Pathologist: Wheat still leads in what producers wish it didn't

"Year in and year out, we have more disease problems with wheat than corn," said Greg Shaner, Purdue University Extension plant pathologist. "Soybean probably runs a close second with some of the emerging problems that we have, such as Sudden Death Syndrome."

As Hoosier farmers prepare to plant soft red winter wheat this fall, they should select wheat varieties able to fend off yield-damaging diseases, Shaner said. They also should avoid planting wheat in fields where disease risk is higher, he said.

Shaner recommends varieties with partial or total resistance to several common fungal diseases.

"Leaf rust is a disease that has been here ever since we've been growing wheat," Shaner said. "It's fair to say that most of our varieties have pretty good resistance to it but there are some that are susceptible."

Leaf rust is spread when wind-blown fungal spores land on wheat fields. The fungal pathogen produces yellow to brown-black spots on upper wheat leaves. The disease thrives in cool, moist conditions.

"Another pair of diseases that look a lot alike, called Septoria leaf blotch and Staganospora leaf blotch, cause premature defoliation and can substantially reduce test weight," Shaner said. "We don't have varieties that are completely resistant to either of these — particularly Staganospora — but there definitely are differences among varieties."

Both diseases are favored by wet weather. Septoria does well when it is cool and wet but Staganospora prefers warmer temperatures.

The granddaddy of Midwest wheat diseases is Fusarium head blight, commonly known as head scab. The fungal disease attacks the wheat head where ears of grain develop. The disease can interrupt a wheat plant's grain-making ability, resulting in low yield. The Fusarium fungus also produces deoxynivalenol (DON), a compound toxic to humans and livestock. Wheat grain with even trace amounts of DON is difficult to market.

Farmers can reduce head scab risk by not planting wheat in fields that just grew corn, Shaner said.

"We've generally recommended not planting wheat into corn residue," he said. "What we've found is that if we have a spring where the weather is really favorable for scab, it doesn't matter if you've got corn residue in the field — there's enough of the fungus around. But if conditions are only marginally favorable for scab, it may be worse where you've got corn residue than where you don't.

"The fungus that causes Fusarium head blight is the same one that causes Gibberella ear rot and stalk rot in corn. Even if you don't have an ear rot or a stalk rot problem in a corn crop, that fungus is part of the group that decays corn residue. So you can pick up most any piece of corn residue and the Gibberella fungus will be there."

Planting date plays a role in the transmission of the barley yellow dwarf virus, a virus that causes yellow dwarf disease. Producers who wait to plant wheat until after the Hessian fly-free date in their area can lower the odds of infection, Shaner said.

The Hessian fly-free date ranges from Sept. 22 in extreme northern Indiana to Oct. 9 in the southernmost Hoosier counties.

"The virus is transmitted by aphids," Shaner said. "If a wheat field is planted before the fly-free date, there's a greater likelihood that aphids will still be active. The aphids will zero in on the little wheat plants that are just emerging. If the aphids are carrying the virus they will transmit the virus as they feed."

Infected crops are stunted and often fail to reach full yield potential. "Fall infection by the barley yellow dwarf virus can be quite damaging to wheat," Shaner said.

Five aphids per foot of crop row represent an economic threshold level for barley yellow dwarf transmission. Producers should consider treating their field with insecticide.

Winter wheat planting usually begins in September, with harvest the following summer. Soft red winter wheat is used in cookies, cakes and similar products.

Indiana's 2006 wheat production is estimated at 32 million bushels, up 31 percent from 2005, according to the Indiana Agricultural Statistics Service (IASS). The 2006 projected average of 71 bushels per acre is one bushel under the 2005 yield record.

By comparison, the state is expected to produce 893.5 million bushels of corn and 273.8 million bushels of soybeans this fall, the IASS projected.

For more information on winter wheat, visit the Purdue Small Grains Management Web site.

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

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

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page

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

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