White mold, a common soybean disease in the Upper Midwest can be detrimental to your yields and can be prevented in large part by seed selection and management.
By Melanie Lundheim
If you see a white, fluffy substance outside the stems and on the pods of your growing soybeans, it’s probably not cotton, spider webs, or snow. Rather, your crop is most likely suffering from Sclerotinia stem rot. Also known as white mold, Sclerotinia stem rot is a fungal disease that threatens soybeans in the Upper Midwest and other northern regions.
White Mold Causes and Symptoms
Sclerotia are the survival structures that germinate to release the spores that cause Sclerotinia stem rot. Resembling rat droppings and containing hardened fungi, sclerotia are dark, irregularly shaped, and ¼ to ¾ inches long. Sclerotia can remain dormant for long periods of time and survive in the soil for as many as seven years or longer.
If sclerotia are present, the pathogen thrives when temperatures are between 68 and 79 degrees Fahrenheit, soil conditions are moist, and the canopy of your high-yield soybean crop is dense. Having a history of white mold also increases the likelihood sclerotia are present in the soil and your crop will come down with the disease.
In addition to white mold growth on your soybean plants, you may find wilted leaves, shedding stems or stems that show lesions that are either gray or white in color. Sclerotia may also be present on or inside your plants.
Impacts of White Mold on Crops
Sclerotia produce fungi-containing apothecia capable of producing millions of spores. So it’s no surprise that, once a soybean crop becomes infested with white mold, the disease can spread to other host plants as soon as it comes into contact with moldy tissue.
As a result of the disease spread, you may lose 40 to 100 percent of yields on your soybean crop, not to mention loss of yields among other crops that contract white mold.
The extent of yield loss is correlated with such factors as the severity of the disease, the susceptibility of your plants to white mold and the length of time your plants have been infested with Sclerotina stem rot.
Preventing White Mold
Since early canopy closure exacerbates Sclerotinia stem rot, don’t plant seeds too close together.
Soybean plant populations of 175,000 or more per acre can increase the likelihood your crops will come down with white mold, so consider reducing your plant population enough to decrease risk while maintaining yield potential.
White mold can increase as the spacing of your soybean plants narrow, so plant them farther apart. Also, be sure to effectively control broadleaf weeds near your soybean plants, since they not only increase the density of your vegetative canopy, but also host white mold.
If your field has a history of white mold outbreaks — and environmental conditions are favorable for development of disease — determine whether it’s warranted to apply a fungicide during flowering of your soybean plants.
According to Mike Staton, senior educator, Michigan State University Extension, chemical control should be combined with other white mold strategies, including cultural control, use of tolerant varieties, irrigation water management, reduced planting rates, row spacings wider than 20 inches, and biological control.
“Varieties vary significantly in their susceptibility to white mold, and planting the most resistant varieties in fields prone to white mold is a key management practice,” said Mike.
He recommends selecting varieties that resist lodging and have a narrower canopy width, which can reduce the incidence and severity of white mold. In addition, planting varieties from a range of maturity groups may also help some fields avoid severe infestations by staggering the susceptible flowering period.
The overall topography of your field makes a big difference in protecting your soybean crop from coming down with Sclerotin,a stem rot, as well. Avoid building — or remove — barriers that impede movement of air over your soybean crop, such as low areas and lines of trees.
Another important step to preventing white mold is ensuring you don’t introduce the pathogen to your crop in the first place. Sources of contamination include wind-borne spores; seed that’s contaminated or infected with sclerotia; harvest equipment, tractors, four-wheelers and pickup trucks that move from field to field throughout the season; and farmers’ boots — to name a few. Choosing certified clean seed, harvesting infected fields last, and thoroughly sanitizing items that come into contact with your crop can also help safeguard your soybean crop from the spread of Sclerotinia stem rot.
Subject Matter Experts:
David Kee, David@MNsoybean.com
Sources of background info: