Plant Diseases
Wheat Diseases in Arkansas
Scab (Head Blight)
Scab (head blight), caused by several species of the Fusarium fungus, is
favored by warm, cloudy weather with abundant rainfall before, during and after
flowering. Symptoms occur on the head after flowering. Individual spikelets or
the entire head appear prematurely bleached. Superficial pink or orange fungal
growth may be associated with the bleached areas. The fungi causing scab are in
every wheat field every year, but favorable weather conditions are needed for
severe disease.
Florets become infected at flowering, and the fungus kills individual
spikelets or the entire head. The bleached spikelets usually contain shriveled
grain. Scabby grain may contain one or more toxins produced by some Fusarium
species, and may be docked at the elevator. Most of the scabby grain is very
light weight and can be separated from the healthy grain. Scabby grain should
not be fed to livestock, especially hogs, because of the potentially lethal
effects.
If a scabby seed lot must be used for planting, a fungicide seed treatment
should be applied before planting. The seed treatment should improve seed
germination and seedling vigor but will have no effect on scab the next year.
All adapted varieties are susceptible to scab. Any differences among
varieties probably are due to differences in flowering time. The weather
conditions around flowering time are critical for infection, and a variety that
flowers earlier or later than others may escape severe infection. However, under
severe conditions such as those that occurred in 1990 and 1991, the head may be
attacked while still in the boot.
Cultural practices such as crop rotation and tillage to bury crop debris
reduce the amount of initial inoculum. Corn and sorghum are very good hosts for
the Fusarium fungi, so wheat following these crops is more vulnerable to scab
outbreaks. Planting varieties with different flowering times may be a way to
escape severe scab in at least part of a crop.
Foliar fungicides have not been effective against Fusarium fungi, especially
when the weather is extremely favorable for disease.
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