White mold (a.k.a. Sclerotinia stem rot) is caused by the soilborne fungus Sclerotinia sclerotiorum. Up until a few years ago, white mold showed up usually only in very localized areas where crops such as green beans, sunflowers, or canola were grown. This tells us that these crops are very good hosts for the fungus, but 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 is hard to miss if you examine the plant. 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 or 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, which is right around the corner.
The sclerotia are the survival structure of the fungus. They can survive in the soil for many years. Sclerotia near the soil surface produces small, mushroomlike structures called apothecia after prolonged wet weather. Apothecia produce windborne spores that then can infect the flowers or the stem or branches.
This fungus actually is a necrotroph; in other words, it infects dead tissue. It infects the 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 produces a toxin that actually kills the tissue in advance of the 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 (here I mean the ability of the plant to yield well despite a susceptible type infection in comparison to a susceptible variety). There are also some partially resistant varieties (that is, reduced rate of disease development in varieties), such as Dassel, Northrup King NK S1990, Corsoy 79, Asgrow A-2506, and DSR-173. Topsin M is registered to control white mold, but it must be applied before infection. So keep an eye on conducive weather conditions. Much work has been done recently at the University of Illinois on management of the disease. Tolerant and partially resistant varieties have been identified and fungicide seed treatments have been shown to control seed transmission. The results of this work are reported on the Web at http://www.whitemold.uiuc.edu/.--Suzanne Bissonnette