Institute of Food and Agricuultural Sciences
EXTENSION PLANT PATHOLOGY REPORT NO. 16
GAINESVILLE, FLORIDA JANUARY 2005
(Revision No. 8)
by Tom Kucharek
Disease Control Program for Soybean
Soybean diseases reduce yields and monetary returns. They do so by retarding desirable
plant development and seed quality. Some diseases such as frogeye leaf spot, downy
mildew, and anthracnose are obvious while others such as root rots may be unnoticed
because the damage they cause is below the soil surface. The yield potential for a field
may be 50-60 bushels or more per acre, but because of soilbome diseases, yields may be
limited to 30-40 bushels per acre. Overall, soilbome diseases cause more yield
depression than leaf, pod, and upper stem diseases in Florida.
Acreage of soybeans in Florida has declined dramatically over the past 10 to 15 years
because of more production in other countries and highly efficient production in the
midwestern United States. In 1980, 480,000 acres of soybeans were reported to be
planted in Florida. Concomitant with this reduction in acreage has been a reduction in the
overall severity of disease in Florida-produced soybeans because more land is available
for rotation with other crops which in turn reduces the total amount inocula for the
various diseases of soybean.
During the 1970s, 1980s, and early 1990s, the primary diseases that caused most
economic loss were caused by fungi. In addition, two bacterial diseases, bacterial pustule
and bacterial blight, and two viral diseases, soybean mosaic virus and peanut stripe virus
(near peanut breeding tests only), have been identified in Florida.
Fungi are organisms that lack roots, leaves, stems, and chlorophyll. Instead they have
hyphae (microscopic threads) of various types that can grow in the soil, organic matter in
or on the soil, or in the host plant. Hyphae absorb food from the plant or organic
material. Individual structures such as a spore are microscopic. Fungi can not make
certain necessary food materials so they live on food products manufactured by other
organisms. Some organisms that cause disease in soybean, such as Pythium spp., are no
longer classified as fungi except in a colloquial sense, but they are similar in appearance
and they can be parasitic.
Fungi and related organisms that cause disease reproduce by various methods. Many of
them produce microscopic spores. Different types of spores may be produced. Some
spores are disseminated by air currents and some are disseminated within water. Some
thick-walled spores are formed within the hyphae and are called chlamydospores; they
are survival structures that typically occur in or out of plant debris within the soil.
Fusarium spp. commonly produce chamydospores.
Some fungi produce survival structures called sclerotia (sing. Sclerotium) which are
compacted masses of hyphae; these structures provide protection for the organism and
thus, they serve as survival organisms. Sclerotium rolfsii and Macrophominaphaseolina
are two parasitic fungi that can infect soybeans and other plant species and they produce
their own characteristic sclerotia. Sclerotium rolfsii typically produces light to dark
brown sclerotia on the lower stem near the soil line or on debris on the soil and
Macrophominaphaseolina typically produces its black sclerotia in the stem or root
tissue. Rhizoctonia spp. also produce sclerotia
When fungi come into contact with a susceptible plant, they grow on it and eventually
enter the plant if it is susceptible. Some fungi do not require wounds for entrance while
others are more aggressive if wounding is provided. Nematodes and field operations such
as cultivation cause wounding. Also, soybeans have natural openings in leaves, stems,
pods, and roots through which organisms can gain access to the inside of the plant.
At this time fungal diseases are the primary pathogens of significance in soybeans grown
in Florida and the southeastern United States. Another fungal pathogen, soybean rust,
will need to controlled if it survives in the southern United States which is highly likely.
Asian soybean rust, caused by Phakopsorapachyrhizi, was first identified in November,
2004 in Louisiana and then Florida on soybean. By the last few days of November, 2004,
asian soybean rust was reported in the additional states of Mississippi, Alabama,
Arkansas, Georgia, Missouri, and Tennessee. This will be a new foliar disease that will
need to be controlled. Prior to the presence of soybean rust, the primary fungal, foliar
diseases we have contended with are frogeye leaf spot, anthracnose, and pod and stem
Bacteria are microscopic one-celled organisms which increase by simple division.
