WEST LAFAYETTE, Ind. — When soybeans are in the bloom stage, cooler, heavy-dew mornings or humidity are a recipe for white mold, said Shaun Casteel, Purdue University agronomy extension soybean specialist.

White mold of soybeans, also called Sclerotinia stem rot, is present throughout most of the Northern states; the areas of greatest risk are north of Interstate 70. It is can cause significant damage in infested fields, according to a Purdue extension bulletin. The disease is often most severe in varieties that have a denser, faster-closing canopy.

White mold first becomes apparent when single plants within a generally healthy canopy wilt and die rapidly in July and August. Leaves remain on the stem but turn brown, and the entire plant dies.

“With white mold you have to have the source, the pathogen, in your field,” Casteel said. “Fields that have had a history of white mold are more prone to have it.

“This disease infects the flower itself whenever the temperatures are cooler and you have wet conditions during bloom.”

The variety of soybean selected is one place to start in prevention of the disease, he said. There is information available on levels of tolerance. White mold, however, is difficult to rate in variety and breeding efforts because it doesn’t show up every year.

“In other words, the disease pressure isn’t nearly as high every year, so it is hard to determine if this is truly a tolerant or resistant line,” Casteel said. “But if you have that information, that is the first step.”

The next step is to have a good crop rotation, since planting corn or small grains tends to help in reducing the population of the pathogen, Casteel said. Also, no-till makes it easier for the sclerotia to germinate, so having a year of corn or small grains in the cycle is helpful.

“Another step in management would be to plant wider rows. Also, you don’t want to be in excessive plant populations,” he said. “That creates this micro-environment of a dense canopy and so it’s not allowing air to flow through, so you have higher humidity, higher-moisture conditions.”

If conditions are right for the disease, growers who live in an area susceptible to white mold should take preventive measures by applying fungicides or other products. “If you learn that you have white mold after the fact, it is really difficult to do any good in terms of reducing the infection,” Casteel said.

Dave Downing, brand manager with MANA Crop Protection, agreed.

“This is a disease that, if you’re not managing for prevention of the disease, then once you already have it, trying to cure it is extremely difficult,” he said. “Once the disease get into the blooms, it goes systemic throughout the plant.”

MANA has developed a weekly email of white mold information it will be sending out from May 1-June 15. Growers can sign up for the newsletter at no charge.

“We will provide to growers, on a weekly basis, an update, a short audio and video clips that will be helpful to them to make smart choices for scouting, for application timing, for treatment expectations in soybeans,” Downing said.

“This will give them the background necessary to make an informed decision on whether or not they need to treat and how to manage this disease in soybeans.”

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