Approximately 30 different diseases are known to cause economic loss
of yield and grain quality to corn in Illinois. To prevent loss from these
diseases, it is necessary to follow a comprehensive integrated corn disease
management program. Such a program should include the use of disease-resistant
hybrids, crop rotations, various tillage practices, balanced fertility,
fungicides, insect and weed control, and other cultural practices. These
practices should relate to the risk potential of the various diseases
and the life cycles of disease-causing organisms (pathogens).
Table 1 lists those diseases known to cause
yield losses in Illinois and the relative effectiveness of various control
measures.
Disease-Resistant Hybrids
The use of disease-resistant hybrids is the most economical and efficient
method of disease control. Although no single hybrid is resistant to all
diseases, hybrids with combined resistance to several major diseases are
available. The maps in Figure 1 portray the areas of Illinois in which
certain diseases are most likely to occur and the approximate expected
yield loss on a regional basis if susceptible hybrids are grown. Yield
losses may be higher in individual fields and losses will vary from year
to year. Corn producers should select high-yielding hybrids with resistance
or tolerance to major diseases in their area.
Crop Rotation
Crop rotation is a very powerful disease-control tool. Many common pathogens
require the presence of a living host crop for growth and reproduction.
Examples of such corn pathogens include the leaf diseases ("Helminthosporium"
leaf diseases, Physoderma brown spot, Goss's bacterial wilt, gray leaf
spot, yellow leaf blight, eyespot) and nematodes. Rotating to nonhost
crops (e.g., soybeans, alfalfa, clovers, and canola) "starves out"
these pathogens resulting in a reduction in inoculum levels and the severity
of disease. Continuous corn, especially in combination with conservation
tillage practices, which promote large amounts of surface residues, may
result in severe outbreaks of disease. In such cases it is highly advisable
to utilize all other disease-control measures.
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Tillage
Tillage programs that encourage rapid residue decomposition, before the
next corn crop is planted, will help reduce populations of pathogens which
overwinter in or on crop debris. Although a clean plow-down is an important
disease-control practice, the possibility of soil losses from erosion
must be considered. Other control measures can provide effective disease
control if conservation tillage is implemented. Examples of diseases partially
controlled by tillage include stalk and root rots, "Helminthosporium"
leaf diseases, Physoderma brown spot, Goss's bacterial wilt, gray leaf
spot, anthracnose, ear and kernel rots, yellow leaf blight, eyespot, and
nematodes.
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Balanced Fertility
Adequate balanced fertility plays an important role in checking the development
of such diseases as Stewart's bacterial wilt, seedling blights, leaf blights,
smut, stalk rots, ear rots, and nematodes. Diseases are often most severe
where there is an excess of nitrogen and/or a lack of potassium. Healthy,
vigorous plants are more tolerant of diseases and better able to produce
a near normal yield.
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Seed Treatment
Essentially all commercial corn seed is treated with a fungicide, and
sometimes with an insecticide, by the seed processor. This important practice
helps to insure high yields of top-quality grain. Seed treatment is most
valuable under adverse soil conditions (cold and wet or very dry) when
seed germination is delayed, or when planting into heavy residues (no-till
or certain conservation tillage practices). Proper fungicide seed treatment
controls seed rots and seedling blights. Failure to control this disease
complex results in spotty stands. The treated seed should be plump, injury-free,
and planted in soils at 50 F or above. The seedbed should be well prepared
with fertilzier, herbicide, insecticide, and seed properly placed.
The recommended seed treatment fungicides are given in Illinois Extension
Service publication: Plant Disease Management Guide for Field Crops. This
circular is revised annually and can be purchased from Ag Services, P345,
1917 S. Wright, Champaign, IL 61820.
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Foliar Fungicides
One or more "Helminthosporium" leaf blights and rust diseases
may occur every year regardless of the precautions taken. If extended
periods of moist, overcast weather occur before or shortly after tasseling,
these diseases may cause losses of 10 to 30 percent. If significant disease
occurs earlier than 2 weeks after tasseling, the application of foliar
fungicides may be justified, especially in seed production fields. The
decision to apply fungicides should be based on the prevalence and severity
of leaf diseases. Leaf blights generally are first seen on the lower leaves.
Rusts first appear on the upper leaves.
In general, fungicide applications are economically feasible only in
seed-production fields. Weekly scouting for "Helminthosporium"
leaf blights and rusts should begin in the tillering stage. If diseases
are present, and weather conditions favor continued disease development
(rainy and overcast), fungicide applications should be considered. Two
to four protectant sprays are suggested, starting when disease appears.
Add a label-recommended spreader-sticker (surfactant) to the spray tank
to insure more uniform coverage. Follow the manufacturer's recommendations
regarding rates, timing of applications, and the time interval between
the last application and harvest. The best method for applying fungicides
is by air (fixed-wing or helicopter), using a minimum of 5 gallons of
water per acre. Fungicides must be applied uniformly. Do not feed fungicide-treated
fodder or forage to livestock.
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Integrated Control
The aim of control measures is to disrupt the combination of factors
necessary for disease development, including a favorable environment,
susceptible plants, sufficient quantities of a virulent pathogen, and
adequate time for disease development. Disease control programs are based
on an understanding of host and pathogen biology and the factors involved
in infection and disease development.
Short-term control may be achieved by a single practice. However, long-term
reduction of disease losses requires the implementation of an integrated
control program, including the use of disease-resistant hybrids, crop
rotation, tillage, balanced fertility, insect and weed control, and, if
necessary, the timely application of disease-control chemicals.
Disease-control practices should be based on knowledge of which diseases
are likely to occur and when they are likely to cause economic loss. The
maps in Figure 1 indicate approximate disease
risk areas. Field surveys can be helpful in identifying diseases and developing
appropriate disease-control measures. For example, fall surveys for stalk
and ear rots will help reduce losses from these diseases and determine
drying and storage strategies.
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