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Copyright 2005, Board of Trustees, University
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-
RESULTS AND DISCUSSION
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.