Bacteria enter plants through natural openings or wounds. Bacterial diseases have
occurred in soybeans in Florida, but they have not contributed to measurable losses in
Viruses are particles typically with a nucleic acid core and a protein coat. No cellular
structure exists as with fungi and bacteria. Viruses must replicate within the cells of the
host. Viral diseases of soybean exist in Florida, but they have been of no significance so
SEQUENTIAL DISEASE CONTROL PROGRAM
STEP 1. CROP ROTATION
Avoid planting soybeans on land that was in soybean or peanut. Several pathogens that
cause disease in soybean also occur in peanut. The fungus Sclerotium rolfsii which
causes southern stem rot (white mold, southern blight) infects and propagates on both
soybean and peanut. Sclerotium rolfsii infects and multiplies on many other broadleaf
plant species (e.g. beans, tomatoes, peppers, carrots, and many more broadleaf plant
species). Cotton is not a preferred host for Sclerotium rolfsii except very occasionally
when seedling blight may be caused by S. rolfsii in cotton.
The following crops are suggested for summer plantings in rotation with soybean.:
Field corn Sweet Corn Bermudagrass
Sorghum Millet Sorghum-Sudan
STEP2. DEEP PLOWING AND HARDPAN DISRUPTION
Deep plowing refers to the use of bottom plows (turn plows) not discs. When turning the
soil, every effort should be made to bury surface trash an stubble at least six inches deep.
This method of control is specifically aimed at reducing white mold and other soilborne
diseases, but it is also beneficial in reducing leaf and stem diseases such as stem canker,
frogeye leaf spot, anthracnose, downy mildew, and pod and stem blight when the interval
between soybean crops is less than one or two years.
If minimum tillage is used, clearing the intended seedling zone of plant residues with a
rolling basket or double disc openers is extremely important to minimize seedling blights
and earlier infections for root rots. Avoid use of minimum tillage in fields where
soybean stem canker has occurred.
Hardpans restrict vertical root development. Such slowing of root growth predisposes the
plant to infection by soilborne fungi (e.g. Rhizoctonia spp., Fusarium spp., etc.). Also,
where the root system is restricted to the upper soil levels, the plant is more apt to
become stressed during dry periods.
STEP 3. MAINTAIN POPULATIONS OF MYCORRHIZAE
Mycorrhizae are beneficial fungi that exist in the soil and upon their infection of root
tissue of host plants, a symbiotic (mutually beneficial) relationship between the fungus
and the host plant is established. The benefits for the host plants include increased
nutrient uptake (e.g. phosphorus) and some degree of protection against parasitic
organisms that invade root tissue. The host plant provides a site for the mycorrhizal
fungus to exist and multiply. Galls are not produced as do beneficial infections from
innoculant bacteria (Bradyrhizobium spp.) in roots of legume plants.
Soil pHs of 6.5 to 6.8 are optimum for development of mycorrhizal relationships in
soybeans, but pHs of 5.8 to 6.2 are generally considered ideal for growth of soybeans.
The higher pHs of 6.5 to 6.8 might allow nutrients such as manganese,k iron, and zinc to
become less available to the soybean plant. So, a compromise must be met and a pH of
6.2 is considered satisfactory for both nutrient uptake and development of mycorrhizae.
Populations of mycorrhizae in the soil are reduced by excessive cultivation. Minimum
tillage supports mycorrhizal populations. Studies have shown that carbofuran (Furadan)
and PCNB (Terraclor) can be inhibitory to mycorrhizae. Therefore products with these
ingredients should be used only if necessary.
STEP 4. WEED CONTROL
Weeds interfere with the growth of the crop and with harvesting operations. The presence
of weed seeds with the final product can cause docking of the price received for the crop.
In addition, the presence of weeds in the crop restricts air flow which lengthens leaf
wetness which enhances foliar diseases such as leaf spots, anthracnose, and pod and stem
blight. Also, the restricted air flow encourages white mold (southern blight). Finally, if
fungicide sprays for suppression of foliar diseases are used, weeds interfere with spray
deposits and the spray does not reach the intended target.
STEP 5. NEMATODE CONTROL
Nematodes cause wounds in roots which provide entrance ways for organisms that cause
root rot. For example, white mold (southern blight) is enhanced by nematode injury.
STEP 6. PURCHASE DISEASE-FREE SEED
Numerous fungal, bacterial, and viral pathogens are capable of being transmitted through
soybean seed (See Table 3). While many of these organisms can carry over in the field
on volunteer plants, weeds, and old crop debris, seed may be the main mechanism of
placing the pathogen in close proximity to the crop. Abundant data demonstrate that
fungi (e.g. Phomopsis spp.) in seed reduce the germination of seed. Your best indicator
for healthy seed is the germination test required to attain certified seed status. Seed that
is discolored or shriveled should not be planted. Seed produced in Florida or other
southeastern states are likely produced during wet seasons and therefore, there is a strong
possibility that such seed will be infected by fungi and the germination may be low.
