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Soybean diseases reduce Illinois soybean yields by 5 to 15 percent annually,
depending on the diseases involved, the varieties grown, the management
practices followed, and various environmental factors. Approximately 15
different diseases are responsible for these yield losses. A comprehensive
soybean disease management program can sharply reduce these losses in
yield and grain quality.
A successful disease-control program could involve just a single practice,
but the long-term reduction of disease losses generally requires the application
of several control measures. A comprehensive and integrated disease-management
program should include the use of adapted, disease-resistant varieties;
high-quality, disease-free seed; a well-drained, fertile seedbed; crop
rotations; various tillage practices; balanced soil fertility and proper
soil reaction (pH) based on a soil test; fungicides; insect and weed control;
following other suggested cultural practices; and scouting and monitoring.
The goal of an integrated disease-control program is to disrupt the combination
of factors necessary for disease development: an environment favorable
for disease-causing organisms (pathogens), susceptible plants, the presence
of sufficient quantities of a virulent pathogen capable of reproducing
and spreading, and adequate time for the disease to develop.
Essential to a disease-control program, therefore, is an understanding
of pathogens, disease cycles, which plant parts are attacked and when,
and the factors involved in the spread and reproduction of pathogens.
Table 1 (pp 6-7) lists the common diseases that are known to cause yield
losses in Illinois.
The maps following the text show the relative risk of losses from individual
diseases on a regional basis. "Relative risk" does not mean
that yield losses may not be higher or lower for individual fields; it
simply means that the diseases commonly pose this potential risk in the
areas shown. In general, disease losses rarely exceed 5 to 10 percent
in any field regardless of how many diseases are present. Therefore, potential
disease losses are not cumulative. Irrigated fields, fields in river bottoms
or other areas where warm, wet conditions prevail, fields planted to narrow
rows, fields where reduced- or no-tillage is used, fields planted with
poor-quality seed, or fields where soybeans are grown continuously have
a greater risk potential for disease loss.
With the knowledge of the "risk potential" of the various diseases
and of the life cycles of the pathogens that cause them, an integrated
and comprehensive disease-control program can be planned. Various control
practices that could be incorporated into a comprehensive program are
discussed in this report.
CULTIVAR MATURITY AND GROWTH HABIT
The relative maturity of soybean cultivars can have a dramatic impact
on the severity of soybean diseases. For example, early maturing varieties
for a given area will generally be more severely affected by pod and stem
blight, Septoria brown spot, anthracnose, Cercospora purple seed stain
and leaf blight, and sudden death syndrome. Late maturing varieties are
affected less because of the generally cooler, drier conditions prevalent
later in the growing season. However, early maturing cultivars are generally
less affected by brown stem rot than late maturing cultivars. To minimize
the damage from charcoal rot, full-season cultivars should be planted
as early as possible. Soybean growth habits can affect disease development.
For example, yields of short determinant varieties are generally more
affected by rain-splashed pathogens than the yields of indeterminant varieties.
Diseases likely to be more severe on short-stature varieties include Septoria
brown spot, pod and stem blight, and Cercospora purple seed stain and
leaf blight. Differences in resistance may negate the effect of plant
height on disease.
SEED QUALITY
Many important pathogens can infect soybean seeds. Seed infection often
results in poor quality (i.e., low germination, vigor, yield, or a combination
of these). The disease most commonly damaging to soybean seed quality
is pod and stem blight.
To reduce losses, soybean growers should plant only seed that has a germination
rate greater than 70 percent in a cold germination test. The cold germination
test is a better indicator of seed quality than the standard warm germination
test. All seed lots should be tested before planting to assure good stands
of vigorous seedlings with a potential for high yield. Plump seed, free
from disease-causing organisms and cracks, is more likely to produce vigorous
stands and sustain fewer losses from seed rot and seedling blight fungi.
In general, seed-rotting and seedling blight fungi cause severe problems
only where diseased, cracked, or low-vigor seed is used or where seedbed
conditions do not favor rapid germination and emergence.
The use of seed-treatment fungicides to increase germination of poor-quality
seed is not recommended. The productivity of stands of poor-quality seed,
even if properly treated, is not equal to that of stands of nontreated,
high-quality seed. High-quality seed, produced in fields with a low incidence
of disease, should be harvested as soon as it is mature, and handled carefully
to prevent mechanical damage. Seed treatment fungicides have no effect
on damaged seed.
