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March 21, 2003

Farmers could lose if soybeans go to 'sudden death,' specialist says

WEST LAFAYETTE, Ind. - Soybean farmers itching to plant early this spring could awaken a sleeping giant.

Growers too hasty with the planter might put their crops at greater risk of the fungal disease known as sudden death syndrome (SDS), said Greg Shaner, a Purdue University Cooperative Extension Service plant pathologist. Farmers can reduce the risk of SDS infection by planting soybean fields later.

"Early planting increases the risk of sudden death syndrome," Shaner said. "If you plant at what we consider the normal period of May 5-20 it doesn't eliminate the risk, but beans that are planted in late April into the first week of May have a somewhat greater risk.

"If a field has a history of sudden death syndrome, then a farmer may want to plant other fields earlier and plant that field a bit later to reduce the risk."

Farmers can further protect their crops by selecting soybean seed less susceptible to SDS, Shaner said.

"If a grower has SDS in fields, then it also may be a good idea to try to find a resistant variety," he said. "In talking to a seed dealer a grower needs to ask some questions about how confident the seed dealer is that the variety really has resistance. Just because a variety looks good in one test does not mean it has reliable resistance. Resistance must be proven by multiple tests where SDS pressure is heavy and uniform."

SDS presents the No. 1 disease threat to soybean fields. The root rot disease, first confirmed in Indiana in 1986, enters the soybean plant early in the crop season and usually manifests itself as the plant nears maturity. SDS causes small yellow blotches to appear on soybean leaves. The blotches grow larger in size and number, and the tissue within the infected area becomes brown and dies. Eventually all the leaves turn brown, severely limiting the plant's ability to produce grain.

The fungus responsible for SDS - Fusarium solani f.sp. glycines - can survive through regular crop rotation cycles. No one knows how many years a field must be planted to something other than soybeans to reduce the soil fungus levels.

To add insult to injury, SDS-infected fields often are infested with soybean cyst nematode, another deadly soybean pathogen, Shaner said.

SDS thrives in wet conditions. Except for the planting period, the 2002 crop season was unusually dry in much of Indiana. As a result, SDS was noticeably absent from most Hoosier soybean fields a year ago.

Farmers should not be lured into a false sense of security this crop season, Shaner said. SDS needs only to be triggered by favorable weather and field conditions to resume its destructive ways.

"We know that the sudden death syndrome fungus, which is a soilborne organism, is widespread in the state," Shaner said. "To that extent we need to be concerned about it every year. There are some weather conditions that determine whether or not and how severe it will actually be in any given year.

"Last year SDS was fairly restricted. It was in a band in west-central Indiana. We didn't see much in the south, which has been the traditional hotspot for SDS, nor in the far northwest part of the state. What SDS we did see was closely related to weather. We had some timely rainfall in west-central Indiana that many other areas missed."

Corn growers could have their hands full with plant diseases this year, as well. Shaner cautioned farmers to be on the lookout for a trio of fungal pathogens that attack corn leaves, stalks and ears.

"In contrast to the situation with soybeans, most of our troublesome corn diseases are caused by fungi that infect the above-ground parts of the plant. They cause leaf blights or ear rots," he said. "The diseases that are of most concern these days are gray leaf spot, Diplodia ear rot and anthracnose, which can cause both a leaf blight and a stalk rot. Those diseases are of particular concern because we don't have as much resistance in hybrids as we do to some of the other leaf blights."

The corn diseases' fungi survive in crop residue, Shaner said.

"This is one of those examples where there's a conflict between the desirable benefits of conservation tillage and the downside, which is that all of this corn residue on the soil surface allows these pathogens to survive in much greater numbers," he said.

"Under those conditions, a grower ought to pay close attention to the resistance ratings of hybrids that are selected, because the disease pressure is going to be greater if you have residue. It doesn't have to be right in the field, either. If you've got corn following soybeans, you don't have to go too far to find a soybean field that had last year's corn residue, and that's the source of the inoculum."

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

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

Related Web sites:

Crop Diseases in Corn, Soybean and Wheat: http://www.btny.purdue.edu/Extension/Pathology/CropDiseases/

Purdue Department of Botany and Plant Pathology: http://www.btny.purdue.edu/

Purdue News Service: (765) 494-2096; purduenews@purdue.edu


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