Symptoms and Signs
Two species of Sclerotinia cause disease on cole crops. Sclerotinia minor only infects stems or leaves in close contact with the soil. Once infection takes place, water-soaked, brown, necrotic areas develop on these structures. The necrotic areas rapidly turn into soft, watery rots. Plants then wilt and collapse. Profuse amounts of white mycelial growth and numerous small (up to 0.125 inch or 3 mm), black, hard resting bodies called sclerotia, form on the outside and inside of the stems. Sclerotinia sclerotiorum can also infect lower leaves and stems, causing the same type of symptoms as S. minor. In addition, S. sclerotiorum forms tiny, brown, mushroomlike bodies (apothecia) that release aerial spores, which can infect any of the upper leaves and flowers. If conditions are right, these spores cause a watery, soft rot of these tissues as well. Sclerotinia sclerotiorum forms sclerotia that are larger (0.25–0.5 inch) on average than those of S. minor.
Comments on the Disease
Sclerotia of both species enable the pathogens to survive in soil for a number of years without susceptible hosts. Wet soil conditions favor disease development. On crucifers, S. sclerotiorum tends to be the more important pathogen, while S. minor is only found infrequently. For S. sclerotiorum, cool and moist conditions are necessary for development of and infection by the spores. The aerial spores usually only infect injured or senescing leaves and flowers.
Crop rotations and deep inversion plowing may be helpful in reducing severity of S. minor infections. Deep plowing or soil inversion reduces the number of sclerotia of S. sclerotiorum in the particular field, but has no effect on incoming aerial spores from surrounding fields and from long distances.
Chemical treatments are usually not required for Sclerotinia diseases in fresh market cole crops, but may be necessary in seed production fields. Currently only iprodione (Rovral) is registered for use on broccoli.