal et al (1994) also reported increased parasitism of eggs of Meloidogyne spp. When used properly, organic amendments not only reduce pathogen pop ulations but also improve soil fertility and induce soil suppressive ness by stimulating activiti es of antagonists in soil. Amendments may also stimulate the germination of S. rolfsii in soil and render them vulnerable to antagonist attack (Beute and R odriguez-Kabana, 1979). Smith (1972) reported that 72

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germinated sclerotia are more susceptible to antagonists than non-germinated ones. Hadar and Gorodecki (1991) found that non-ger minated sclerotia can have thei r viability decreased by some amendments and render them sensitive to antagonistic action by other organisms. The beneficial effect of bahiagrass rotation on the reduction of peanut diseases including Sclerotium rolfsii has been extensively demonstrated in the field. However, the mechanism of such suppression in the case of S. rolfsii has not been fully investig ated. The objectives of this study were to: 1) monitor SSR occurrence on peanut in a bahiagrass a nd conventional rotation, and 2) investigate the possible mode of Sclerotium rolfsii suppression in soils amended with bahiagrass residue. Material and methods Field Studies The study was conducted on a Dothan sandy loam (fine loamy siliceous, thermic Plinthic Kandiudult) at the North Florid a Research and Education Ce nter, Quincy, FL from 2003. Rotation plots were established in year 2000 and consisted of a ba hiagrass rotation with peanut and a conventional cotton-peanut rotation. Th e cropping sequence for the bahiagrass rotation involved growing cotton in the first year and then followed by bahiagrass for two consecutive years and in the fourth year the plots were cultivated to pe anut for one year (CBBP). The conventional rotation consisted of growing peanut in the first year with cotton in the two subsequent years followed by peanut in the fourth year (PCCP). Bahiagrass cover crop was killed in Decem ber 2003 by spraying recommended herbicides. The winter oats (Avena sativa L.) cv. Florida 501 cover crop was planted at 125 kg/ha, killed 124 days after planting (DAP) in 2003 by broad cast spraying glyphosat e (Roundup WeatherMAX; Monsanto, Kansas City, MO). The seedbeds for peanut were prepared in both years by striptilling with a KMC (Kelly Mfg. Corporation, Ti fton, GA) Georgia Green peanut cultivar was 73

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planted on 7 May 2003 with a Monosem pneumatic planter (ATI, Inc. Lenexa, KS) at six seeds per 31 cm of row and 91-cm row spacing. Phor ate (Thimet 20-G; Micro Flo Company LLC. Memphis, TN) at 7 kg/ha was applied in furrow at planting for thrips control. Twenty plants were randomly assessed for signs (presence or absence) of SSR at 46, 61, 75, 100, 137 DAP in all plots during 2003. Isolation and Maintenance of Micro-organisms Peanut ( Arachis hypogaea L.) plants showing stem rot symptoms with sclerotia were collected from peanut fields at the NFREC, Quincy. The pathogen wa s isolated on potato dextrose agar (PDA) medium. Cultures were in cubated in the laboratory until they produced sclerotia and were identified according to their morphology and colony characteristics. Soil Treatment Field soil was collected and then solarized for one month under greenhouse conditions to heat-kill most soil organisms befo re the start of the experiment. Th e soil was spread thinly (2 cm thick) on a white polythene sheet with periodic s tirring to allow drying. The dry soil was sieved to remove clods and other large debris. Pot cult ure assay of bahiagrass roots and leave pieces on sclerotia survival was conducted in 16-cm diam eter plastic pots containing 1.2 kg of field. Bahiagrass leaves and roots that had been prev iously cut into 2 cm pieces and dried were weighed in different ratios to c onstitute 1% organic matter conten t (wt:wt) of the measured soil. The various treatments and amendment ratios were as follows: T1 Only sclerotia inoculation, T2 -1:1 Leaves to roots (12 g of leaves and 12 g of roots), T3 1:2 Leaves to roots (8 g of leaves and 16 g of roots), T4 2:1 Leaves to roots (16 g of leaves to 8 g of roots), T5 1% dry cut root (24 g dry root), T6 1% OM dry cut bahiagrass l eaves (24 g dry cut leaves), T7 1% OM (24 g) dry finely ground bahiagrass leaves, and T8 1% ( 24 g) dry finely ground root were added per pot and thoroughly mixed into th e soil. Twenty sclerotia of Sclerotium rolfsii were harvested 74

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from PDA by brushing the propagules onto sterile Petri dishes. These sclerotia were deposited into 25 micron nylon mesh pouches 2 cm x 2 cm and buried at 2-cm depth of soil in pots. Except for T8 that was carried out in dupl icate, all other treatments were carried out in tr iplicate in 4-6 replicate pots. The pots were imme diately watered and left for eigh t days before infesting with sclerotia and thereafter when necessary. The pots we re left to incubate for two and a half months. For the first experiment, ten incubated sclerotia per replication for each treatment were plated onto PDA, whereas twelve were plated for each pot of six replications for the second and third experiments. Determination of Sclerotia Survival in Soils Buried sclerotia in nylon mesh were retrie ved from each pot and soil was washed off under tap water and rinsed in three changes of sterile di stilled water. The mesh with the sclerotia was air dried and then opened under sterile conditi ons and sclerotia tran sferred onto PDA and incubated under laboratory conditions. In the ev ent where most sclerotia were macerated during incubation, pieces of the sclerotia were plated to investigate possible surviv al or the presence of other microorganisms. Data Analyses The field data and each experiment in the gr eenhouse were analyzed separately and the greenhouse data later pooled for analysis to investigate how the treatments performed using Statistical Analyses System (version 8.0, SAS Institute Inc Cary, NC) GLM approach. The ANOVA was used to evaluate the efficacy of ame ndments on survival of sclerotia and presence of antagonists. A comparison among treatment means was done using Tukeys Studentized Range (HSD) test at (P 0.05). 75

