The interveinal yellowing and browning on the upper trifoliates, shown in this field of soybean plants, is a characteristic symptom of sudden death syndrome. (Purdue Department of Botany and Plant Pathology photo/Kiersten Wise)

A team composed of a Purdue University researcher and others from several Midwest universities and Canada is studying soybean sudden death syndrome to help farmers better protect their crop from the recurring disease.

Most Indiana soybean fields had some level of the disease last year, the most severe since 2010, noted Kiersten Wise, an associate professor of plant pathology. Even though the disease reduced yields, Indiana soybean farmers still produced a record crop last year, at 307.4 million bushels.

"Yields may have been even greater if it weren't for SDS," she said.

Wise and other researchers from Iowa State University, Michigan State University, the University of Illinois and the Ontario Ministry of Agriculture have been looking for answers beyond typical production practices, which include crop rotation and planting disease-resistant varieties. That hasn't been enough to stop the disease.

"We wanted to look at how a combination of various production practices affected SDS to determine a better management strategy," she said.

Those practices include early to late planting, use of varieties resistant to both SDS and soybean cyst nematode, and fungicide seed treatments. They also examined how weed killer glyphosate and pre-emergence herbicides affected SDS severity.

Sudden death syndrome, first reported in Arkansas in the 1970s, is now in nearly every state where soybeans are grown. Contrary to its name, infected plants do not die suddenly. Plants are typically infected in the early vegetative stages of growth, and symptoms appear in the mid-to-late reproductive stages, usually late August in Indiana.

SDS is an annual problem because the disease-causing fungus - Fusarium viguliforme - survives in the soil over the winter. The fungus also can live on corn debris.

Cool and wet conditions in the spring make the soybean plant susceptible to early infection, and frequent rains in the reproductive stages allow the toxin produced by the fungus to move up the plant and into its leaves. Although symptoms are usually most severe in soybeans planted in April, Wise said it is important to remember that later-planted soybeans are still at risk for disease development.

"It's the environment rather than the calendar that determines whether the conditions are conducive to infection," Wise said.

She said preventive measures before planting are very important, since there are no in-season management options to manage the disease.

Funded by the North Central Soybean Research Program, a farmer-funded checkoff, Wise last year conducted field trials at the Pinney Purdue Agricultural Center in northwest Indiana. The study included the additional option of seeds treated with a new chemical, fluopyram (floo-oh-PEYE'-ram), designed to protect the root system against the SDS fungus in the seed zone. The chemical is marketed by Bayer, which contributed financial support for the research.

The Indiana trial results showed that:

* The SDS-resistant variety had lower disease levels at all planting dates with and without the fluopyram seed treatment.

* The resistant variety produced higher yields with and without fluopyram in all but one planting date.

* Fluopyram significantly reduced disease severity and increased the yield in both resistant and SDS-susceptible varieties compared with a standard seed treatment. Greatest disease reductions were noted in soybeans planted before May 15.

Wise said fluopyram is another tool that farmers could consider in managing SDS. This year will be the first year the seed treatment is available for use.

In addition, the researchers recommended that farmers plant soybean varieties resistant to SDS and soybean cyst nematode, avoid compaction and maintain notes on which fields are affected by the disease for future management decisions.

"We're still learning about new ways to manage SDS," Wise said. "There will be more questions to answer this year and beyond."

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