CROP ROTATION AND TILLAGE
Crop rotation and clean tillage can be important practices in controlling
diseases since many soybean varieties have little or no resistance to
many common diseases. Practically all of the important fungal and bacterial
diseases of soybeans survive between cropping seasons on and in soybean
crop debris. Few of the fungi, bacteria, and nematodes that attack soybeans
infect other crop plants. Therefore, when soybean crop residue is removed
or thoroughly decayed and/or rotations with corn, sorghum, small grains,
or forages are used, the disease-causing organisms lack a host on which
to feed and reproduce, and eventually die.
Crop rotation and tillage programs that promote soybean residue decomposition
before the next crop of soybeans is planted will help reduce diseases
such as pod and stem blight, anthracnose, stem canker, Alternaria leaf
spot, Phyllosticta leaf spot, powdery and downy mildew, bacterial blight,
bacterial pustule, Septoria brown spot, Cercospora leaf blight (purple
seed stain), brown stem rot, Sclerotinia white mold and several other
fungal and bacterial leaf diseases. Soybean cyst nematode populations
can be reduced up to 50 percent for each year an alternate crop is planted.
Producing soybeans with practices that do not allow crop residue decomposition
may result in an increase in certain diseases. In these situations it
is important to use all other available disease-control practices. These
include planting resistant varieties, rotating to nonhost crops, scouting
fields to determine the presence of diseases, using pesticides when needed,
and planting high-quality, certified seed. Tillage practices that increase
soil compaction may increase the severity of diseases such as Pythium
and Phytophthora root rot and brown stem rot.
ROW WIDTH
Little experimental data exists on the effect of row width on disease
incidence. However, as row width narrows, higher humidities and slower
drying conditions are encountered. Therefore, as row width narrows, diseases
such as powdery mildew, downy mildew, pod and stem blight, stem canker,
and anthracnose, Sclerotinia white mold, and Cercospora purple seed stain
and leaf blight may increase in severity. Research on Septoria brown spot
has shown no difference in that disease's development as a result of row
width. Charcoal root rot impact will be less severe where narrow rows
are used with early planting because of the rapid canopy development and
resultant lower soil temperatures and higher soil moisture levels.
DATE OF PLANTING
Date of planting can have a profound effect on development of certain
soybean diseases. For example, late planting increases losses to charcoal
root rot and soybean cyst nematode. Losses to charcoal root rot increase
rapidly as soil temperatures increase. Early planted beans, however, appear
more susceptible to sudden death syndrome.
Planting into cold soils (less than 55 to 60 F) can delay emergence and
increase seed decay and seedling blight. Soybean seeds may swell at lower
temperatures but will not germinate, giving pathogens an extended opportunity
to attack the seed. The period of high susceptibility to these diseases
is from time of planting until the plant has 2 to 3 trifoliate leaves.
The longer this period, the more severe these diseases may become. Planting
high-quality seed in a warm (60 F or more), moist, well-drained, and fertile
seedbed at the proper depth and spacing will ensure stands of vigorous,
high-yielding seedlings.
FERTILITY
Adequate, balanced fertility can be important in reducing disease losses.
Inadequate phosphorus or potassium can increase losses from soybean cyst
nematode, charcoal rot, other root rots, and pod and stem blight. Vigorous
plants are more tolerant of pathogens and are better able to produce an
almost normal yield despite diseases.
DISEASE SCOUTING AND MONITORING
Scouting and monitoring soybeans is an important practice in long-term
disease management. Scouting fields is especially helpful since diseases
will probably be important only where they were present in recent years.
Accurate surveys and diagnoses of problems through scouting thus permit
more effective use of controls in succeeding crops, and early detection
can improve the chances of reducing disease losses.
Begin scouting your fields early in the season to detect areas that may
cause problems as the crop matures. Low or flooded areas, and any part
of the field where plants appear to be weak or to lack vigor should be
checked more frequently. Many soybean diseases begin as circular areas
in fields where the plants appear chlorotic or weakened. These areas often
develop into disease centers as the plants mature. Periodic scouting of
five areas of each field should begin in the seedling stage and continue
throughout the season.
Use the tables, maps, and other resources listed earlier to help you
identify the important diseases, their relative risks, and their appearances.
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SPECIFIC SCOUTING PROCEDURES FOR SOYBEAN
CYST NEMATODE
RACE DETERMINATION
Experienced personnel can determine the most prevalent race present in
a field by planting one or several small areas (two to three rows, 5 to
10 feet long in areas of the field showing damage) to a Race 3-resistant
variety, to a Race 3- and 4-resistant variety, and to a susceptible variety
normally used by the grower. If no cysts appear on the roots of the Race
3- and Race 4-resistant varieties after six to eight weeks but cysts can
be seen on the roots of the susceptible variety, the predominant race
is 3. If cysts are obvious on the roots of the susceptible and the Race
3-resistant variety, Races 3 and 4 are both present. If cysts are obvious
on all varieties, a race other than Race 3 or 4 is present.
