Issue No. 14, Article 5/June 30, 2006
White Mold of Soybean
White mold (aka Sclerotinia stem rot) is caused by the soilborne fungus Sclerotinia sclerotiorum. Until a few years ago, white mold usually showed up only in very localized areas where crops such as green beans, sunflowers, or canola were grown. This tells you that those crops are very good hosts for the fungus, but the whole story is that there are over 300 hosts for white mold.
The first symptoms of the disease are usually wilted leaves on the top of the plant. Very obvious white mold can be seen growing on the stem, branches, and pods. The white mold is very frequently seen on the stem near the soil line, extending down from a node. It is quite a dramatic sign, and hard to miss if you examine the plant.
White mold mycelia on soybean stems (photo courtesy of Department of Crop Sciences, UIUC).
As the disease progresses, the fungus produces hard black survival structures called sclerotia. They are about the size of a soybean seed, although more irregularly shaped (reminiscent of a rat dropping, actually). They are produced in and on the stem, and can even be found in pods. Typically, the fungus girdles the stem and the plant dies prematurely; seeds may be infected as well.
Why talk about a stem disease now? Because white mold typically infects at flowering, and that time is right around the corner. Bill Lindenmier, Extension crop systems educator in Ogle County, reports that several fields in his area are under close watch for white mold, because they have a history of the disease and weather conditions have been fairly conducive.
The sclerotia are the survival structure of the fungus. They can survive in the soil for many years. Sclerotia near the soil surface will produce small mushroom-like structures, called apothecia, after prolonged wet weather. Apothecia produce windborne spores that then can infect the flowers or the stem or branches.
This fungus is a necrotroph, meaning it infects dead tissue. It infects flowers as the petals die or other plant tissue that may have a dead area, such as a wounded stem. After a successful infection, the fungus actually produces a toxin that kills the tissue in advance of the fungus. (Okay, it's a little gruesome, but it's a successful fungus.)
Management recommendations have changed somewhat in the last few years. Reducing humidity in the canopy by such things as wide row spacing and planting nonlodging varieties may help reduce infection. Not planting bin-run seed from infected fields is a very important method of management, because infected seed and the sclerotia spread the disease.
Some varieties show tolerance to the disease, by which I mean the plant can yield well despite a susceptible type infection in comparison to a susceptible variety. There are also some partially resistant varieties (i.e., reduced rate of disease development in varieties).
Topsin M fungicide is registered to control white mold, but it must be applied before infection. So keep an eye on conducive weather conditions. For information on varieties, check out the disease ratings for white mold on the VIPS (Varietal Information Program for Soybeans) Web page.--Suzanne Bissonnette