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Results Field Results In 2003 rotations significantly (P 0.05) affected the incidence of stem rot. Peanut in the PCCP rotation had 30% of sa mpled plants showing of S. rolfsii whereas only 5% were observed in the CBBP rotation. Incidence of the disease fluctuated during 2003 due to weather conditions (Figs. 2.6 and 2.7, in chapter 2 of this dissertation), but generally remained higher for the PCCP rotation than the CBBP rotation (Fig. 4.1). Greenhouse Results Amendment of field soils with bahi agrass cuttings significantly (P 0.05) reduced survival of sclerotia of Sclerotium rolfsii in all three experiments when sclerotia were buried for two months in amended soils (Figs. 4.2, 4.3, and 4.4). This was irrespective of the plant part though there was variability in the extent of reduc tion depending on the proportions of which plant combination were used. Most of the recovered sclerotia were eith er disintegrated or simply did not regenerate when plated on PDA. A significant number (P 0.05) of non-germinated sclero tia recovered from the amended soil showed signs of colonization by Trichoderma spp, bacteria, and other unidentified fungal species. Survival of sclerotia in non-amended soils varied between 80% in the first experiment and 100% in both the second and third experiments, whereas those in the amended soils were 2050% in the first experiment, and 8-75% in bot h the second and third experiments. Among plant parts, few significant differences were found in th eir ability to affect survival of sclerotia. Similarly, though amended soils had comparable nu mbers of colonized sclerotia, soils amended with higher proportions of bahiag rass leaves significantly (P 0.05) encouraged more bacteria colonization than those with root pieces and reduced sclerotia survival the greatest. Cut or ground bahiagrass leaves reduced sc lerotia survival in all three experiments, and amendments 76

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with higher proportion of leaves performed better than those with roots. Trichoderma spp. were found to grow on root pieces of bahiagrass collected from the field and those surfaced sterilized in 5% Chlorox and 2% lactic acid and were incubated in Petri di shes under laboratory conditions (data not presented). Some of the b acterial isolates recovered from amended soils exhibited inhibition zones when co-plated on agar medium. Sclerotia that were incubated in soils amended with bahiagrass germinated faster (6 h ours after plating) on media than those incubated in only field soil (18 hours). Discussion Incidence of SSR in the field study was found to be consistent with previous studies on the ability of bahiagrass to suppress peanut SSR (Johnson et al., 1999; Brenneman et al., 2003). Incidence of SSR was significantly lower on the CBBP than the PCCP rotation for most of the season, and the fluctuations were attributed to changing weather during the season. The sharp decline in incidence between 75 and 100 DAP was attributed to a pronounced dry period (Fig. 2.6). However, the improved leaf retention by peanut in the CBBP ro tation 100 DAP provided a conducive microclimate for survival of S. rolfsii even though there was a dry period, thus resulting in the slightly hi gher incidence on the CBBP rota tion (Fig. 4.1). Signs of SSR on peanut in the field under the CBBP rotation were atypical for SSR, as they appeared disintegrated. Increasing rates of chitin in the soil led to reduced sclerotia germination possibly due to the enhancement of chitinolytic microor ganisms (Rodriguez-Kabana et al., 1987), and similarly inhibitory effects of grape com post was attributed to the high numbers of Penicillium isolated from embedded sclerotia (Hadar and Gorodecki, 1991). In this study, the two most common organisms isolated from sclerotia were Trichoderma spp. and bacteria. Although data were not taken on individual eff ects on sclerotia, obser vations on their cultural interaction when co-plated with sclerotia suggest th eir role in sclerotial decay. Cut pieces of roots and leaves of 77

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bahiagrass performed better in suppressing sclero tia survival and encouraged more of the antagonists than powdered components. This obser vation suggests that during decomposition of large debris the diversity of mi croorganisms in the soil will be enhanced. In addition, bahiagrass produces large quantities of leaves over time, and when dead, leaves form a thick mat on the soil surface that can serve as suitable substrate for an tagonistic organisms to act on and subsequently degrade sclerotia. Field observations of peanut plants attacked by S. rolfsii showed signs of degeneration in the cottony hyphae of the pathogen suggesting that some form of microbial antagonism might be taking place. There have be en no reports of bahiagrass root colonization by Trichoderma spp., but the results from this research s uggest that could be a possibility since Trichoderma grew exclusively on root pi eces of bahiagrass collected from the field that were surface sterilized with Clorox and lactic acid. Adoption of new cropping patterns such as minimum tillage and twin-row planting pattern will result in significant changes in microclimat e within a peanut field that could enhance S. rolfsii survival. Porter (1980) repor ted increased severity of Sclerotinia minor on peanut in fields with lush canopy as a result of fungicide sprays to control leaf spot. Other reports also suggest that efficient management of peanut leaf spot through fungicide sprays could create a microclimate that will favor SSR (Shew and Beute, 1984). Although current fungicides can control both leaf spots and SSR at the same time, investing in a cropping system that will naturally lower peanut diseases as in the case of bahiagrass rotations will be more profitable and sustainable. Results from this research indica te that bahiagrass rota tion may reduce SSR by way of encouraging the activities of antagonistic microorganisms in soils. It is important to study how bahiagrass rotation influences soil microbial diversity, since that will give a clearer understanding of why soils planted to bahiagrass are suppressive. 78

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0 10 20 30 40 50 60 70 46 61 75 100137 Days after planting (DAP)SSR incidence (%) CBBP PCCP Figure 4-1. Effect of bahiagrass (CBBP) and conventional (PCCP) rotati on on the incidence of southern stem rot (SSR) in Quincy, FL during 2003. Incidence represents the percentage number of plants out of 20 showing pathogen signs. Data represents means of a minimum of 4 replications. Standard error bars are displayed for each rotation and assessment time. 0 10 20 30 40 50 60 70 80 90 T1T2T3T4T5T6T7 TreatmentOrganisms recovered (%) S. rolfsii Trichoderma Bacteria Figure 4-2. Experiment 1: Eff ect of bahiagrass amendments on survival of sclerotia of Sclerotium rolfsii T1 Infestation with only sclerotia, T2 -1:1 Leaves to roots (12 g of leaves and 12 g of roots), T3 1:2 Leaves to roots (8 g of leav es and 16 g of roots), T4 2:1 Leaves to roots (16 g of leaves to 8 g of roots), T5 24 g dry cut roots, T6 24 g dry cut bahiagrass leaves, and T7 24 g dry finely ground (powder) bahiagrass leaves. Bars represent percentage of sclerotia recove red that produced S. rolfsii or were colonized by Trichoderma spp. and bacteria. 79

