How to Manage Pests
UC Pest Management Guidelines
Under high humidity the fungus produces a mass of cottony hyphae or mycelia on the soil or plant surface. Later, large (5–10 mm), black sclerotia (hard, dark masses of hyphae) are formed on infected plant parts. Frequently the sclerotia are found inside dead 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 smaller than those of Sclerotinia; it also doesn't produce 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 primarily a disease of vegetables, such as beans, carrots, celery, and lettuce. The fungus often infects the plant near the soil line but infections can occur on any aboveground part. 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.
Sclerotinia sclerotiorum does not produce 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, the sclerotia germinate to form apothecia (saucer-shaped, dime-sized structures on stalks) that produce spores. The spores 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 occurs. 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. Infections frequently occur at the soil level because plants can be infected directly by sclerotia in the soil that germinate to produce vegetative strands (hyphae).
Protective fungicides, as well as steam, solarization, or fumigation can be helpful. Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically treat growing medium. 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.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries