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Disease Alert!

December 7, 2010
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
F10465 Field
Sudden death syndrome was widespread in 2010, making it a disease to plan for next season. Planting SDS-tolerant varieties and changing cultural practices can help reduce outbreaks.   

Soybean diseases are like the proverbial box of chocolates—you can’t predict which ones are going to show up next year. What you can do is understand each field’s history, know what conditions foster each disease, select resistant or tolerant varieties and be prepared to apply a fungicide if necessary.

“Really look at the resistance package as you choose varieties for 2011,” urges Ohio State University plant pathologist Anne Dorrance. “In Ohio, the past two years were good examples of different weather environments. Since we can’t predict next season’s weather and which pathogens it will favor, having resistance to several diseases will help maintain yields.”

Plant pathologists in several major soybean growing areas cite an array of disease threats that popped up this year and that may or may not return, largely depending on the weather.

In Ohio, Dorrance says, seedling diseases such as phytophthora, pythium and fusarium hit early, and the effects were aggravated by flooding injury. Later, drought aggravated the effect of soybean cyst nematodes.

The heat also fostered charcoal rot. “Some southern Ohio growers have experienced three or four years of drought, so they need to learn more about charcoal rot,” Dorrance says.

Sudden death syndrome (SDS) showed up in many Ohio soybean fields. “The majority of those fields contained soybean cyst nematode [SCN], combined with the fungus Fusarium virguliforme, which causes sudden death syndrome,” Dorrance says. “These two pests have been linked since the pathogen was first identified in the U.S. The fungus colonizes the roots and leads to root rot, causing the roots to decay.”

Overall, Illinois experienced only average disease pressure, says University of Illinois plant pathologist Carl Bradley. But individual growers suffered yield losses from a variety of pathogens.

Sudden death spreads. “Sudden death syndrome continued its sprawl across the state,” Bradley says. “It was most severe in west-central and northern Illinois, especially in areas that stayed wet and in fields that were planted earlier than normal.”

Cool, wet conditions and early planting also set the stage for seedling diseases and for frogeye leaf spot, which was present in some Illinois fields. “We observed frogeye leaf spot across the state, but it was severe only on highly susceptible varieties,” Bradley says.

Frogeye leaf spot is becoming less of a threat in the Midwest as growers adopt resistant varieties, Dorrance and Bradley agree. But there may be a cloud on the horizon. Bradley’s detection of fungicide-resistant strains of Cercospora sojina, the causal agent of frogeye leaf spot, suggests the disease should not be taken for granted.

Bradley found the strobilurin fungicide–resistant strains in plant samples from a Tennessee soybean field. The active ingredient strobilurin is used in several popular fungicide products.

So far, the fungicide-resistant strains have not been found to be widespread. But their existence reaffirms the importance of planting varieties resistant to frogeye leaf spot. “Resistant varieties remain the best way to manage the disease,” Bradley says.

If you plant a susceptible variety and are considering applying a fungicide, choose a triazole fungicide rather than a strobilurin product, Bradley advises. If you need to spray a strobilurin fungicide for other foliar diseases as well as frogeye leaf spot, apply a product or a tank mix containing both classes of fungicides.

Assess the risk. To reduce the chance of developing resistance to frogeye leaf spot, apply foliar fungicides only when they are needed, based on disease risk and scouting observations, Bradley adds.

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FEATURED IN: Farm Journal - December 2010

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