Quincy NFREC Research Report 93-2
FUNGICIDE CONTROL OF
LEAFSPOT ON COMMERCIAL
PEANUT CULTIVARS UNDER
LONG ROTATION CONDITIONS
F.M. Shokes, D.W. Gorbet, and T.A. Kucharek
Florida Agricultural Experiment Stations
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
FUNGICIDE CONTROL OF LEAFSPOT ON COMMERCIAL PEANUT CULTIVARS
UNDER LONG ROTATION CONDITIONS
F. M. Shokes, D. W. Gorbet, and T. A. Kucharek
North Florida Research and Education Center (NFREC), University of
Florida, Quincy, FL. 32351; NFREC, University of Florida, 3925 Hwy
71, Marianna, FL. 39244; Department of Plant Pathology, University
of Florida, Gainesville, FL. 32611. Research NF 93-2.
Leafspot diseases can cause severe defoliation of peanut
(Arachis hypogaea L.) and up to 70% loss of pod yield if unchecked.
Yield loss is greatly determined by the amount of healthy leaf area
maintained by the plant during the pod development phase of growth.
If a leafspot epidemic reaches a high level and causes significant
defoliation during early pod development, it is more harmful than
if it reaches a high level near harvest. If harvest is delayed for
any reason, even late epidemics can cause serious yield losses.
Harvest timing is critical when leafspot diseases are severe. With
leafspot-susceptible cultivars (varieties) delaying the beginning
of the preventive fungicide program can be detrimental. It is most
harmful to the crop if only a one or two-year rotation out of
peanut is practiced. With a short crop rotation the initial
leafspot inoculum may be sufficient to begin an early epidemic that
can be severe by the end of the season.
Leafspot diseases are very weather dependent. Temperatures
during the growing season in North Florida are almost always within
the range required for the leafspot pathogens to reproduce (Figure
1). If frequent rain events or long leaf wetness periods from dew
occur, leafspot infection will follow.
In recent years an increased number of peanut cultivars have
become available to growers. Most of these cultivars are very
susceptible to both early and late leafspot diseases. Therefore a
preventive fungicide program must be used to sustain adequate pod
yields. Some of the available cultivars have not been widely used
in the southeastern peanut belt and little is known about their
yield response when early season, preventive fungicide
applications are skipped.
The program recommended for leafspot management with most
peanut cultivars grown in Florida includes initiation of fungicide
sprays by 35 days after planting (DAP) followed by applications on
a 10-14 day interval to within two weeks of harvest. Such a pro-
gram will allow adequate protection of new foliage and if properly
implemented should insure maximum healthy leaf area throughout pod
fill. Fungicide applications cost $8 to $10 per acre per applica-
tion and growers often try to save in this area by skipping an
application or by delaying the initiation of the spray program.
Therefore, tests were designed to determine the effect of skipping
the 1st, 2nd, or 3rd fungicide applications with 12 commercially
available cultivars when grown in a long crop rotation situation.
I t I
Jun Jul Aug Sep Oct
Figure 1. Rainfall and temperature extremes for the Dozier test
sites, Jackson County, Florida, 1989-1991.
MATERIALS AND METHODS
Twelve peanut cultivars were grown for three seasons (1990 -
1992) on various sites at Dozier Boys School, Mariana, Florida.
Five runner market types of medium maturity (135-145 days to
maturity), two runner and one Virginia type of early maturity (125-
130 days), one late maturing runner (>150 days), and three Virginia
types of medium maturity (Table 1) were tested. Seed of all the
cultivars tested are commercially available in the US. Peanut seed
were planted (4 seed/ft, 80-120 Ib/A), in 20 ft rows on 3 ft
centers on 31 May 1990, 11 June 1991, and .2 June 1992. The
planting sites had not had peanuts within the previous five or more
years and were the equivalent of fields with an excellent rotation.
