Watch Out for White Mold

Wet-Loving Fungus Could Thrive This Year

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Growers should be on the lookout for white mold infections this year after many states saw wet, mild conditions during flowering. (Photo courtesy Carl Bradley, University of Illinois)

ST. LOUIS (DTN) -- Pods may not be the only thing sprouting on your soybean plants this summer.

With more than half the nation's soybeans piling on pods this week, plant pathologists are warning growers to be on the lookout for a more sinister growth: white mold.

White mold thrives in high humidity and cool temperatures, and is therefore especially prominent in northern soybean-producing states like Michigan, Wisconsin and the Dakotas. However, thanks to wet, mild conditions during flowering, plant pathologists from more southern states like Ohio and Illinois are also recommending that growers keep an eye out for it.

For most growers, it is too late to treat white mold infections, but knowing that the fungus is present in your fields can give you valuable intelligence when it comes to variety selection and crop management for next year, said North Dakota State University plant pathologist Michael Wunsch.

"For dryland growers, variety selection, row spacing, and plant population will all impact white mold," he said. "But you have to know your risk."

Irrigated growers, however, can manage irrigation to minimize the disease damage and progression if they spot it in their fields, he said. "We know even if the total amount of irrigation is unchanged, the frequency of irrigation events is critical for determining white mold levels," Wunsch said. "Irrigating high amounts of water infrequently will be less favorable for white mold than irrigating often with small amounts of water."

White mold's most striking and visible symptom -- the frost-colored fungus that coats wilted plants -- is also a sign that the disease is well underway and mostly immune to fungicide treatments, which only prevent new infections.

Instead, growers must make a decision to spray at early bloom (R1 to early R2), when the fungus' spores initially attack the plant through dead blossoms, and well before the disease is easily identified.

"Fungicide applications after R1 or R2 might help some, but you're unlikely to get an economic return," Wunsch said. Not only will the fungicide have no effect on an ongoing infection, but the fuller canopy of later-maturity soybeans will limit its coverage, he said.

Scouting now for the disease and keeping track of a field's history of it are therefore extremely important for future control.

Fields with a history of the disease are always at highest risk and most likely to benefit from timely fungicide treatments in the future, Wunsch said. Once the fungus colonizes the internal tissue of a plant, it produces hardened black structures called sclerotia that drop into the soil after harvest or plant death.

In subsequent growing seasons, the sclerotia germinate and produce tiny mushrooms loaded with spores, and the infection cycle begins again. Sclerotia can survive in soils for many years, and the more your plants produce each year, the greater your risk of future infections, Wunsch said.

Growers can plot out fields where the disease is present, keep track of yield losses there, and select more resistant varieties next year.

Research on the disease suggests that for every 10% increase in the incidence of white mold, a field is likely to lose 2 to 5 bushels per acre, according to an Iowa Soybean Association fact sheet on the disease. No soybean varieties will completely resist the disease, but some will yield better in the disease's presence and succumb to plant death more slowly.

Soybean varieties with reduced susceptibility to white mold are often narrow or relatively short, Wunsch said. Wider row spacing and lower plant populations can also help reduce your risk.

"Anything that facilitates more airflow within the canopy and dries it out more will lead to less infection," Wunsch said.

Multiple years of crop rotation to corn, small grains, or forage legumes can reduce the number of sclerotia, but growers should avoid peas and sunflowers, the ISA publication noted.

For more information on lessening the impact of white mold infections, see the ISA fact sheet here:…, and this comprehensive guide from the North Central Soybean Research Program here:….

The latest fungicide efficacy trials for white mold from Purdue here can be found here:…. See also Michigan State University's guide to making fungicide decisions here:….

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Emily Unglesbee