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White Mold

August 2, 2005
It seems odd to be talking about a wet-weather disease when some areas of the state are still experiencing serious drought. But it has appeared in wet areas in Illinois and may occur where plants are heavily irrigated. Common names include white mold, watery soft rot, cottony rot, and Sclerotinia disease, but white mold is the most common.

The causal fungus--Sclerotinia sclerotiurum, S. minor, or S. trifoliorum--is favored by wet weather, as some areas have just experienced. These fungi can infect over 370 species of plants. In Illinois we might see white mold on such vegetables as green bean, carrots, tomatoes, and peas. We have seen it on ginger and on soybeans at the Plant Clinic this year. In addition, look for it on aster, begonia, columbine, dahlia, delphinium, larkspur, peony, snapdragon, and other garden plants. It also occurs on some woody plants, but that does not seem to be a problem in Illinois.

Symptoms of white mold may occur on any above- or belowground portion of the host plant. You’ll see the problem where plantings are dense and the soil is wet. Look for brown lesions, especially on the stems, which become covered with a fluffy white mycelial growth (mold). This growth is very white and wispy compared to the dense gray mold of Botrytis. As the disease develops you will see large black resting bodies of the fungus that look much like irregular rabbit pellets. They grow in or on the plant tissue. Although black on the outside, these resting bodies are white internally. As the infected plant tissue dries down, the white mycelium may dry up, but the canker will still be bleached or tan, as if grabbed by a very hot hand or glove.

This disease, including a list of susceptible plants, is discussed in Report on Plant Disease 1008, available in Illinois extension offices and at http://www.ag.uiuc.edu/%7Evista/horticul.htm (Sclerotinia Disease, White Mold or Watery Soft Rot, RPD No. 1008). Correct identification is important to disease control because the resting stage of white mold can remain in the soil for many years and will cause problems at that location on many other hosts. The sclerotia in the top 2 to 3 inches of soil germinate each year given a favorable environment. There is no cure for plants once they are infected. Many plants are hosts of this disease, so choose a nonhost crop for the infected area if plants decline.

Author: Nancy Pataky


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