Standard recommended cultural practices were used except for
leafspot management practices. Fertilizer, 500 Ib/A of 0-9-18 with
minor elements was applied before planting and turned under with
a moldboard plow. Bradyrhizobium inoculant (7 lb/A) was applied in
the furrow at planting. Disyston (7 lb/A) was applied at planting
and other insecticide use was based on need as determined by weekly
scouting for pests. Weed control was accomplished with recommended
herbicides at labeled rates as required for the specific weed
problems. Gypsum (1000 lb/A) was applied at early pegging (35-45
days after planting (DAP)) each year.
Table 1. Peanut cultivars planted at Dozier Boys School, Marianna,
Days After Planting of Harvest
Relative' 1990 1991 1992
Cultivar Maturity H1 H2 H1 H2 H1 H2
Marc I (McI) E 127 134 122 132 129 136
AgraTech 127 (AT127) E 127 134 122 132 129 136
Florunner (FR) M 134 144 132 143 136 146
Sunrunner (SR) M 134 144 132 143 136 146
Okrun (OkR) M 134 144 132 143 136 146
Tamrun 88 (TR88) M 134 144 132 143 136 146
GK7 M 134 144 132 143 136 146
Southern Runner (SoR) L 144 151 143 153 146 153
NC9 E 127 134 122 132 129 136
NC7 M 134 144 132 143 136 146
NC10C M 134 144 132 143 136 146
Florigiant (FG) M 134 144 132 143 136 146
1 E= early maturity, 120-130 I
L= late maturity, >145 DAP.
)AP; M= medium maturity, 131-145 DAP;
Chlorothalonil as Bravo 7200, 1.5 pt/A was applied in 18 GPA
at 40 PSI for leafspot control on 14-day intervals. All treatments
were applied with a high-boy sprayer using two nozzles per row
(8002 flat fan). Treatments were applied according to the
schedules given in Table 2. Treatments were replicated three times
in a split-plot design in which fungicide initiation dates were
main plots and cultivars were sub-plots. All four rows of each
plot were sprayed with fungicide according to the treatment
Table 2. Initiation date of fungicide treatment in days after
planting (DAP) and number of applications made to 12 peanut
cultivars in.1990 through 1992.
Treatment Beginning of
Number Spray Program No. of Applications
1 35 DAP 7
2 49 DAP 6
3 63 DAP 5
4 77 DAP 4
Foliar diseases were assessed a minimum of four times during the
growing season at 80 DAP, 91-100 DAP, 102-109 DAP, and at 127 DAP.
The Florida 1-10 scale (Table 3) was used to assess leafspot. Late
leafspot caused by Cercosporidium personatum was the predominant
foliar disease each year. Early leafspot caused by Cercospora
arachidicola was present each year but was in low incidence in the
first two years. In 1992 early leafspot was a major factor
throughout the season but no attempt was made to differentiate
between the two diseases. Some leaf scorch and pepper spot caused
by Leptosphaerulina crassiasca occurred each year, usually late in
the season when daily temperatures had decreased. Web blotch
caused by Phoma arachidicola occasionally occurred but the
incidence was very low. Rust (Puccinia arachidis) was negligible
all three years.
White mold (southern stem rot or southern blight) caused by
Sclerotium rolfsii and limb rot caused by Rhizoctonia solani were
evaluated after the plants were dug and inverted each year. White
mold was assessed as hits/40 row ft and limb rot was assessed as
the percentage of limbs with symptoms of the disease. The
incidence of these diseases was very low in 1991 but was sufficient
to be determined the other two years.
Although no irrigation was available at the test sites rainfall
was adequate each year (Figure 1). In 1991 the abundance of rain
events in May delayed planting until the second week of June. Two
harvests were taken from each plot by digging rows 1 and 2 for the
first harvest and rows 3 and 4 for the second harvest. Digging
dates were set so that the first harvest would be close to optimum
maturity with the second harvest about 10 days later. Plants were
inverted in windows at digging and allowed to dry for three days
before picking pods with a modified commercial peanut combine.
Pods were dried to 10% moisture and weighed for yield
Table 3. Florida 1-10 scale for leafspot disease assessment.
