White Mold


Irregular shaped dark survival structures (sclerotia) produced by the white mold fungus. Sclerotia can be formed within the stem or on the stem surface. Irregular shaped dark survival structures (sclerotia) produced by the white mold fungus. Sclerotia can be formed within the stem or on the stem surface.
White cottony mycelial growth on a stem infected with white mold. White cottony mycelial growth on a stem infected with white mold.

White mold, or Sclerotinia sclerotiorum, 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 fungicide soil drench in the affected areas before planting.  Follow label directions.


Precautionary Statement: Utah State University and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks, and not all are registered for edible crops. “Restricted use” pesticides may only be applied by a licensed applicator. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed in this publication.