Soybeans produced for seed should be harvested as early as possible and the field should
be sprayed with a foliar fungicide twice (R3 & R5 growth stages) to minimize adverse
effects from fungal infections in pods and seeds.
STEP 7. USE RESISTANT VARIETIES
Read the literature from the various seed companies to determine what varieties have
resistance to what diseases. In the past the diseases that have caused damage and for
which resistance may be available are frogeye leaf spot, pod & stem blight, purple seed
stain, and anthracnose.
STEP 8. SUPPRESSION OF SEEDLING BLIGHTS.
The primary source of the fungi that cause seedling blight is the soil, particularly non
decomposed organic matter from prior crops or weeds. Rhizoctonia spp. and Pythium
spp. are the fungi that are typically associated with pre-emergent and post emergent
seedling blights. Other parasitic fungi such as Macrophominaphaseolina are sometimes
With replicated on-farm tests in northern Florida with a seed treatment fungicide where
the comparisons were made in large areas requiring use of tractor-drawn planters and
combine harvesting, yield increases from 2.6 to 7 busheles/acre occurred across 4/5 tests
by reducing the incidence of seedlings with non-lethal lesions in stems caused by
Rhizocotonia solani. Plant stand was increased to some degree in some of the tests.
Thus, the effects of seedling blights in soybean are evident from the time of stand
establishment through yields at harvest time.
Seedling blight can be reduced using several tactics. First, crop rotation and double
cropping with non-legume crops is preferred. Rhizoctonia spp. and Pythium spp.infect
and multiply in grass and non-grass plant species, but over the years rotations and double
cropping with legumes in the system tend to result in more seedling blight. Regardless of
the presence of legumes in the cropping system, the old crop debris must be decomposed
before planting the soybean crop. Typically, if the old crop debris is decomposed, as
opposed to having visible stems roots, etc., beneficial microorganisms (fungi and
bacteria) build up and compete against the disease-causing organisms. Typically, about
30 days between plowing down old crop or weed debris and the planting of a new crop
should be done. Where the old crop or weed debris is incorporated into the soil with a
bottom plow or a highly parabolic cutting disk and then allowed to rot, the increase of
beneficial microorganisms is more likely to occur.
Every effort to utilize the above-mentioned cultural techniques to minimize the impact
from soilborne pathogens must be done. To further enhance disease on young plants,
certain chemical treatments are available.
TABLE 1. CHEMICAL TREATMENTS FOR SUPPRESSION OF SEEDLING
BLIGHTS AND YOUNG PLANT SOILBORNE DISEASES.***
FUNGICIDE METHOD RATE REMARKS
Demosan 65 W Seed Treatment 4 oz/cwt Optional hopper box
treatment is available
on the label. This
product is effective
Demosan 2.9 FL Seed Treatment 7 fl oz/ cwt against Rhizoctonia
FUNGICIDE METHOD RATE REMARKS
Apron XL LS Seed Treatment 0.16-0.64 fl oz/cwt This product is
ApronMaxx RTA Seed Treatment 5 fl oz/cwt Activity will be
against Pythium spp.
& Rhizoctonia spp.
Allegiance 2.65 FL Seed Treatment 0.10-0.375 fl oz/cwt This product is
42-S Thiram Seed Treatment 2 fl oz/cwt General fungicide
SoyGard Seed Treatment 0.32-0.43 oz/cwt This product is
Pythium spp. &
Captan 30-DD Seed Treatment 2 fl oz/cwt General fungicide
Captan 400 Seed Treatment 1.5 to 2.5 fl oz/cwt General Fungicide
Vitavax 34 Seed Treatment 3-4 fl oz/cwt This product is
VitaFloTM-280 Seed Treatment 4 fl oz/cwt Same as Vitavax 34
plus some other fungi
Vitavax-200 Seed Treatment 4 fl oz/cwt This product is
Maxim XL 2.7 FL Seed Treatment 0.167 to 0.334 fl oz Pythium and maybe
some other fungi
Ridomil Gold 4EC Soil Treatment 3/8-3/4 pint/treated Apply as a 7-10" band
Acre over the row center
*** These seed and soil treatments should not be expected to kill fungi already in the
seed. The purpose of these treatments is to protect the seed and young emerging
seedlings from infection by fungi in association with the soil. If a seed treatment is low
in germination, these treatments should not be expected to increase the germination rate.
CAUTION: DO NOT FEED FUNGICIDE-TREATED SEED TO MAN
OR BEAST UNDER ANY CIRCUMSTANCES.