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0 10 20 30 40 50 60 70 80 90 100 110 T1T2T3T4T5T6T7T8 TreatmentOrganisms recovered (%) S. rolfsii Trichoderma Bacteria Figure 4-3. Experiment 2: Eff ect of bahiagrass amendments on survival of sclerotia of Sclerotium rolfsii T1 Infestation with only scleroti a, T2 -1:1 Leaves to roots (12 g of leaves and 12 g of roots), T3 1:2 Leaves to roots (8 g of leav es and 16 g of roots), T4 2:1 Leaves to roots (16 g of leaves to 8 g of roots), T5 24 g dry cut roots, T6 24 g dry cut bahiagrass leaves, T7 24g dry finely ground (powder) bahiagrass leaves, and T8 24 g dry finely ground ( powder) bahiagrass root s. Bars represent percentage of sclerotia recovered that produced S. rolfsii or were colonized by Trichoderma spp. and bacteria. 0 10 20 30 40 50 60 70 80 90 100 T1T2T3T4T5T6T7T8 TreatmentOrganisms recovered (%) S. rolfsii Trichoderma Bacteria Figure 4-4. Experiment 3: Eff ect of bahiagrass amendments on survival of sclerotia of Sclerotium rolfsii T1 Infestation with only scleroti a, T2 -1:1 Leaves to roots (12 g of leaves and 12 g of roots), T3 1:2 Leaves to roots (8 g of leav es and 16 g of roots), T4 2:1 Leaves to roots (16 g of leaves to 8 g of roots), T5 24 g dry cut roots, T6 24 g dry cut bahiagrass leaves, T7 24 g, dry finely ground bahiagrass leaves, and T8 24 g ground (powder) bahiagrass roots. Bars represent percentage of sclerotia recovered that produced S. rolfsii or were colonized by Trichoderma spp. and bacteria. 80

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CHAPTER 5 EFFECT OF BAHIAGRASS ON NEMATODE POPULATION, REPRODUCTION, AND MOVEMENT IN THE FIELD, GREENHOUSE AND LABORATO RY CONDITIONS Introduction Peanut is an important crop worldwide and was listed as one of the crops standing between man and starvation (USDA 2007). Among the attributes of the peanut plant is the ability to grow and produce on marginal soils and produce seeds that are rich in calories and contain 25% protein. Peanut is extracted for oil and th e protein rich by-product is used for human consumption or as animal feed. Peanut hay is also used as animal feed in both developing and developed countries. Peanut is one of the most im portant nut crops in the United States (US) and a major crop for farmers in the southeastern US. Farmers in the state of Florida produce approximately 6 percent of th e total crop. Approximately 30-50 % of input costs in peanut production are allocated to managing weeds, insect s, and disease (Garciac asellas, 2004); hence the quest by southeastern farmers is to find ways to reduce input costs in order to boost profit. The current trend of escalating production costs associated with low commodity price exerts tremendous pressure on farmers who seek alterna tive but sustainable production methods to cut costs and preserve the environment. One opportunity is the use of crop rotation with compatible crops to reduce pest pressures, and additio nally, to improve and sustain soil fertility. Nematodes Diseases of Peanut in the Southeastern US and their Management Plant-parasitic nematodes are damaging to pea nut and cause an estimated 12% loss in crop yield and quality annually (Sasser and Freckma n, 1987). Several nematodes species attack peanut but the most prevalent include Meloidogyne spp., Pratylenchus brachyurus, Belonailamus longicaudatus (Shama 1985). Sasser (1977) listed three species of Meloidogyne that are damaging on peanuts: M. arenaria M. javanica and M. hapla Among these, M. arenaria (the most dominant species in the US) and M. javanica occur in warm and hot regions of the world 81

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and M. hapla occurs only in cooler regions (Dic kson and Waele, 2005). Nematodes are widespread and destructive pe sts on peanuts and are sometimes described as the hidden enemy since their damage are often impercepti ble to farmers. In Alabama, 5 to 10% of potential peanut yield is estimated to be lost due to nematodes (Hagan, 1994; Koenning et al., 1999). Typically, nematodes infect small areas of fields but can occasionally destroy crops when the infection is widespread. Root-knot nematodes cause galling on peanut roots, pegs, and pods and severely infected plants have stunted growth Root-lesion nematodes a ffect roots, pegs, and pods and can be identified by the presence of small spots that are tan with darker centers in color on pods (Rich and Kinloch, 2007). According to Hagan (1994), nematode populations are generally highest in light and sandy soils and th e more often peanuts ar e grown in a field, the greater the risk of damage caused by nematodes, especially peanut root-knot nematode. Nematode-damaged peanuts typically show yellow foliage and may wilt at midday, even if soil moisture levels are adequate fo r good plant growth. Vines may be so stunted that they do not lap or shade out the row middles, making the peanuts more sensitive to drought. Severely stunted peanuts frequently die if stressed by hot, dry weather. Damaging nematodes are not evenly distributed across a field and scat tered patches with damage can ra nge in size from a few feet to several acres. Control of a nema tode pest of peanut can be accomplished with crop rotation and nematicides. Organic Amendments and Nematode Suppression Organic amendments have been widely studied in the management of soil pests such as nematodes and other soil-borne pathogens that cause disease on agronomic crops. Nematode populations have been negatively or positively correlated with soil organic matter content (RodriguezKabana et al., 1987; Akhtar and Mahmood, 1994). Chem ical composition of organic matter determines the efficacy and the type of microorganisms that develop during degradation. 82

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Rodriguez-Kabana et al. (1995) reported the re lease of nematicidal co mpounds such as organic acids, hydrogen sulfide, nitrogenous ammonia, phenols, and tannins during degradation of amendments. Soler-Serratosa et al. (1996) reported th e occurrence of vola tile compounds with nematicidal and fungicidal properties on incor poration of plant residues, while ChavarriaCarrayal et al. (1994) reported in creased parasitism of eggs of Meloidogyne spp. upon incorporation of plan t residues into soil. A considerable amount of research has been de voted to the use of cover crops in nematode management (Haroon and Smart, 1882; Widmer and Abawi, 2000). Whether cover crops are incorporated as green manure or as killed rotation crops, the pro cess of decomposition releases a large array of active compounds that tremendously impact on the diversity of soil microbial populations (Haroon and Smart, 1983; McLeod a nd Steel, 1999; Widmer and Abawi, 2000). Koon-Hui et al. (2002) extens ively reviewed the use of Crotalaria as a cover crop in the management of nematodes. Wang (2000) suggested that the reduction in R. reniformis population in a pineappleC. juncea intercropping cycle might be due to the enhanced activities of bacterivorous nematode population as well as nematode-trapping fungi.. Several mechanisms are proposed to be responsible for the reductio n of plant-parasitic nematodes by cover crops including: 1) cover crops act as non-host or poor host as re ported by (Rodriguez-Kabana et al.,1992, 1994); 2) production of allellochemicals that are toxic or inhibito ry (Haroon and Smart, 1983; Gommens and Baker 1988; Halbrent, 1996); 3) cover crops could provide a niche for antagonistic flora and fauna (Cas well at al., 1990; Kloepper et al., 1991); 4) cover crops may act to trap nematodes (Gardner and Caswell-Chen, 1994; Lamondia, 1996). 83