1.0 No disease
1.1 Trace only a lesion or 2 in canopy hard to find.
1.5 Very few lesions in bottom canopy but easier to find than
2.0 Very few lesions (none on upper canopy); a few spots on most
2.5 A few more lesions and some on mid canopy.
3.0 Few lesions (very few on upper canopy); spots getting easy to
find and often distributed from bottom to top.
3.5 Spots easy to find, well distributed, more than a 3.0 but
noticeable defoliation has not occurred.
4.0 Some lesions with more on upper canopy and slight defoliation
noticeable; roughly 5-10% defoliation.
5.0 Lesions. noticeable even on upper canopy with noticeable
defoliation; you can see lesions from end of plots.
6.0 Lesions numerous and very evident on upper canopy with
significant defoliation (50+%); defoliation a major
7.0 Lesions numerous on upper canopy with much defoliation (75-
90%); defoliation a major determinant.
8.0 Upper canopy covered with lesions with high defoliation (90-
96%); defoliation a major determinant.
9.0 Very few leaves remaining and those covered with lesions; some
plants completely defoliated (99-100%).
10.0 Plants dead.
All data were statistically analyzed using MSTATC on a
personal computer. Data were submitted to an ANOVA procedure for
a randomized complete block split-split-plot design in which year
was the main plot, fungicide treatment the sub-plot and cultivar
the sub-sub-plot. If the year by treatment interaction was
significant (P<0.05) the statistic was reanalyzed within years as
a split-plot design. Since leafspot ratings were made either four
or five times each year disease progress could be evaluated by
determining the areas under the disease rating curve (AUDRC). The
AUDRC were determined and statistically compared. Mean separation
was performed for all parameters using the least significant
Leafspot Disease Ratings: Foliar diseases were always minimized
with seven applications of fungicide with initiation of the spray
program by 35 days after planting (Figure 2). Differences in final
disease were. slight between the five and six-spray treatments
across the three years. Southern Runner consistently had the
lowest foliar disease ratings for the four treatments in each year.
It was the only cultivar with a disease rating of <5 on the FL 1-10
scale for all treatments. Areas under the disease rating curves
were consistently lowest for this treatment ranging from 131 for
Southern Runner to 174 for Sunrunner (Table 4).The AUDRC for
Southern Runner were always significantly (PF0.05) lower than those
of the other cultivars for all treatments. The cultivars NC9 and
NC10C had similar disease ratings for the four-spray treatments and I
had slightly lower leafspot amounts than all of the runner
cultivars except Southern Runner and GK7. From best (lowest
leafspot disease) to worst (most leafspot disease) the 12 cultivars
are ranked as shown in Table 5.
Table 4. Areas under the disease rating curves (AUDRC) for
leafspot diseases on 12 peanut cultivars over three years I
at the Dozier location, Jackson County, Florida.
AUDRC for different Spray Treatments'
Cultivar Seven2 Six Five Four
Florunner 1593 190 224 278
Sunrunner 174 198 246 294
Southern Runner 131 149 169 216
Marc I 167 206 250 297
AgraTech 127 174 211 242 295
Okrun 167 199 231 292
Tamrun 88 163 190 217 273
GK 7 159 186 220 263
NC 7 158 196 234 293
NC 9 156 186 206 266
NC 10C 155 176 214 267
Florigiant 166 193 227 274
LSD05 9.3 11.0 9.3 10.6
'Relative areas under the disease rating curves are based on four
or five disease ratings during the season.
2Number of chlorothalonil applications made during the season.
3The lower the value the less the leafspot disease development on
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White Mold and Limb Rot Diseases: White mold disease loci ranged
from <1 hit/40 row ft to 7.7 hits/40 row ft when averaged across
the three years (Table 6). Southern Runner consistently had fewer
hits of white mold than any other cultivar. Limb rot was generally
too low (<8%) to be a major yield-determining factor in the test
areas. The virginia market type NC10C exhibited a trend toward
slightly higher levels of limb rot than all of the other cultivars
across treatments and years with an average of 10.2%.
