Front Cover
 Results and discussion

Group Title: NFREC, Quincy Research report ;, NF-85-3
Title: Effect of foliar fungicides on soybeans in North Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00073719/00001
 Material Information
Title: Effect of foliar fungicides on soybeans in North Florida
Series Title: NFREC, Quincy Research report
Physical Description: 7 p. : ; 28 cm.
Language: English
Creator: Cobb, L. C
University of Florida -- Agricultural Experiment Station
Publisher: Florida Agricultural Experiment Station, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville FL
Publication Date: 1985
Subject: Soybean -- Diseases and pests -- Control -- Florida   ( lcsh )
Soybean -- Effect of pesticides on -- Florida   ( lcsh )
Fungicides -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 7).
Statement of Responsibility: by L.C. Cobb ... et al..
General Note: Cover title.
Funding: Research report (North Florida Research and Education Center (Quincy, Fla.)) ;
 Record Information
Bibliographic ID: UF00073719
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: oclc - 84655542

Table of Contents
    Front Cover
        Front Cover
        Page 1
        Page 2
        Page 3
    Results and discussion
        Page 4
        Page 5
        Page 6
        Page 7
Full Text


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NFREC, Quincy Research Report NF-85-3

Effect of Foliar Fungicides

on Soybeans in North Florida

by L. C. Cobb. F M. Shoes, 1 M. Snell,
and D. L. Wright

Florida Agricultural Experiment Stations
Institute of Food and Agricultural Sciences
University of Florida, Gainesville


Effect of Foliar Fungicides on Soybeans in North Florida

L. C. Cobb, F. M. Shokes, J. M. Snell, and D. L. Wright

Several foliar diseases caused by fungi, commonly occur on soybeans

in Florida. Among these are brown spot (Septoria leaf spot), frogeye

leaf spot, and downy mildew. Brown spot is primarily a leaf spot disease

causing irregular dark brown spots on leaves. This fungus causes some

defoliation of soybeans late in the growing season. Downy mildew gener-

ally causes pale green to light spots on the upper surface of young

leaves but does not seem to cause enough damage to be of great concern in

Florida. Frogeye leaf spot causes small reddish brown circular to angu-

lar spots which develop gray centers as the spot enlarges. The fungus

that causes this disease has several races and some of the varieties

grown in Florida are susceptible to the races that are prevalent. This

fungus has been reported on stems, pods and seeds as well as leaves. It

is not known yet whether any economic damage occurs due to this disease

in Florida soybeans but yield reductions of 5-10% have been reported on

susceptible varieties elsewhere (2).

Two fungal diseases of major concern are those which infect pods and

seeds, anthracnose, and pod and stem blight. Symptoms of anthracnose are

irregular brown areas on pods, stems, and leaf petioles. Premature leaf

defoliation may occur and infected plants may be shorter than healthy

plants. This disease is most noticeable when plants are nearly mature

and can cause serious losses if wet weather prevails during the maturing

process. Seeds may be underdeveloped, may not develop at all, or may be

moldy, dark, and shriveled. Pod and stem blight is a disease of pods,

seeds, stems, and leaf petioles. It can be recognized by tiny black

fruiting bodies on petioles, pods, or other diseased tissue. On stems

these appear to be in lines. Infected plants may be stunted and infected

stems may die prematurely. Heavily infected seeds may be badly cracked

or shriveled while lightly infected seeds may be normal in appearance.

Infected seed produce low quality oil and flower and may lead to a grade

reduction due to their appearance.

Foliar diseases may not always decrease soybean yields because there

is sufficient leaf area remaining to compensate for losses from leaf

damage and low to moderate levels of defoliation. Pod and stem diseases

may decrease yields because they stunt the plants and/or interfere with

pod or seed development. If yield losses do not occur, pod and stem

diseases may decrease the germination percentage of the seed.

Fungicides can be used to decrease foliar and pod and stem diseases

of soybean. Since the fungi which cause pod and stem diseases usually

infect soybeans at some time from bloom to pod fill, fungicides are

generally applied at stage R3 (beginning pod) and about 14-21 days later

at stage R5 (beginning seed) (4,5). Cultivars that mature in late

October or November, the dry period in Florida, have a greater probabil-

ity of escaping pod and stem diseases. If weather conditions are wet

prior to and during seed maturation, the use of a fungicide is important

particularly with soybeans grown for seed. Backman (1) reported that

fungicide applications based on wet periods can reduce the number of

sprays needed without reducing yields. Gershey (3) found that one appli-

cation of 0.5 Ib/A benomyl prior to stage R6 might produce yields equal

to two applications of 0.25 Ib/A if the timing is right in relation to

the weather conditions that are favorable for infection. Since there is

no practical disease forecasting system available, fungicide scheduling

is still based on growth stage of the crop.

Since the diseases mentioned do not always cause a yield loss, their

control has not always resulted in an economic benefit. Phillips (6)

looked at the potential benefits of foliar fungicides over a ten-year

period in Georgia. Levels of damaging diseases were usually low and in

greater than 90% of the tests no significant yield increase was obtained

from the use of a foliar fungicide. Several tests were conducted in

Florida at the North Florida Research and Education Center from 1979 to

1983 and similar results were obtained. There are, however, some prob-

lems involved with fungicide testing in small plots of soybeans. Plot to

plot variations in yield and other factors may reduce the practicality of

statistical tests. Larger plots and/or more replications can be used to

improve statistical ;reliability. Also, varieties that are well adapted

to an area may develop very little disease or delay development of dis-

eases so that they have little or no effect on yield even with poor

weather conditions. A test was conducted for four years using large

plots and several different varieties, to determine the effect of a

fungicide application on yield of soybeans.