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Cover crops influence soil nematode populations in several ways such as their failure to reproduce in non-hosts and in some cases produc ed fewer eggs (Rich and Rahi, 1995, McSorley, 1999). Greenhouse studies on the influence of plant residues incorporation on the survival of pathogens and reproduction of nematodes have been well studied, though the successful application of such studies in large scale in field has been limited (Haroon and Smart, 1983; Widmer and Abawi, 2000). Organic amendments and other naturally occurring compounds can effectively suppress a number of plant parasitic nematodes (Yeates and Coleman, 1982; Rodriguez-Kabana, 1991. Another option for nemat ode management will be the use of plantgrowth promoting substances that have the abi lity to suppress plant pathogens. Timper et al., (2001) observed a cropping system effect on M. arenaria and its antagonist Pasteuria penetrans, where populations of P. penetrans endospores were higher in a continuous peanut compared to peanut after two years. Role of Root Exudates in Rhizosphere Interactions Several researchers (Buchholtz, 1971; Fisher et al., 1978) implicate the role of leachates from residues incorporated into the growing medium, or residues in natural undisturbed condition in allelopathic interact ions. Different plants produce va rious metabolites that act as allelopathic compounds against nema todes with little environmenta l impact (Soler-Serratosa et al., 1996). Crotalaria spp. reduced root-knot galling in green house test and this plant is known to produce pyrrolzidine alkaloids and monocrotaline wh ich could be toxic to nematodes (Rich and Rahi, 1995). Fassuliotis and Skucas (1969) reporte d that exposure of root-knot nematode juveniles to monocrotaline caused them to jerk and reduced thei r infectivity. Allelopathy was demonstrated in bahiagrass when 84

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Fisher and Adrian (1981) obser ved that as the percentage of ground covered with bahiagrass increased the height of a 3-year-old pine decreased markedly, and further demonstrated that both living and decaying bahiagrass residu e were allelopathic to pine. Crop rotation is generally beneficial to cr op production and suggestions are that these benefits are due to improved nutrition, decreased disease levels and improved soil structure. Johnson and Pfleger (1992) indicated that the bene ficial effects of a rotation may be due to the population dynamics of VAM fungi Bahiagrass root exudates have been reported to enhance mycelia and spore production in VAM fungi (Cruz et al., 2003; Ishii et al., 1996). It is, therefore, probable that the presence of VAM fungi in bahiagrass may be res ponsible for the higher yields and lower disease levels in a bahiagrass-peanut rotation system. Higher hyphal as well as high root infection, and higher spore numbers of G. margarita were found in a bahiagrass and millet root environments than in an adjacent papaya root environment when each was separated from each other where Gigaspora margarita spores were placed (C ruz et al., 2003. Similarly, methanol extraction of bahiagrass and millet roots eluates showed a similar result as above (Cruz et al., 2003), an indication that bahiagrass produced compounds that stimulate VAM fungi development. Bahiagrass root ex tracts stimulated the growth of Gigaspora ramisporophora in axenic culture (Ishii et al., 1996), and the substances later id entified as flavonoids: eupalitin and two other unidentified compounds (Is hii et al., 1997). It is well es tablished that rotating peanut with bahiagrass suppresses plant-parasitic nemat odes, and other soil-borne pathogens (Johnson et al., 1999; Rodriguez-Kabana et al., 1994; Summer 1982), however, the mechanisms of such activity is not fully understood due to th e complexity of the soil ecosystem. 85

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The objectives of this research were to: 1) monitor soil nematode population on peanut in a bahiagrass and conventional rotation, 2) inves tigate further the effects of bahiagrass stem cuttings on the reproduction of M. arenaria and 3) investigate nema tode behavior on agargrown bahiagrass seedlings. Materials and Methods Rotation and Cultural Practices Field e xperiments were conducted at the North Flor ida Research and Education Center in Quincy, Florida from 2003 to 2006. Rotation plots were first established in the year 2000 and consisted of a b ahiagrass (B) rotation with peanut (P) and a conventional cotton-peanut (CP) rotation. The cropping sequence for the bahiagrass rotation involved the growing of cotton in the first year and then followed by bahiagrass for two consecutive years and in the fourth year the plots were cultivated to peanut for one year (CBBP). The conventional rotation consisted of growing peanut in the first year with cotton in the two subseque nt years followed by peanut in the fourth year (PCCP). Crop management practices were conducted according to the Florida Cooperative Extension Services recommendations. Each plot measured 22.8 m in length by 18.4 m (20 peanut rows). The bahiagrass cover crop was killed in the fall of each year by spraying recommended herbicides. A winter oat ( Avena sativa L.) cv. Florida 501 cover crop was planted at the seeding rate of 51 kg/A and was killed 124, 97, 123, and 120, DAP in 2003, 2004, 2005, and 2006, respectively by broadcast spraying glyphosate (Roundup WeatherMAX; Monsanto, Kansas City, MO). Seedbeds were prepared in all years by strip-tilling all plots with a KMC (Kelly Mfg. Corporation, Tifton, GA). Georgia Green pea nut cultivar was planted on 7 May, 2003 and 10 May in 2004with a Monosem pneuma tic planter (ATI, Inc. Lenexa, KS) at 6 seeds per 31 cm of row and 91-cm row spacing and phorate (Thimet 20-G; Micro Flo Company LLC. Memphis, 86