Table 5. Ranking of 12 peanut cultivars according to leaf spot
ratings over three years of testing at the Dozier location, Jackson
1 Southern Runner
5 Tamrun 88
11 Marc I
12 AgraTech 127
Table 6. White mold disease on 12 peanut cultivars over three
years at the Dozier location, Jackson County, Florida.
White Mold Hits/40 row ft
Cultivar Seven' Six Five Four Mean
Florunner 2.2 2.2 1.0 3.7 2.3
Sunrunner 1.3 2.4 3.1 3.2 2.5
Southern Runner 1.6 1.1 0.9 0.9 1.1
Marc I 2.6 1.3 2.6 3.9 2.6
AgraTech 127 2.2 1.9 2.7 3.4 2.6
Okrun 3.7 2.7 3.4 5.0 3.7
Tamrun 88 2.6 2.4 2.2 2.8 2.5
GK 7 2.2 2.4 1.3 4.2 2.5
NC 7 3.4 2.8 4.1 7.7 4.5
NC 9 3.3 2.7 3.1 3.6 3.2
NC 10C 3.9 2.4 2.2 4.2 3.2
Florigiant 3.6 2.4 3.3 6.1 3.8
LSD05 2.7 1.8 2.2 2.5
INumber of chlorothalonil applications during the season.
Pod Yields: Pod yields were generally good for all cultivars
ranging from 3000 to 5200 lb/A (Table 7). Yields exceeded 2 tons
for the first harvest for the 5, 6, and 7-spray treatments for all
cultivars except Tamrun 88. First harvest yields decreased to
<3500 lb/A for the 4-spray treatment for all cultivars except
Southern Runner, GK7, and NC10C. The second harvest further
differentiated the cultivars, particularly for the 4-spray
treatment. Only Southern Runner maintained a second harvest yield
>4000 lb/A for this treatment, and only NC9 and NC10C maintained
yields near 2 tons for this treatment.
Table 7. Yield of 12 peanut cultivars grown under four leafspot
spray initiation programs at the Dozier location in
Jackson County, Florida, 1990-1992.
1st Harvest Yield (Ib/A)
Cultivar Seven1 Six Five Four Mean
Florunner 4168' 4213 4101 3440 3980
Sunrunner 4212 4454 3996 3331 3998
Southern Runner 4846 4750 5269 4655 4880
Marc I 4391 4235 3728 3337 3923
AgraTech 127 4078 4430 4118 3386 4003
Okrun 4286 4329 4087 3338 4010
Tamrun 88 4119 3967, 3607 3389 3770
GK 7 4909 4372 4419 3691 4348
NC 7 4445 4709 4335 3328 4204
NC 9 4735 4412 4536 3743 4357
NC 10C 4895 5048 4467 4158 4642
Florigiant 5014 4789 4435 3845 4521
Mean 4508 4476 4252 3637
LSD.5 481 453 526 325
2nd Harvest Yield lb/A
Florunner 4247 3749 4149 3197 3836
Sunrunner 4490 4164 3686 3323 3916
Southern Runner 4279 4476 4416 4217 4347
Marc I 4293 4110 3471 2816 3672
AgraTech 127 3859 4133 3741 3191 3731
Okrun 4468 4344 3780 2967 3890
Tamrun 88 4220 3788 3648 3376 3758
GK 7 4440 4510 4289 3548 4197
NC 7 4526 4475 3780 3092 3968
NC 9 4842 4687 4418 3990 4484
NC 10C 4503 4615 4489 3753 4340
Florigiant 4499 4439 4424 3641 4251
Mean 4389 4291 4024 3426
LSD.O 566 404 464 501
'Number of sprays of chlorothalonil during the season.
