While this season is progressing much more quickly than last year, one thing remains the same – lots of rain. Wet conditions may be ideal for an unwanted soybean disease – white mold, warns Daren Mueller, Iowa State University plant pathologist.
White mold, also called Sclerotinia stem rot, typically starts to show up during the middle of the reproductive growth stages. Last year’s cool, wet weather led to the first major outbreak of white mold in almost a decade. While this year has been warmer, Mueller says there is still a risk of white mold developing.
“Fields at a higher risk of getting white mold are those that had disease in previous years and are in high-yielding sites where the canopy closed early,” Mueller says. “Also, fields that have had plenty of soil moisture, high humidity and little airflow have increased chances of getting white mold.”
What good would it be to identify white mold this late in the season? “In general, fungicides are not effective, therefore not recommended, after symptoms have developed,” Mueller says. “Fungicides are more effective if applied before disease gets established in a field. So, while little can be done to stop infection once the disease can be seen in the field, there still is some value in scouting.”
First, locating “hot spots” of white mold may trigger management strategies to reduce the number of sclerotia for subsequent years. “If a particular field has hot spots, you may want to consider burying the sclerotia that drop to the soil surface at the end of the season,” Mueller says. “Another way to reduce inoculum is to apply a biological control to kill the sclerotia.”
Second, knowing what fields are prone to getting white mold may influence what cultivar you plant the next time that field is in soybeans. “Remember: those sclerotia, the small, black survival structures, can survive more than two years in the soil,” Mueller says.
Third, Mueller adds, “You can take extra precautions to keep your combine clean of soybean stems and residue after harvesting fields with white mold. This will help prevent spreading the fungus to new fields.”
All of these are reasons he encourages farmers to be on the lookout for white mold in the coming weeks.
David Wright, director of contract research and strategic initiatives for Iowa Soybean Association, agrees that managing white mold in soybeans is very challenging. “There are limited tools available to farmers to reduce yield loss to white mold,” Wright says. “Applications of spray products have had varying results. Soybean checkoff investments are targeting the development of genetic resistance to white mold. We anticipate the release of breeding lines with greatly improved resistance to white mold in the next one to two years.”