White mold is caused by the fungal pathogen Sclerotinia sclerotiorum. It is most common on winter crops such as cabbage, collards, kale, and other winter-time vegetables. Disease development is favored by wet, cool conditions. Damage from white mold can occur in the field, in storage, in transit, and in the marketplace. The pathogen has a wide host range that includes over 350 species of plants including popular vegetables such as tomato, pepper, and green beans. The disease has also been a sporadic problem on tomatoes grown in high tunnels and greenhouses during the winter in Alabama.

Infected Plants

White mold on a cabbage head.

Most cabbage plants are infected in the field after midseason. Infections may occur at the soil line at the stem, at the base of leaves, or where foliage contacts infested soil. Symptoms start as tan, water-soaked, circular areas which become covered by white, cottony fungal growth. The host tissue becomes soft and watery as the disease progresses as the fungus overtakes the entire head.

In crops like tomato and pepper, white mold is not always observed externally on infected plants, but more commonly inside the blighted stem where sclerotia form. Sclerotinia sclerotiorum survives over the summer and long term in soil as sclerotia. Sclerotia are large, black, seed or stone-like structures up to a centimeter in diameter that can be observed on the mold-like fungal mycelium on infected plant tissue. Sclerotia may also be visible in the soil around the base of heavily infected plants. Finding sclerotia in rotting cabbage heads or other plant parts is a simple way to diagnose white mold.

Management

Fungicides can be used to manage the disease. However, control is compromised by the limited number of fungicide applications that can be made per season. Crop rotation helps reduce white mold inoculum and consists of rotating infested fields with non-hosts such as grains and grasses, including sweet corn. Cabbage rows should be planted in the direction of the prevailing winds to promote air movement to assist with drying of foliage and the soil. Wide row spacing (36 inches) will also reduce moisture in the plant canopy. Cabbage heads need to be handled gently as any type of wound or bruise can be a point of colonization by the pathogen. In small-scale systems, it would be wise to remove affected plants so that sclerotia are not left to survive in the soil for the following crop. Deep till (plow) affected fields immediately after harvest to bury sclerotia and speed up the decomposition.