Leafspot disease levels were low in the test area until late
in the season (early September) each year. This was probably due
to two factors; 1) the predominant leaf spot pathogen was C.
personatum which does not usually increase until late in the
growing season in North Florida and 2) the excellent rotation kept
initial inoculum levels of the pathogen to a very low level,
delaying the onset of disease. It is known that low initial
inoculum levels can delay a late leafspot epidemic as much as three
weeks. If initial inoculum.levels had been such as would occur
with peanut after peanut or only one year out of peanut, we would
have expected Florunner with the 4-spray treatment to sustain 60 to
70% defoliation with pod yield losses up to 1000 lb/A. Previous
work with Florunner indicates that it may lose from 200 to 300 lb/A
of pod yield for each omitted spray even with a three year rotation
Table 8. Effect of spray initiation dates on yield of Florunner
peanut from seven replicated tests in 1975, 1976, and 1977.1
Spray Number of
Initiation (DAP) applications Yield (lb/A)
34 7 3447
48 6 3228
62 5 2973
76 4 2669
90 3 2103
104 2 1893
128 1 1690
untreated 0 1660
1All tests were planted at the North Florida Research and Education
Center, Marianna, Florida, under a 3-yr rotation. (Data from |
Shokes, et al., Plant Disease 66:574-575)
If the fields in the present study had been cropped to peanut
the preceding year (no rotation), initial levels of the leafspot
pathogen would have been higher, the epidemic would have built up
sooner and disease losses would be much greater. The excellent
rotation (5 years or more) in these studies allowed missing the
first two fungicide applications (6 and 5-spray treatments) without
suffering major yield loss (Table 7) on susceptible cultivars when
harvest was near the optimum time. Missing only the first
application did not cause appreciable losses even at the second
harvest. Again this was attributed to the lateness of the disease
buildup. If disease levels had increased sooner such as might
occur with a poor rotation, losses of >500 lb/A would have been
expected for the second harvest with only one or two missed sprays.
The medium and late maturing cultivars generally tolerated one
or two missed sprays better than the early maturing cultivars. The
early maturing cultivars, Marc I and AgraTech 127, seem to be the
most susceptible to leafspot diseases. The cultivar NC10C
maintained a good yield (4158 lb/A) for the first harvest with the
4-spray treatment. Southern Runner was the only cultivar that did
not have an appreciable yield loss by the second harvest with the
4-spray treatment. Average yields of 4880 lb/A were obtained with
this cultivar for the first harvest and 4347 lb/A for the second
harvest across all treatments. This may be attributed to the value
of the resistance that Southern Runner has to late leafspot.
White mold and limb rot levels were not sufficient to
appreciably affect yields. It is interesting to note that Southern
Runner had extremely low levels of white mold across all treatments
in all three years. It is known to have some resistance to S.
rolfsii, the causal agent of white mold. Appreciable limb rot
levels were noted on NC10C for three of the four treatments across
the three years of testing. This cultivar is known for its
resistance to cylindrocladium black rot (Cylindrocladium
crotalariae) a disease that was not present in our test locations.
The long rotation in these studies further reduced the potential
for yield losses from soilborne diseases.
With excellent crop rotation it may be possible to skip one or
two sprays for leafspot control in some years on many of the
cultivars we tested. This is not without risk, however,
particularly on the early maturing runners. It would not be
advisable to skip any fungicide applications if a rotation out of
peanut of less than three years is used. Even with a rotation of
three or more years all volunteer peanut plants need to be
destroyed before leafspot developed each year. Volunteer plants
can carry on disease inoculum and greatly decrease the value of
Four applications of fungicide are inadequate to control
leafspot diseases on susceptible cultivars in any given year in
North Florida. Only Southern Runner can sustain enough yield to
warrant such limited leafspot prevention and even this cultivar
would have benefitted enough from a fifth fungicide spray to pay
the cost of all the applications. If seven applications of
fungicide are applied to a peanut crop the cost will range from $56
to $70 per acre. Only 160 to 200 lb/A of additional yield is
required at $.30/lb to cover the cost of all applications. If for
any reason harvest is delayed the benefit derived from additional
fungicide applications heavily outweighs the cost. Most growers do
not have the luxury of extended rotations and harvesting at the
optimal time for each field is difficult to schedule. Therefore
maintaining leafspot diseases at minimal levels is very important
and must be a major concern when growing peanuts. Leafspot control
is not a safe category for cost-cutting unless some means is used
to insure that disease increases will not negatively impact yields.