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TN) at 2.3 kg/A was applied in furrow at planti ng. AP3 peanut variety was planted on 13 May 2005and 17 May 2006 with a Monosem pneumatic pl anter (ATI, Inc. Lenexa, KS) Twin Row Planter at 3 seeds per 30 cm of row with simultaneous applica tion of phorate (Thimet) at 2.5 kg/A into seed furrows. Soil nematode population in the fields were monitored by randomly taking 10 soil cores (2.5 cm diameter 20 cm deep ) at harvest in Octobe r of each year within peanut rows from each plot for extraction of M. arenaria second-stage juveniles (J2). The 10 soil cores were combined, and nematode s were extracted from a 100 cm3 sub-sample from each plot by centrifugal flotation (Jenkins, 1964). Effect of Bahiagrass Cuttings on Populations of M. arenaria Inter-Planted with Tomato The effect of bahiagrass root a nd leaf cuttings on survival of M. arenaria was investigated in the greenhouse using tomato cv. Rutgers as the test plant. Field soil (fine loamy siliceous, thermic Plinthic Kandiudult) was collected a nd dried under greenhouse c onditions to heat-kill nematodes present before experiment initiation. The soil was spread thinly (2 cm thick) on a white polythene sheet with periodic stirring to allow drying by solarization for one month. The dry soil was sieved to remove cl ods and other large debris. Pot cu lture assays of bahiagrass roots and stem cuttings on survival and reproduction of nematodes were carried out in six 16-cm diameter plastic pots containing 1.2 kg of the dry field soil. Bahiagrass leaves and roots that had been previously cut into 2 cm pieces and dried we re weighed in different ratios to constitute 2% organic matter content (w:w) measured soil. The M. arenaria eggs for the bioassay were extracted from galled roots of tomato ( Lycopersicon solanacerum ) cv. Rutgers using NaOCl (Hussey and Barker, 1973). The various treatment and amendment ratios were as follows: T1Field soil without nematode inoculation, T2 Only nematode egg inoculation, T3 -1:1 Leaves to roots (12g of leaves and 12 g of roots), T4 1:2 Leaves to roots (8g of leaves and 16g of roots), T5 2:1 Leaves to roots (16g of leaves to 8g of roots), T6 2% dry cut root (24g dry root), and 87

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T7 2% OM dry cut bahiagrass leaves (24g dry cut leaves)were added to each pot and thoroughly mixed into the soil. Soil incorporated w ith the amendments were watered and allowed to drain for 24 hours before addition of nematode eggs. Three holes (1 cm diameter by 2 cm deep) were created using a spatula into the potted soil. Egg suspensions of M. arenaria (10,000 eggs per pot) were deposited into the holes and then covere d with soil for three days to allow eggs time to hatch. Water was added to soil in the pots to pr event drying.. Pots were maintained for 10 days in this manner to allow for residue decomposition before three-week-old Rutgers tomato seedlings were transplanted. The plants were watered as necessary for two months when the experiments were terminate d. Plant roots were removed from each pot by washing under tap water. Fresh weight of the shoo ts and roots were determined after the plant parts were allowed to air dry under laboratory cond itions to remove excess water. The plant roots were rated for root-knot galling using the 0 10 scale where, 0 = no galling, and 10 = complete root galling (Zeck, 1971). Nematode egg numbers on the roots were determined as described above (Hussey and Barker, 1973). The expe riments were repeated three times. Nematode Juvenile Movement on Agar-grown Bahiagrass and Tomato Seedlings Bahiagrass cvs. Pensacola, Paraguay, Argen tine, and tomato cv. Rutgers seeds were surface sterilized twice in 100% sodium hypochlorite for 30 minutes and thereafter washed in several changes of sterile distilled water. The surface sterilized seeds were air dried in the laboratory under sterile conditions and later plated on 0.6% water ag ar (3 g agar in 500 ml deionized water) and incubated in Petri dishes (150 by 20 mm) under laboratory conditions for two weeks. The plates were inoculated with approximately 500 eggs of Meloidogyne arenaria suspended in sterile distilled water by placing them at measured distances ( 2-10 cm) from bahiagrass and tomato seedling roots. Similarl y root pieces of tw o-month-old bahiagrass seedlings grown on agar were excised and placed into holes that were dug in the agar medium in 88

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Petri dishes. Egg suspensions of M. arenaria were deposited 10 cm from each root piece as described above. The number of juveniles that migr ated to living and exci sed roots were counted under a stereoscopic microscope at 40X magnification and their be havior along roots noted. Five days after inoculation, and ther eafter weekly, roots of both bahiagrass and tomato were decolorized in NaOCl (Clorox). The decolorized roots were later stained in red food dye (Thies et al., 2002) to detect any pene tration by nematode juveniles. The experiment was terminated after three months. Data Analyses Field data were analyzed by SAS PROC GLM st atistical analysis pr ograms and Fishers least significant diffe rence was used for means separation. The greenhouse data were transformed using (log 10) for large numbers greate r than 100 before analyses using SAS version 9.1. Treatment means for the greenhouse and the laboratory data were separated by Tukeys Studentized (HSD) test at P 0.05. Results Soil populations of ring, spiral, reniform, and r oot-knot nematodes in the rotations varied from year to year. Across all the four year s (2003-2006) and regardless of peanut variety populations of ring nematodes were higher in the bahiagrass than in the conventional rotation (Table 5.1). On the other hand, soil populations of spiral, reniform, a nd root-knot nematodes remained consistently higher in the PCCP than in CBBP rotation soils th roughout the four years. Except during 2004, populations of ring nematode s were highest in the bahiagrass (CBBP) rotation than in the conventional (PCCP) (Table 5.2). Both reniform and root-knot nematodes, the most damaging to cotton and peanut, respectiv ely, were lower in the bahiagrass rotation that in the conventional rotation. 89

