White mold on soybeans has been an increasing problem in the Northeast. Due to the longevity of this disease organism, it's a perennial threat to yields once it attacks.
"If our current (dry) weather pattern holds into the soybean flowering period," says Gary Bergstrom, Cornell University plant pathologist, "we may avoid serious white mold problems this year. But it has been an increasing problem in New York as in areas of the Midwest and Ontario."
The disease can live up to 10 years in a field, even with crop rotation. White mold starts in the soil, moving up the soybean stem and into the canopy.
During the growing season, you have few control options, warns Zach Fore, Pioneer area agronomist. If conditions during flowering are moist for seven to 14 days with moderate temperatures at or below 85 degrees, you'd better be scouting for it, he adds.
What to watch for
Close inspection can reveal symptomatic plants three to four weeks after flowering. It begins with tan, mushroom-like structures called apothecia. Spores from the apothecia infect maturing soybean flowers.
Examine stems for signs of gray to white lesions at the nodes. Lesions vary and can be three to 18 inches long and rapidly progress above and below the nodes.
One management option for growers who've experienced the disease in the past is to select varieties having a higher tolerance to the disease, Fore says. "There are varietal differences. Pioneer scores all its varieties adapted to locations where white mold occurs. These scores reflect differences in the rate at which infection develops and the extent of the damage it causes."
But avoidance is the best strategy for most farms through crop rotation, clean seed, selection of varieties with some resistance/tolerance, and avoiding plant populations that are excessively high, asserts Bergstrom. In fields that become infested with fungal sclerotia, reduced plant populations and longer term rotations with cereals become even more important.
Few, if any, fungicides are completely effective rescue treatments for standing soybeans. Some growers with a history of white mold, he adds, apply thiophanate-methyl fungicides beginning at flowering for partial control, though it often requires multiple applications. Cornell researchers are investigating Contans, a commercial biological control product, to reduce the inoculum potential in infested fields.
"Our current toolbox for white mold control is inadequate for the future," concludes Bergstrom. "We'll need more effective controls, including more resistant varieties and more effective fungicides, as soybean production expands and intensifies in the Northeast."