While we do not have complete resistance to leafspot diseases,
partial resistance will allow us to minimize the risk of cutting
the number of fungicide applications. Further research is underway
toward development of even better resistant cultivars than Southern
Runner. We are also developing peanut lines with multiple pest I
resistance. Currently Southern Runner has some resistance to
tomato spotted wilt virus, rust and web blotch in addition to its
partial resistance to late leafspot and white mold. Use of i
multiple pest resistance lessens the overall risk of losses to
diseases and should decrease our dependence on pesticides.
Use of a disease forecasting model to schedule fungicide sprays
would also allow us to decrease the risk of skipping leafspot
fungicide applications. With a forecasting model sprays can be
applied based on the knowledge of conditions favorable for
infection. When conditions are unfavorable for infection an
application can be skipped. Additional research on disease
forecasting models is being conducted to determine whether we can
safely use such a system in North Florida without jeapordizing
Kucharek, T. A., F. M. Shokes, and D. W. Gorbet. 1991.
Considerations for spraying foliar fungicides to control plant
diseases as exemplified by studies in Florida from 1968 to 1989 on
the control of peanut leafspot. Florida Coop. Ext. Serv., Bull.
Shokes, F. M., D. W. Gorbet, and G. E. Sanden. 1981. Effect of
planting date and date of spray initiation on control of peanut
leafspots in Florida. Plant Dis. 66:574-575.
Smith, D. H. and R. H. Littrell. 1980. Management of peanut foliar
diseases with fungicides. Plant Dis. 64:356-361.
Gorbet, D. W., D. A. Knauft, and F. M. shokes. 1990. Response of
peanut genotypes with differential levels of leafspot resistance to
fungicide treatment. Crop Sci. 30:529-533.
Shokes, F. M. and D. W. Gorbet. 1990. Comparing field performance
of leafspot-resistant peanut breeding lines. Soil and Crop Sci.
Soc. of Fla. Proc. 49:180-184.
Gorbet, D. W., A. J. Norden, F. M. Shokes, and D. A. Knauft. 1986.
Southern Runner A new leafspot resistant peanut variety. Univ.
of Fla. Institute of Food and Agricultural Sci. Circ. S-324, 13
The North Florida Research and Education Center
The North Florida Research and
Education Center (renamed in 1977) at
Quincy is a unit of the Institute of
Food and Agricultural Sciences,
University of Florida. The Research
Center was authorized in May of 1921.
It is a diversified unit that
includes field crops, vegetable
crops, fruit crops, pecans, woody
ornamentals, beef cattle and swine.
Research was expanded to include
peanuts at Marianna in 1941 and a
swine unit in 1971 under the
administration of the North Florida
Experiment Station. Reorganization
into an integrated systems approach
to solving agricultural problems
began in 1977 when Extension
specialists were housed at all three
units. The Agricultural Research and
Education Center, Monticello (woody
ornamentals, fruit and pecans), came
under the North Florida umbrella in
1984 and the North Florida
Demonstration Beef Unit at Chipley in
The Center's mission is to
develop new and expand existing
knowledge and technology, and to
disseminate new scientific knowledge
in Florida via the Cooperative
Extension Service so that agriculture
remains efficient and economically
" The North Florida Research and
Education Center, University of
Florida, located at Quincy,
Marianna and Monticello.
* A statewide center dedicated to
teaching, research and extension.
* Twenty-one faculty are under the
NFREC unit: twelve at Quincy, six
at Monticello and three at
Marianna. County extension
offices for District I are
administered from Quincy along
with the Beef Demonstration Unit
at Chipley and the Aquaculture
Farm at Blountstown.
* A partnership in food and
agriculture, natural and renewable
resource research and education,
funded by state, federal and local
government, and by gifts and
grants from individuals,
foundations, government and
* An organization whose mission is:
Educating students in the food,
resources, and related
Strengthening north Florida's
diverse food and agricultural
industry and its environment
Enhancing for north Floridians,
the application of research and
knowledge to improve the
quality of life through IFAS
This research was paid for by peanut checkoff monies and reproduced
at a cost of $ 1.00 per copy. m