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Incorporation of bahiagrass plant parts had significant effects (P 0.05) on tomato shoot weight, galling index, mean number of nematode e ggs per root, and per gram root but not on the root weight (Table 5.3). Tomato plants amended with 16 g of leaves to 8 g of roots (T5) had the highest shoot weight (67.5 g) but least root weight (27.6 g) whereas, those planted into pots amended with 8 g of leaves and 16 g of roots (T4) showed least shoot weight (Table 5.3). Plants grown in pots un-amended with bahiagrass cuttings had a significantly hi gher galling index (P 0.05), than those amended with cutt ings (Table 5.4). Total number of eggs per root and per gram root were significantly higher (P 0.05) for plants grown in un-amended soil than in amended soil regardless of plant part propo rtions Table (5.3). No signifi cant differences in egg production on tomato were observed from incorporating di ffering ratios of bahiag rass leaves androots. However,, plants grown in soil amended with hi gher proportions of bahiagrass leaves had the most impact on reducing egg production Table (5.3). In the tests utilizing water agar, second-stage M. arenaria juveniles were observed moving on the surface of the agar with in three days after inocula tion of egg suspension. Nematode juveniles moved equally to root zones of tomato and bahiagrass seedlings on agar. There were no significant differences (P 0.05) in the number seedlings of bahiagrass and tomato assessed for nematode juvenile movement in agar. However, significant differences (P < 0.05) were recorded for the maximum distances traveled by nematode juveniles towards seedling roots in media (Table 5.4). Juveniles moved on average 3.7 cm to living roots of tomato, and 2.5, 3.1, and 2.6 cm towards bahiagrass cvs. Argentine, Paraguay, and Pensacola, respectively (Table 5.4). The maximum distance moved (6.3 cm) by juveniles was recorded on cut bahiagra ss root pieces that were embedded in agar (Table 5.4). Juveniles colonized tomato roots and produced galls that were absent on bahiagrass seedlings. 90

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Discussion Soil nematode population densities were lo wer in the bahiagrass (CBBP) than the conventional (PCCP) rotation, particular ly in relation to juveniles of Meloidogyne spp. These data are consistent with those previously repo rted by Rodriguez-Kabana et al., 1994; Timper et al., 2001). Previous studies have de monstrated population reductions in Meloidogyne spp in peanut after bahiagrass rotation (Johnson et al., 1999). Mechanisms of nematode reduction under a bahiagrass rotation were attributed to the non-ho st status of bahiagrass, and the encouragement of the nematode antagonists such as Pasteuria penetrans (Timper et al., 2001). The bahiagrass rotation did not suppress the population of ring ( Mesocriconema spp) nematode populations, although Nyczepir and Bertrand (2000) successfu lly used pre-plant bahiagrass to suppress populations of ring nematodes in young peach orchar ds. Similarly, Zehr et al (1990) reported that bahiagrass did not support M. xenoplax populations under greenhouse conditions when seedlings were inoculated with the nematode. The high populations of ring nematodes could not be explained from present data. The non-host status of bahiagrass to Meloidogyne spp and other nematode species has been well studied (Rodriguez-Kabana et al., 1989); Dickson and Hewllet, 1989). However, the behavior of nematode juveniles around the root zones of bahiagrass has not been documented. In the present study, that juveniles of M. arenaria actively moved towards both living and excised bahiagrass roots in water agar. Since no feeding or root penetration was observed when juveniles moved onto roots, these results confirm previous studies on the non-host status of bahiagrass to Meloidogyne spp. The movement of juveniles to cut root pieces in media in large numbers is an indication that exudates from bahi agrass roots may be acting as an attractant to nematodes. Thus root exudates may act to reduce nematode populations in soils by trapping juveniles into root zones where they can not feed and therefore may die. In addition, bahiagrass root exudates may 91

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be encouraging the diversity of other antagonistic organisms in the rhizosphere, which could contribute to nematode population reduction. This research has demonstrated that juveniles of M. arenaria are strongly attracted to bahiagrass, a nd no evidence of repellency was found. It suggests that nematode stimuli in finding host may be different than the response to enter roots and feed. Incorporation of plant residues into soil and their role in nematode suppression has been studied and reported to be succe ssful (Sikora, 1992). Soil amen dment with either fresh or decomposed plant residue alters soil physical, chemical, and biological equilibrium and will affect the diversity of microbial populations that will enhance nematode suppression. Development of M. incognita was inhibited in soils amended with digitgrass (Haroon and Smart, 1983). Similarly, chopped leaves of brassicas have been reported to successfully lower M. javanica numbers when incorporated into soils (M cLeod and Steel, 1999). The incorporation of bahiagrass residues into soil in this study successfully reduced M. arenaria reproduction on tomato. Both roots and leaves were equally e ffective in reducing egg production regardless of plant part proportion. The mechanism of nema tode suppression when using bahiagrass amendments was not investigated in these st udies. However, the results suggested that mechanisms previously implicated in other st udies were involved (Haroon and Smart, 1983; McLeod and Steel, 1999; Widmer and Abawi, 2000) which include the release of volatile compounds and encouragement of antagonists may be playing some role. It is concluded from these studies that bahiagrass rotations re duced populations of Meloidogyne spp., Helicotylenchus spp., Rotylenchulus spp.but not for Criconemoides spp. Bahiagrass root exudates may also be acting to actively attract M. arenaria juveniles. 92

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When attracted to the root zone, the juveniles of M. arenaria become trapped and in the absence of feeding and root penetration, they either die or become more exposed to antagonists in the rhizosphere. 93

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Table 5.1. Effect of rotations on so il nematode populat ions during 2003-2006s Nematode population/100 cm3 soilw Year/Variety Rotationt Ring Spiral Reniform RKN 2003, Georgia Green CBBP 239.0 a 18.0 b 8.0 b 3.0 b PCCP 83.0 b 32.0 a 122.0 a 20.0 a LSD (P 0.05) 99.8 20.4 66.9 13.0 Standard error 30.6 4.8 21.4 3.8 2004, Georgia Green CBBP 85.0 a 8.0 b 23.0 b 23.0 b PCCP 190.0 a 30.0 a 343.0 a 26.0 a LSD (P 0.05) 120.3 26.7 252.2 Standard error 29.3 6.0 68.0 5.0 2005, AP3 CBBP 180.0 a 13.0 b 6.0 b 5.0 b PCCP 81.0 b 34.0 a 97.0 a 37.0 a LSD (P 0.05) 78.7 22.1 38.6 11.8 Standard error 22.0 5.6 14.9 5.1 2006, AP3 CBBP 81.0 b 34.0 a 97.0 a 37.0 a PCCP 81.0 b 36.0 a 538.0 a 40.0 a LSD (P 0.05) 86.6 22.6 248.9 11.5 Standard error 20.8 4.8 84.2 4.4 s Means within a column followed by the same letters are not significantly different according Fishers LSD test (P 0.05). Each value in table re presents the mean for a minimum of 8 replications. t Yearly rotation sequences were bahiagrass (B), cotton (C), and peanut (P). w Ring nematode ( Criconemoides spp.), Spiral nematode (Helicotylenchus spp.) Reniform nematode ( Rotylenchulus spp.), and RKN ( Meloidogyne spp.). 94

