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June 17, 2003

Disease pressure coming to a head in wheat, pathologist says

WEST LAFAYETTE, Ind. – Harvest is approaching for Indiana's winter wheat crop, but several diseases threaten to arrive before the combines do, said a Purdue University Extension specialist.

A rainy spring that's provided an ideal environment for harmful fungal pathogens could wash away high yields, said plant pathologist Greg Shaner. Farmers should begin harvesting the state's estimated 450,000 acres of soft red winter wheat by month's end.

"The main concern this year is head scab, or Fusarium head blight," Shaner said. "This is going to be a problem mainly in the southern third of Indiana."

Head scab attacks the wheat head, where grain develops, and can cause significant yield losses.

The percentage of blighted heads in Purdue variety trials in southern Indiana this spring ranges from 20 percent to more than 60 percent. Fields with comparable scab levels produce a poor yield of low-quality grain, Shaner said.

"This is a disease caused by a fungus," he said. "It infects the flowers directly and doesn't appear until after flowering. The disease develops during the grain-filling period. The symptom that you see first is a premature bleaching of the wheat heads. At a stage when heads should be entirely green, blighted heads will be all or partially white. That's a good indication that it is scab."

When scab infects a wheat crop it destroys the size and quality of grain, Shaner said.

"There are two ways that a farmer takes a loss on this disease," he said. "First, the kernels that are infected, especially early in development, shrivel up so much that the lightest ones will blow right out the back of the combine with the chaff. Even what is harvested will typically have a pretty low test weight.

"The other big concern with scab is that the fungus produces a toxin in the grain. The technical name for it is deoxynivalenol – we usually refer to it as DON. Another name for it is vomitoxin. It's a very stable toxin, so it survives through milling and baking. If a load of wheat has a lot of the toxin in the grain, an elevator may not even buy it."

Because head scab occurs late in a wheat crop's development, there is nothing a farmer can do to rid a field of the unwelcome intruder. Researchers are working to develop wheat varieties resistant to Fusarium graminearum, the fungus that produces scab, Shaner said.

Farmers should be on the lookout for other wheat diseases, as well.

"Other diseases we're seeing associated with a lot of wet weather are leaf blotch and glume blotch. They both are caused by the same fungus," Shaner said.

"Leaf blotch kills leaf tissue prematurely. In the case of glume blotch, the fungus gets up on the head. In contrast to scab, glume blotch turns the heads a dark, chocolaty brown. The fungus doesn't produce any toxin, but because it's killing so much foliage prematurely, the stress on the plant causes poor grain filling, so you get lower yield and test weight."

Disease pressure is not as great for wheat crops in the northern two-thirds of Indiana. Many wheat fields there are in good shape, Shaner said.

Indiana's 2003 wheat acreage is up 29 percent from a record-low 350,000 acres in 2002, according to the Purdue-based Indiana Agricultural Statistics Service. Hoosier farmers produced 17.5 million bushels of soft red winter wheat in 2002, at an average yield of 53 bushels per acre.

Soft red winter wheat is used in cookies, cakes and similar products.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Greg Shaner, (765) 494-4651, shaner@btny.purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web site:
Purdue University Department of Botany and Plant Pathology


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