Cottony Rot

Under high humidity the fungus produces a mass of white cottony mycelia on the soil or plant surface. Later, large (1/4 to 1 inch long), black sclerotia (hard, dark masses of hyphae) are formed on and inside infected plant parts. Frequently the sclerotia are found inside hollow stems. Plant tissues killed by the fungus often take on a bleached appearance. Gray mold (Botrytis cinerea) causes a similar bleaching and also has black sclerotia, but they are much smaller than those of Sclerotinia. Additionally, Botrytis produces gray mycelium and spores instead of the mass of white, cottony growth that Sclerotinia does.

Cottony rot, also called Sclerotinia rot or white mold, affects many kinds of plants. It is also a disease of vegetables, such as beans, carrots, celery, and lettuce. Moisture and high humidity are necessary for development of the disease and this is one reason the disease is found lower in the plant canopy. Infection can either be soilborne or airborne.

Sclerotinia sclerotiorum does not produce asexual conidia. Sclerotia formed by the fungus undergo a dormant period that is broken by low temperatures (optimal is 56° to 59°F) and high soil moisture. In fall and spring, when temperatures are in the optimal range, sclerotia germinate and can infect the plant near the soil line either directly by producing vegetative strands (hyphae) or by forming apothecia (saucer-shaped, dime-sized structures on stalks) that produce ascospores (sexual spores). Ascospores are discharged forcibly into the air and are carried by air currents. They do not directly infect healthy tissue, but if they land on injured tissue in the presence of moisture, infection can occur on any aboveground part. Flower petals of many plants are susceptible. Foliage may become infected if there is an injury or if the tissue is senescent. If diseased tissue comes in contact with healthy tissue, the fungus can invade the healthy tissue.

Protective fungicides, as well as steaming (at 140°F for 30 minutes), solarization (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or fumigation of the growing medium can be helpful. For flower production in open fields, solarization in warmer climates has been successful for control of Sclerotinia diseases in many crops. Solarization and steaming are acceptable for organic production. In open fields airborne spores can blow in from outside the field, so soil treatment may be limited in its effectiveness.