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Table 5.2. Effect of rotations on so il nematode populations across 2003-2006 s Nematode population/100 cm3 soil w Years Rotationt Ring Spiral Reniform RKN 2003 2006 CBBP 188.0 a 17.0 b 15.0 b 11.0 b PCCP 82.0 b 33.0 a 275.0 a 31.0 a LSD (P 0.05) 45.2 10.3 103.9 8.2 s Means within a column followed by the same letters are not significantly different according to Fishers LSD test (P 0.05). Each value in table represents the mean for a minimum of 8 replications. t Yearly rotation sequences were bahiagrass (B), cotton (C), and peanut (P). w Ring nematode ( Criconemoides spp.), Spiral nematode ( Helicotylenchus spp.) Reniform nematode ( Rotylenchulus spp.), and RKN ( Meloidogyne spp.). 95

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Table 5-3. The influence of bahi agrass residues on development of tomato plants infected with M. arenariaw Treatment Fresh weight (g) Galling index Log number of eggs Shoot Root Total per root per gram root T2 48.2 a 28.4 a 6.7 a 5.6 a 4.2 a T3 59.5 a 32.0 a 4.1 b 5.2 ab 3.7 b T4 48.1 b 28.7 a 4.5 b 5.1 a 3.6 b T5 67.5 a 27.6 a 3.9 b 5.0 b 3.6 b T6 54.7 ab 31.4 a 4.5 b 5.2 b 3.7 b T7 59.0 b 32.0 a 4.2 b 4.9 b 3.4 b LSD (P 0.05) 14.1 8.3 1.6 0.4 0.4 Standard error 1.5 0.8 0.2 0.1 0.1 w Means within a column and followed by the sa me letters are not significantly different according to Tukeys HSD test (P 0.05). Each value in table represents the mean for 90 observations for three experiments. x Treatment represents the propor tion of bahiagrass shoot and root ratios: T2 Only nematode egg inoculation, T3 -1:1 Leaves to roots (12g of leaves and 12 g of roots), T4 1:2 Leaves to r oots (8g of leaves and 16g of roots) T5 2:1 Leaves to roots (16g of leaves to 8g of roots), T6 1% dry cut ro ot (24g dry root), and T7 1% OM dry cut bahiagrass leaves (24g dry cut leaves). Soils of al l the treatments were infested with 10,000 eggs of M. arenaria y Galling index scored on a scale of 0-10 where 0 = no galling, 10 = complete root galling. 96

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Table 5-4. Effect of bahiagrass and tomato roots on the behavior of juveniles (J2) of M. arenaria on water agarw Plant x Number of seedlings Number of juveniles per seedling Mean number of juveniles on roots Distance moved by juveniles to roots (cm) Argentine 11 a 0.9 a 9 b 2.5 a Paraguay 13 a 0.3 ab 4 a 3.1 a Pensacola 14 b 1.4 a 16 b 2.6 a Rutgers 9 b 1.3 a 12 3.7 a Bahiagrass root pieces 7.0 b 6.3 b Standard error (P 0.05) 0.76 0.13 1.25 0.32 w Means within a column followed by the same letters are not significantly different according to Tukeys HSD test (P 0.05). Numbers within a column followed by the same letter are not significantly different (P 0.05) from each other. x Rutgers is a variety from tomato whereas, Argentine, Paraguay, Pensacola are varieties of bahiagrass ( Paspalum notatum ), bahiagrass root pieces were regardless of variety. 97

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CHAPTER 6 SUMMARY AND CONCLUSIONS Severity of early leaf spot ( ELS) and late leaf spot (LLS) on peanuts that were planted after two years of bahiagrass rotation was lower than th ose planted after two ye ars of cotton. Results during 2003-2006 showed significant reductions in ELS and LLS severity in a consistent manner. By utilizing the Florida 1 10 leaf spot assessment scale, [where 1 = no leaf spot; 2 = very few spots on leaves with none on upper canopy leaves; 3 = few lesions on the leaves, very few on upper canopy; 4 = some lesions with mo re on the upper canopy, 5 % defoliation; 5 = lesions noticeable on upper ca nopy, 20% defoliation; 6 = lesions numerous and very evident on upper canopy, 50 % defoliation; 7 = lesions num erous on upper canopy, 75 % defoliation; 8 = upper canopy covered with lesions, 90 % defoliati on; 9 = very few leaves remaining and those covered with lesions, 98 % defoliation; and 10 = pl ants completely defoliated and killed by leaf spot], final disease severities in the bahiagrass rotation were 5, 6, 6, and 6 compared to 7, 8, 7, and 8 in the conventional system during 2003, 2004, 2005, and 2006, respectively. Defoliation of peanut as result of ELS and LLS was greater (near 90%) in some years in the conventional system compared to 60% in the bahiagrass ro tation. Onset of ELS and LLS was delayed on the bahiagrass rotation compared to the conventiona l rotation, and the rate of disease increase remained higher on the conventio nal rotation in all years. Inoculations of peanut leaves taken from pl ants growing in the two rotations had 83% of leaflets from the conventional compared to 33% from the bahiagrass rotation showing symptoms of ELS after four weeks of incubation under laboratory conditions. It is likely the delayed onset of disease may be accounting for the variability in ELS and LLS epidemics that were observed in the field. 98

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Peanut yield in the bahiagrass rotation was greater (2,935; 3,053; 2,250; and 4,504 kg/ha) than the conventional (2,229; 2,297; 1,703; and 3,278 kg/ha) rotation during 2003, 2004, 2005, and 2006, respectively when peanut plants were not sprayed with fungicides. Similarly, higher yields were obtained in the bahiag rass rotation than in the conven tional rotation in all years when the plots were sprayed with fungicides thr oughout the period of the study. Peanut grade was higher in the bahiagrass rotation than in the conv entional system. The incidence of peanut rust ( Puccinia arachidis Spegg), however, was higher on the bahiagrass rotation than in the conventional system during 2003, and the reasons fo r this observation could not be explained. The impact of bahiagrass rotation on the in cidence and severity of tomato spotted wilt (TSW) on peanut was studied during 2003-2006 in the same rotational system described above. The peanut cv. Georgia Green wa s planted in 2003 and 2004 in a si ngle row pattern, whereas, cv. AP3 was planted in 2005 and 2006 in a twin-row pattern as recommended for the management of thrips population on peanut. Pea nut grown after two years of bahiagrass rotation reduced the incidence and severity of TSW in all years comp ared to the conventional system irrespective of the variety or row planting pattern, although ther e was variability in se verity and incidence among years. Incidence of TSW at the beginning of the season in all years was higher in the conventional system than in the bahiagrass ro tation, and remained high er at all times of assessment in all years of the study (2003-2006). The incidence of TSW 32 DAP in 2003 was 22% on peanut in the bahiagrass rotation compar ed to 39% in the conve ntional rotation, and 24% and 38% 40 DAP, respectively during 2004. Final TS W incidence was similarly lower at 17 and 32 % for the bahiagrass compared to 21 and 72% for the conventional rotation during 2003 and 2004, respectively. 99

