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Dealing with Crop Diseases During Drought
The classic drought-related disease problem, according to University of Maryland Extension plant pathologist Dr. Arv Grybauskas, is aflatoxin in grain. Aflatoxins are toxic by-products of fungal growth that can develop especially in corn when droughty conditions exist during pollination and grain fill. Detailed information is contained in an online Extension fact sheet entitled Aflatoxins
While considering possible alterations in agronomic practices as a result of droughty conditions, farmers should evaluate the possible impact of diseases, says Dr. Arv Grybauskas, plant pathologist for field crops with the University of Maryland Cooperative Extension.
"Many farmers assume there won't be any diseases to deal with, and that would be true if we had a season-long drought," says Grybauskas. "But I’d like to think that we are already looking at an improved moisture situation, which means that some disease development could impact the crop this season."
Agronomists recommend that producers facing a droughty season reduce corn populations. While Grybauskas agrees with these recommendations, he cautions that producers need to know what impact they might have on a common disease, gray leaf spot.
"If we have a long droughty period then gray leaf spot is not an issue," he says. "But if we have periods of disease-favorable [rainy] conditions, then low corn populations actually favor disease development. The more open stand permits greater spore movement, which means that more plants can be infected. It also allows more sun to reach the fungus, which has a unique light-activated toxin that helps it make the characteristic rectangular lesion."
Grybauskas advises growers to plant a gray-leaf-spot-resistant hybrid if they have a source of gray leaf spot inoculum nearby. Such a hybrid is an absolute necessity if they are no-tilling corn into corn stubble because the fungus that causes gray leaf spot overwinters in infected corn debris. Producers not planting directly into corn stubble need to assess how close their field might be to a source.
What about soybeans?
One of the most common diagnostic mistakes producers make, according to Grybauskas, is to ignore soil-borne diseases in a droughty year.
"People will attribute poor yields to the dry conditions alone and miss the fact that a combination of dry conditions plus root damage from diseases may have been involved," he explains. "Soybean cyst nematode and root rots, for example, will not produce above-ground symptoms at early stages of disease development or when damage to the root or vascular system is not extensive enough. Yet those same low-level damaged plants will suddenly and dramatically wilt when droughty conditions follow infection."
Grybauskas recommends that growers use cyst-resistant varieties whenever possible, except when they know as a result of soil testing that cyst is not present in the soil. Producers must also realize that the most common race of cyst nematode is still race 1, although races 3, 5, 6 and 9 have been found. This means that most of the group III and group IV cyst-resistant soybean varieties are actually fairly susceptible to the most prevalent race. Poor performance in a cyst-resistant variety may mean a grower is dealing with a race of cyst nematode for which the variety has no resistance.
Soybean sudden death syndrome- first identified on the Eastern Shore in 2000- is another root rotting disease that may not show until some additional stress later in the season exacerbates symptoms. It requires cool, wet soil conditions at planting but won't produce symptoms until flowering. Fortunately, says Grybauskas, there are varieties available that have some resistance. "We don't know how widespread a problem this disease might be, but since resistance is pretty widely available among commercial varieties, it doesn't hurt to pick one that has some resistance rather than wait for a problem to develop and get compounded by droughty conditions."
For more information contact:
Arv Grybauskas, 301-405-1602 or ag31@umail.umd.edu
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