At the spring extension meetings, a commonly asked question was whether white mold will be widespread in 1996. In 1992 and 1994, Iowa growers, especially those in northern Iowa, witnessed wide spread of white mold. Most of the fields that had white mold in 1994 will be in soybean in 1996, resulting in concerns about this disease. It would be great if we could predict white mold occurrence before planting, which would allow us to make management decisions efficiently. However, it is practically impossible to make a good prediction on the epidemics of white mold before planting because the level of white mold occurrence highly depends on weather conditions during soybean flowering. On the other hand, we can still minimize white mold risk by planning ahead, although we can not predict its occurrence.
Soybean white mold is a fungal disease caused by Sclerotinia sclerotirum. The infections occur during flowering stages, and start by entering soybean flowers. Wet and cool conditions during blooming are necessary for its spread. The 1992 and 1994 weather conditions were wet and cool. If the weather conditions are dry and hot during the soybean flowering season, the disease risk will decrease significantly. That is why one cannot predict the disease. Cultural practices also affect the disease. Often, we see more white mold in drill planted soybeans than 30-inch soybean because the canopy has higher moisture in narrow-rowed soybeans than wide-rowed soybeans.
There are a few things we can do now to minimize the disease risk. If your fields had high levels of white mold in 1994, avoid using susceptible varieties and consider using white mold tolerant varieties. High yielding, white mold tolerant varieties are available. Some of them are listed in a new ISU Extension publication, Pm-1649, Soybean: Disease-Resistant Varieties for Iowa (PDF format). Also, avoid planting soybean in narrow rows (15 inch or less) in fields that had white mold in 1994.
According to a publication from University of Wisconsin, fungicides Benomyl (also called Benlate) and Topsin (also called thiophanate) provide control for soybean white mold if applied at early flower. Topsin has been registered to control soybean white mold. Benomyl is not specifically registered for control of soybean white mold. Benomyl, however, is registered for control of other soybean diseases and to control dry bean white mold, caused by the same fungus as soybean white mold. We tested Benomyl in 1995 and our experimental results showed that Benomyl can reduce soybean white mold damage by 50 percent.
This article originally appeared on page 19 of the IC-476 (3) -- March 25, 1996 issue.