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Monitoring of thrips populations on peanut seedlings was conducted in 2005, and it was observed that thrips per peanut seedling under the bahiagrass system was lower (6) than those in the conventional system (22). Seedling damage due to thrips feeding was 100% in the conventional system compared to 45% in the bahiagrass rotation during 2005. The number of thrips per seedlings on two years of continuous p eanut was 42 compared to 4 per plant on peanut grown after one year of bahiagra ss rotation. Feeding damage of thrips positively correlated (r = 0.60) with the number of thrips per seedling, and the final TSW incidence (r = 0.94). Similarly, a strong correlation existed for the number of da maged seedlings and final TSW incidence (r = 0.84). During 2006, differences in thrips popula tions were observed on volunteer peanut seedlings in plots planted to winter oat cove r crop (45), compared to 21 and 7 on peanut seedlings in killed and green bahiagrass plots, respectively. Bahiagrass inflorescences harbored 4 thrips per head compared to 18 thrips per pani cle and 6 thrips per seed of winter oat. The incidence and severity of TSW on peanut planted in those plots pr eviously planted to winter oat had both higher initial (40%) and final TSW (53 %) incidence compared to plots not planted to winter oat that had 22 and 23% TSW, respectivel y in 2006. These data indicate that oat may be acting as a host to thrips when compared to bahi agrass, and thus harbor th rips that subsequently attack peanut grown afterwards. Investigations into the impact of bahiagrass on southern stem rot (SSR) of peanut caused by Sclerotium rolfsii was monitored on peanut in the ba hiagrass and conven tional rotations during 2003. Incidence of SSR wa s variable during the season but remained higher on the conventional than on the bahiagrass rotation. Fl uctuations in the incidence of SSR were attributed to changing weather conditions prevailing during 2003. The average incidence of SSR in the conventional rotation was 30% compar ed to 5% in the bahiagrass rotation. The 100

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mechanisms involved in the suppression of SSR by bahiagrass was investigated in the greenhouse in field soil amended with different proportions of bahiagrass root pieces and leaves. Sclerotia of S. rolfsii were buried in both amended and nonamended dried field soil in the greenhouse and incubated for ten weeks. Surviv al of sclerotia in non-amended soils ranged from80 to 100% compared to 8 to 75% in amended soils. Sclerotia recovered from soils were disintegrated and were colonized either by Trichoderma spp., bacteria or by both which resulted in inhibition zones in media. Ba hiagrass seedlings co-grown with S. rolfsii on media were killed and sclerotia were produced, whiles Trichoderma spp. grew on incubated bahiagrass roots that were surface sterilized u nder laboratory conditions. Bahiagrass rotations reduced soil nema tode populations of Meloidogyne spp., Helicotylenchulus spp., Rotylenchulus spp., in exception of Criconemoides spp through 20032006. Incorporation bahiagrass roots and leaves in varied proportions reduced egg production by M. arenaria on tomato under greenhouse conditions. Bo th roots and leaves were equally effective in reducing egg production regardless of plant part proportion however; residues higher in bahiagrass leaves were more effective in nematode suppression. The mechanism of nematode suppression was not investigated when bahiagrass residues were incorporated in this study. Juveniles of M. arenaria actively moved onto roots of bahiagrass seedling grown in agar, and did not feed nor penetrated the root s. Juveniles moved equal distances to roots of both live tomato and bahiagrass seedlings as well as cut root piec es of bahiagrass seedlings embedded in media. There were no differences in the numbers that colonized root zone for both plants, although significantly higher numbers were found on cut root pieces that living plants. When attracted to the root zones the juveniles become trapped, and in the absence of feeding and root penetration, they either die or become more exposed to antagonists in the rhizosphere. 101

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It is concluded from these studies that bahiagrass suppresses ELS, LLS, TSW, SSR, and nematodes of peanut under natural conditions without the assistance of chemical inputs. Suppression of ELS and LLS may be attributed to delayed dis ease onset as well as increased tolerance of peanut to leaf s pots, while the possible modes in the suppression of TSW is in the reduction of initial thrips popul ation. Reduction of SSR in a bahiagrass rotation could be attributed to the enhancement of an tagonists that degrade sclerotia of S. rolfsii Bahiagrass root exudates may be acting to actively attract M. arenaria juveniles to the root surfaces where they can not feed and may be prone to anta gonistic attack by other soil organisms. Further in-depth studies are needed to dete rmine how bahiagrass rotation influences soil microbial diversity, and mechanisms of disease suppression. 102

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117 BIOGRAPHICAL SKETCH Francis Kodjo Tsigbey was born in Hoviefe, in the Volta Region of Ghana, in 1963. After attending village schools at the primary and mi ddle school levels, he attended Peki and Mawuli Secondary High schools. Francis Ts igbey later went to the Univer sity of Ghana, Legon, where he earned his Bachelor of Science ( 1990) and Master of Science (1996) degrees in crop science. He later took on a job as a resear ch plant pathologist near Tama le in Ghana at the Savannah Agricultural Research Institute Nyankpala, an institute under the Counc il for Scientific and Industrial Research (CSIR, Ghana) in 1996. Until 2003 his research focus was on the development of disease management strategies on legumes. Francis worked with the Peanut Collaborative Research Support Program (Peanut CRSP) at North Carolina State University and the University of Florida. Francis took up a rese arch assistantship with Dr. Jim Marois where he investigated the impact of bahi agrass rotation on peanut diseases. Francis completed his Ph.D. in December 2007.