White mold typically rears its ugly head in late spring and early summer around the time that both soybeans and dry bean plants begin to bloom. The disease thrives in cool temperatures ranging between 59 and 75 degrees Fahrenheit and moisture-rich soil conditions caused by heavy spring rains. Combined with a dense crop canopy that can stay moist until the late morning hours, Markell says the environment can become “a perfect storm" for the disease to infect throughout a field. Infection occurs when ascospores, released from the white mold pathogen land on senescing flower petals, then germinate and form mycelium.

“The spores are generally unable to infect the plants directly, but can easily digest the senescing flower petals,” Markell says. “Shortly after the infection occurs, the fungus will form a lesion, causing significant damage to the plant and disrupting nutrient uptake.”

Portions of the plant stem, above the area or infection, die and dry up. The disease then hollows out and shreds the stem at the spot of the lesion. At this point, the plant is non-productive, if not dead, resulting in significant yield loss for that season and sclerotia in the soil for years to come.

Land grant universities across much of the Midwest and Northern regions are placing greater attention on the disease. Many are completing yield impact studies to determine the changing significance of the issue across varying geographies. The results have shown that yields can be reduced by 10, 20, 30 and even as high as 50 percent depending on disease severity, conditions and control practices. With a low-end reduction of just 20 percent on an expected 60 bushel per acre crop means 12 bushels will never be harvested. And at $12 per bushel as an average soybean price, that quickly equates to $144 an acre eliminated from a grower’s profit potential.  

“A grower only has to look back as far as the 2009 season to see the economic impact white mold can have on a crop,” says Markell. “There is traditionally a white mold outbreak at some point and on some crop in North Dakota due to our climate. But in 2009, growers as far south as Iowa and Illinois experienced white mold pressure and the severity it can have on yield.”

Iowa State University reports about the 2009 white mold outbreak pointed to losses totaling more than $10,000 per field for soybeans alone. In North Dakota, growers saw significant yield reductions in soybeans and total crop destruction in some dry bean fields.

Markell adds that the 2009 epidemic was an eye-opening experience for a lot of growers, and with a repeat in 2010 of high white mold pressure in certain geographies, he says every soybean and dry bean grower should have the disease on their radar. He continues by saying that there is still plenty of white mold inoculum in the soil, making it necessary to take precautions to minimize the disease risk for the 2012 crop.