I've noticed several cases of White Mold (Sclerotinia sp.) over the past few days. While at a field day last week, I noted several different annual flowering plants dying as a result of this fungus. And another email today concerning green beans looked like a similar problem. This fungus infects more than 370 species of plants, including many common landscape annuals and perennials.
White Mold on Green Beans- email submission
According to the University of Illinois Plant Clinic the first symptom usually noticed on infected plants is wilting. Upon closer inspection, a white, fluffy mycelium can be seen on infected plants. This white growth soon (in 7 to 10 days) develops into sclerotia, fungal resting structures specialized to withstand harsh environments such as winter. The sclerotia initially are white but mature as hardened, black structures ranging in size from 1/16 to 1/2 inch. At the field day, we cut open several of the dried stems of infected stems and found sclerotia inside petunias and zinnias.
It is these sclerotia that germinate and cause new infections in future years, so diseased plants should be immediately removed from the garden. Sclerotia can overwinter in the soil for several years.
The Plant Clinic also notes that other characteristic symptoms caused by this disease are water-soaked, brown lesions on stems. The characteristic white mold develops only during wet weather and often disappears as the plant dries down, leaving tan (bleached-looking) lesions and stems. For that reason, diagnosis can be tricky. Look for those sclerotia for confirmation.
There is no control for white mold and infected plants should be removed from the garden. Report on Plant Disease, no. 1008, Sclerotinia Disease, White Mold, or Watery Soft Rot, provides a full list of susceptible plant species to avoid planting in that spot next year.
The likely reason we're seeing more white mold this year is the cool and wet conditions we received in early summer. Initial infections need moisture and relatively cool temperatures to occur which we had had both of earlier this summer.
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