Disease Management: Weather favors development of soybean diseases

Conditions are favorable for a yield-robbing fungus to plague soybean fields in the Midwest.

Soybean specialists urge farmers be on the lookout for white mold, and be prepared to combat the disease. Development is bolstered by wet, cool, cloudy and humid weather at flowering.

With most soybeans just starting to or about to reach the R1 (flowering) stage, experts suggest farmers spray fungicides now in fields with a history of white mold. A severe infection will weaken plants and cause wilting, lodging and death.

Daren Mueller, Iowa State University Extension and Outreach plant pathologist, said a preemptive strike is warranted since saturated soils and cool temperatures throughout much of the Great Lakes region makes white mold a viable, widespread threat. Once the disease is on plants, treatments are considerably less effective.

Yield losses of 10 to 30 percent or more in untreated fields with white mold are typical.

“It’s a very aggressive pathogen that can affect large sections of fields,” Mueller said. “This is the perfect environment for white mold.”

Farmers will want to assess the risk of white mold development to judge whether fungicides are warranted. The disease is more problematic in high-yield environments with high plant populations and narrow row spacing.

No single management strategy is 100 percent effective for white mold, and in-season options are limited. Several products that have been rated as “good” include Approach, Endura and Proline.

For information on which fungicides are labeled for disease control and recommendations on fungicide efficacy, consult this table.

At best, a fungicide application will manage 50 to 60 percent of a white mold outbreak in a field, experts say.

Costs can range from about $15 to $30 per acre, officials said. The high end of the scale is for aerial application, which may be needed for saturated fields.

Weather conditions are also conducive to foliar diseases such as frogeye leaf spot, cercospora leaf blight and septoria brown spot, according to Tristan Mueller, operations manager-agronomic research for the Iowa Soybean Association’s On-Farm Network®. Fungicides that work on white mold are effective on these pathogens as well, he said.

“With the current weather patterns, it is certainly more likely that fungicides will pay compared to the last two drought years,” Tristan Mueller said.

Timing is everything when it comes to combating white mold. Damon Smith, University of Wisconsin Extension crops pathologist, said farmers often want to wait until soybeans are in the R3-R4 stage to see if there’s a problem.

“That’s too late. There’s no rescue treatments,” Smith said.

To kill white mold, he said spraying will need to be done by mid-July.

The disease is caused by the fungus Sclerotinia sclerotiorum, which lives in the soil. It isn’t a problem every year. The last widespread issue was in 2010, which also had a wet, cool June.

As the name suggests, the disease is easily distinguished by the presence of white, moldy growth on plants. Once that’s found, fungicides are of little help.

Scouting for signs of white mold before it’s a problem is difficult. At the beginning of the disease cycle, the fungus will grow very small mushroom-like structures on the soil surface that will release spores that enter plants. The fungus disrupts the transport of water and nutrients, which reduces seed numbers and weight.

Scouting is still beneficial, officials say. Farmers can document which fields are susceptible for future management considerations.

“The risk will be fairly high this year,” Smith said.

If a soybean field has high levels of white mold it should be harvested last. To reduce fungus movement, harvesting equipment should be thoroughly cleaned.

Several resources are available to help farmers and agribusiness personnel learn about management of white mold on the new Soybean Research & Information Initiative website. Extension plant pathologists have developed a publication in collaboration with the North Central Soybean Research Program to describe the disease and optimal management strategies. “Management of White Mold in Soybeans” is available at the following here.

A podcast series to facilitate learning about white mold on-the-go is also available here.

Matthew Wilde is a senior writer with the Iowa Soybean Association.

This article originally appeared on Iowa Soybean Association’s website and has been reposted with permission.