SOIL ANALYSIS
Soil analysis for soybean cyst nematodes should always be done before
a susceptible variety is planted. At present, a soil population below
5 mature cysts or 150 eggs and larvae per 100 cubic centimeters of soil
is considered the threshold below which a susceptible variety can be planted
without significant damage. Growers who want to plant a susceptible variety
as soon as possible in the rotation should sample each nonhost crop to
determine the remaining population. If populations increase after cropping
to a resistant variety, the buildup of a new race should be suspected.
Soil samples are collected to a depth of six inches from the field in
question in the fall. Ideally, approximately 10 subsamples per five acres
should be taken. Subsamples should be mixed and a composite sample should
be sent to the University of Illinois Plant Clinic, 1401 W. St. Mary's
Rd., Urbana, IL 61802, or to another designated soybean testing laboratory,
for analysis. A nominal charge will be made for processing samples.
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FUNGICIDES
SEED TREATMENTS
Fungicide seed treatments will generally improve stands regardless of
quality. However, the greatest benefits will be found (1) where low seeding
rates are used; (2) where seed that is of poor quality because of low
rates of fungal infection must be used; and (3) where seed is planted
in a seedbed in which delays in germination or emergence are likely (reduced
till or no-till fields).
Fungicide seed treatments are not a substitute for high-quality seed
and will not improve the performance of seed that is of low quality due
to mechanical damage or physiological factors. Treated seed of low quality
will not produce stands and/or yields equal to untreated high-quality
seed. Therefore, only high-quality seed should be considered for planting.
A fungicide seed treatment should be used on seed planted to produce
seed. Research has shown that fungicide-treated seeds produce seeds that
have a lower incidence of pod and stem blight infection. Fungicide seed
treatments will not improve germination of seed that is of low quality
because of mechanical damage or physiological factors. There are many
excellent seed treatment fungicides available.
Table 2 (p. 8) is designed to assist in determining the need for seed
treatments, to control Pythium and Phytophthora. Selection of the proper
seed treatment is very important because of the specificity of certain
fungicides for controlling only Phytophthora and Pythium.
FOLIAR TREATMENTS
Foliar fungicide treatments may reduce losses from Septoria brown spot,
Cercospora leaf blight (purple seed stain), anthracnose, pod and stem
blight, and stem canker. These diseases are most damaging when the weather
is warm (70 to 80 F) and wet from early pod fill to maturity or when harvest
is delayed. Foliar sprays of fungicides may increase yields 10 to 15 percent,
increase seed quality, and reduce disease losses when such fields are
planted to soybeans the following year. The use of fungicides should be
based on expected disease severity. The six diseases just listed will
not be as severe in cool, dry seasons and where adequate rotations have
been used.
The checklist in Table 3 (p. 9), can be used at early bloom to determine
whether fungicide controls for the six diseases mentioned previously should
be considered. A key factor in this checklist is the presence of black
specks (pycnidia) on fallen petioles. Only brown, fallen petioles should
be assayed, and more than two-thirds to three-fourths of these petioles
should show pycnidia. If growers use the checklist and apply fungicides
correctly, maximum benefits should be achieved. Less than optimal benefits
will be achieved if fungicides are applied incorrectly or if disease severity
does not warrant spraying.
FUNGICIDE APPLICATION
At present, aircraft are the best vehicles for applying fungicides to
agronomic crops. Some aircraft may not be equipped or calibrated to do
this job. It is therefore important to select an aerial applicator who
is familiar with disease control and whose aircraft has been properly
calibrated for uniform, thorough coverage of all aboveground plant parts.
With the equipment now available, a reasonable job of applying fungicides
requires a minimum of 5 gallons of water carrier per acre. Superior coverage
may be obtained with more water, but the cost may be prohibitive. Conversely,
a lower volume (under 3 to 4 gallons per acre) gives correspondingly poorer
control. Five gallons of water can be applied uniformly using approximately
30 to 70 properly spaced nozzles, depending on the aircraft. The nozzles
should be D-8 to D-12, hollow cone, with No. 45 or No. 46 cores. The final
decision on nozzle number, size, swath width, and placement depends on
the air speed, pressure, and volume desired. Droplet size is also important.
Ideally, droplets should be 200 to 400 microns in size for thorough and
uniform coverage.
NEMATICIDES
Nematicides may be useful in controlling soybean cyst nematodes and other
plant-parasitic nematodes. Nematicides are suggested only where (1) crop
rotations are not possible, and (2) resistant varieties cannot be used.
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