Utah Plant Disease Control No. 19
Revised February 1993

White Mold of Ornamentals

White mold, a disease common to many vegetable crops, is also a problem on ornamental flowers, causing them to wilt and die. Many common bedding plants, including petunia, zinnia, and marigold, are affected.

SYMPTOMS

The fungus Sclerotinia sclerotiorum invades the stems of plants near the soil line and initially causes brown, water-soaked lesions. White cottony mycelium grows quickly around the infection site, spreading rapidly in one or two days. Leaves and flowers wilt and die as the stem rots. Inside the stem or on the surface of the affected area, hard irregular, black fungal bodies called sclerotia may be formed. They range in size from about 1/8 to 1/3 inch or larger. These sclerotia fall to the ground and are the survival structures for the fungus during cold or dry weather. They can persist in the soil for several years and, therefore, the disease reappears in the same area year after year. Warm, wet conditions favor germination of the sclerotia and development of the disease.

CONTROL MEASURES

Controlling white mold is best accomplished by prevention. The disease is most serious when plants are closely spaced, resulting in high relative humidity. Therefore, plants should be widely spaced and furrow irrigated to prevent wet foliage. Irrigating in the morning rather than evening will provide conditions for the foliage to dry out.

White mold survives from year to year by sclerotia in soil and plant debris. Therefore, reducing the populations of sclerotia each season will help control the disease in subsequent years. Infected plants should be culled and destroyed by burning or discarding; composting of this material is discouraged because most compost piles do not reach and maintain temperatures high enough to kill sclerotia. Deep plowing which inverts the soil layers will also help control white mold because only those sclerotia in the top 2 - 3 inches of the soil will germinate.

In areas where white mold is a perpetual problem, chemical control measures may be justified. If
you know or suspect the disease was present in past years, apply a soil drench in the affected areas before planting. Some effective drenches are listed in the table. Apply one of the foliar sprays as a preventive spray in areas where the disease has been a problem. Follow label directions.

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

Listing of commercial products implies no endorsement by the authors or the Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Persons using such products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its prope
use. Always read and follow the label.

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