STEP 9. DELAYED PLANTING
Planting soybeans as late as possible up to June 15 reduces stem canker, increases seed
germination because the soil is warmer, and reduces seedling blights because the warm
soil fosters faster germination. These trends are enhanced further by using varieties in
the higher maturity groups (e.g. Groups 7 & 8) for our area
STEP 10. AVOID DEEP PLANTING IF SOIL MOISTURE PERMITS
Planting excessively deep will offset benefits from seed treatment and cultural controls
that may have been used. The longer it takes for a seedling to emerge, the longer the
entire plant is exposed to soilborne fungi. Recommended planting depths are 1 /2 to 2
STEP 11. FOLIAR APPLICATIONS OF FUNGICIDES
This control measure has been directed at frogeye leaf spot, anthracnose, purple seed
stain, and pod and stem blight. Use of a foliar fungicide spray program is advised for
soybeans grown for seed because the fungi that cause anthracnose, pod and stem blight
and purple seed stain infect the pods and seed. Their presence in seed reduces seed
quality and in turn, germination is heavily reduced.
In November and December of 2004, asian soybean rust caused by Phakopsora
pachyrhizi was identified in Louisiana, Florida, Mississippi, Alabama, Georgia, South
Carolina, Tennessee, Arkansas, and Missouri. So, it is likely that soybean rust will need
to be considered into the overall fungicide spray program for soybeans. At this point in
time, we do not have a specific spray program for rust because we do not know when
such a program will need to be initiated.
Foliar sprays can be applied with ground or aerial equipment provided the label of a
product does not prohibit some method of application. Where aerial applications are
used, the spray volume should not be less than 5 gallons per acre.
Table 2. Foliar fungicides for soybeans grown in Florida. ***
for Mature Pre-
Rate/Acre/ Appl. Plant Harvest
Fungicide Application No. Sprays Interval Remarks
Bravo Weather Stik 6F 1 to 2 pints 3 6 pints 4 days Make 1st application 1
Or wk after flowering
Echo 720 6F (R1) & use 14 day
intervals which should
coincide with small
pods (R3) & then early
seed fill (R5)
1 /2 to 2 1% pints 2 6 pints 4 days Make 1st appl. when
(2/2 for pods are 1-1 /2" (R3)
Echo 720 long & make last appl.
14 days later at early
seed fill (R5)
Table 2 continued
for Mature Pre-
Rate/Acre/ Appl. Plant Harvest
Fungicide Application No. Sprays Interval Remarks
Table 2 continued 7/8 to 1 5/8 lbs 3 4.875 lbs 42 days Same as for Echo 720
Echo 90 DF with 3 sprays
1 1/4 to 2 bs 2 4.000 lbs Same as for Echo 720
with 2 sprays.
Folicur 3.6F 3-4 fl ozs 2 8 fl oz 21 days Section 18 for rust
Laredo 2 EC 4 to 8 fl ozs 2 16 fl ozs 28 days Section 18 for rust
Laredo 1.67 EW 4.8-9.6 ozs 2 19.2 fl ozs 28 days Section 18 for rust
Quadris 2.08 FL 6.2 to 15.4 fl ozs 2 2.88 qts. 14 days
Headline 2.09 FL 6 to 12 fl ozs 2 24 fl ozs 21 days
Tilt 3.6 EC 4 to 8 fl ozs 2 Early Section 18 for rust
Or seed fill only
PropiMax 3.EC R5
Topsin 4.5 FL 10-20 fl ozs 2 40 fl ozs 21 days Apply 1st appl. at full
bloom to when pods
are up to 1 long. Do
not make the 2nd appl.
later than when beans
start forming in the
material is not
effective for rust.
Topsin M 70 WP /2 to 1 lb 2 21 Same as for Topsin
***** Bravo, Echo, and Topsin products are labeled and useful for control of
anthracnose, brown spot, frogeye leaf spot, pod and stem blight, and purple seed stain.
Bravo and Echo products are labeled and useful for rust.
Topsin products are useful for suppressing Rhizoctonia aerial blight.
All sprayable fungicides prohibit or have strict limitations about feeding sprayed crop
to livestock. Read the label for each material for such information.
Step 12 EARLY HARVESTING
Soybeans should be harvested as soon as possible especially if they are grown for seed.
Infection of pods and seed by the fungi that cause anthracnose, pod and stem blight, stem
canker, and purple seed stain increases daily in our wet climate.
Group 4 and Group 5 soybeans sustain faster and more severe seed degradation with
delayed harvest than Groups 6, 7 or 8. Fungal colonization of seed reduces seed quality
for seed intended for market and such colonization reduces seed germination for seed
intended for the seed market.