Soybeans were planted in Jackson County, Florida, from 1980 to 1983

on land that had been planted to soybeans for six years prior to the

beginning of the test. Ten varieties common to all four years are in-

cluded in this report. Plots were 0.9A in size and one-half of each plot

was sprayed with benomyl at 0.5 Ib/A at stages R3-4 and R5-6 using a

high-boy sprayer with an eight-row boom. Three flat fan nozzles per row


applied 10 gallons of spray per acre at 50 psi. Soybeans were planted in

May of each year and harvested at maturity in October or November. Each

treatment-variety combination was replicated across years for analysis of

variance and only those varieties used each of the four years are includ-

ed. A t-test was used to compare treated to untreated plots for each

variety. Disease ratings for foliar diseases and pod and stem diseases

were made at stages R6-7 (full seed) and R7-8 (full maturity), respect-



Yields were higher for the treated half of the ten varieties compared

to the unsprayed half (Table 1) over the four years. Average yield

increases ranged from 1.4 to 4.4 bu/A. Based on a price of $7.00/bu, an

increase of 2.8 bu/A would be required to return the cost of the two

fungicide sprays. Only Bragg, Centennial, Coker 237, and Hutton would

have benefitted economically from the fungicide applications for three of

the four years. Fungicide treatment also would have been beneficial to

Braxton, Coker 338, and RA 701 for two of the four years. Seed germin-

ation increases due to fungicide protection ranged from -2 to 12% (Table

2). Percent germination was generally higher for treated than for un-

treated soybeans while disease ratings of foliar and pod and stem dis-

eases were always lower in plots receiving fungicide sprays.

Use of a foliar fungicide should be based on a logical evaluation of

a particular growing situation. If soybeans are grown for seed, it is

generally a good idea to use a foliar fungicide in Florida. Even if

soybeans are not grown for seed a foliar fungicide might be helpful in

wet years or when soybeans are grown continuously on the same land.

Regardless of whether a fungicide is used or not, care should be taken to


harvest the crop as soon as possible after maturity, since rain aids dis-

ease development which will decrease seed quality appreciably. Planting

soybeans of maturity groups VI, VII, and VIII within the recommended

dates (May 15 June 15) will often insure that maturity occurs during a

period when the weather is normally dry in North Florida (7). This tends

to minimize infection by organisms which could decrease seed quality.

Table 1. Average Yields of

Ten Soybean Varieties Over Four Years.

Average Yield (bu/A) % Yield Yield (bu/A)
Variety Unsprayed Sprayed Increase Increase

Bragg 35.2 38.0 8.0 2.8
Braxton 36.2 38.3 5.8 2.1
Centennial 41.6 45.1 8.4 3.5
Cobb 38.8 40.2 3.6 1.4
Coker 237 36.4 40.8 12.1 4.4
Coker 338 33.7 35.7 5.9 2.0
Coker 448 37.5 39.5 5.3 2.0
Hutton 34.1 37.4 11.1 3.8
RA 701 39.9 42.0 5.3 2.1
RA 800 38.8 40.5 4.4 1.7

Table 2. Average Pod and Stem Blight Ratings1 and Percent Germination of
Ten Soybean Varieties Over Four Years. 1980-1983.

Pod and Stem Blight % Germination % Germination
Variety Unsprayed Sprayed Unsprayed Sprayed Increase

Bragg 3.8 2.5 63 70 7
Braxton 3.5 2.5 60 65 5
Centennial 2.8 2.0 75 79 4
Cobb 3.5 2.3 78 79 1
Coker 237 4.0 2.5 66 78 12
Coker 338 1.0 1.2 67 65 -2
Coker 488 4.0 1.8 66 67 1
Hutton 2.5 1.3 68 73 5
RA 701 3.1 1.3 71 75 4
RA 800 3.0 1.5 71 75 4

Disease ratings on 1 to 10 scale.

1 = no disease.


1. Backman, P. A., M. A. Crawford, and J. M. Hammond. Foliar Fungi-

cides for Soybeans. Prediction Systems Make More Money. High-

lights of Agr. Res., Vol. 29, No. 3, Fall 1982.

2. Compendium of soybean diseases. 1982. J. B. Sinclair, editor.

Pub. by the American Phytopathological Society. 104 pp.

3. Gershey, J. S., G. T. Berggren, Jr., and M. E. Pace. Timings of

Foliar Fungicide Applications on Soybeans in Louisiana. Proc.

SSDW. March 1984. Dept. of Plant Pathology and Crop Physiology,

Louisiana State University.

4. Kucharek, T. A. Soybean Disease Control Program for Florida. 1980.

Report No. 16. Fl. Coop. Ext. Serv. IFAS, University of Florida.

5. Kucharek, T. A., and R. S. Mullin. Florida Plant Disease Control

Guide. Fl. Coop. Ext. Serv., IFAS, University of Florida.

6. Phillips, D. V. Foliar Fungicides in Georgia: A Ten Year Summary.

Proc. SSDW. March 1984. University of Georgia.

7. Shokes, F. M., D. C. Herzog, and D. L. Wright. 1983. Seed Quality

of Soybean Maturity Groups as Affected by Planting Dates in North

Florida. Soil and Crop Sci. Soc. of Florida Proc. 42:117-122.

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