Soybean white mold update

In 1994, Iowa growers, especially those in northern Iowa, witnessed widespread white mold. Most of the fields that had white mold in 1994 are in soybeans this year. The good news is that July weather has not been conducive for the occurrence of soybean white mold, although we have observed activities of white mold fungi in our research plots.

White mold apothecia.

Soybean white mold, also known as Sclerotinia stem rot, is a disease caused by a soilborne fungus. At soybean flowering stages, the fungus can germinate from a special structure called sclerotium forming apothecia, a mushroom-like structure (click on the small picture above). Infection takes place when spores are projected into the air from apothecia and land on flowers where the fungus enters the plant. This week, we have observed the production of these "mushrooms" in a commercial field.

Scouting for these small "mushrooms" is not recommended because it may be difficult for an untrained person to identify the "mushrooms." Detection of a large amount of these white mold "mushrooms" is an early warning for the occurrence of white mold if weather in August becomes wet and cool. If you are interested in finding them, check fields that had severe white mold in the last two years. The white mold "mushrooms" also can be found from corn fields that had been rotated with white mold infested soybeans (click on the small picture below).

White mold apotecia in a cornfield.

When the disease risk is high, the use of fungicides is a control option. Unfortunately, no threshold number is available to guide the fungicide applications. Supported by check-off dollars, we are trying to determine the relationship between the number of apothecia and the number of diseased plants.

According to a publication from the University of Wisconsin. Benomyl (also called Benlate) and Topsin (also called thiophanate) provide control for soybean white mold if applied at 25-50 percent flowering. Topsin has been registered for control of soybean white mold. Benomyl is not specifically registered for soybean white mold. Benomyl, however, is registered for control of other soybean diseases and to control dry bean white mold, which is caused by the same fungus as soybean white mold. We tested Benomyl in 1995 and our results showed that Benomyl can reduce soybean white mold damage by 50 percent.

This article originally appeared on page 149 of the IC-476(21) -- August 5, 1996 issue.

Updated 08/04/1996 - 1:00pm