Sclerotinia leaf drop of Lettuce

Symptoms. Two species of Sclerotinia infect lettuce in California and cause a disease known as lettuce drop: in coastal growing areas Sclerotinia minor is the primary species of importance, while in other areas S. sclerotiorum is more prevalent. Sclerotinia minor only infects the stems and leaves in contact with the soil. Once infection takes place, the fungus will cause a brown, soft decay that eventually destroys the plant crown tissue. Older leaves then wilt and later the entire plant will wilt and collapse, making it unharvestable. Plant collapse usually occurs when lettuce is near maturity. Profuse amounts of white mycelia and small (up to 0.125 inch or 3 mm), black, hard, resting bodies (sclerotia) form on the outside of the decayed crown.
Sclerotinia sclerotiorum can also infect lower leaves and stems, causing symptoms similar to those of S. minor. In addition, S. sclerotiorum has an aerial sporevthat can infect any of the upper leaves. Spores usually infect damaged or senescent tissue when the weather is cool and moist. Infection results in a watery, soft rot that is accompanied by white mycelial growth and formation of sclerotia. Sclerotinia sclerotiorum forms sclerotia that are larger (0.25–0.50 inch) than those of S. minor.
Comments on the disease. Sclerotia of both species enable the pathogens to survive in the soil for 2 to 3 years without susceptible hosts. Wet soil conditions favor disease development of both species. For S. sclerotiorum, cool and moist conditions are necessary for development of the fruiting structure (apothecium) that produces the airborne spores. In California, S. minor does not have a spore-producing stage. Symptoms caused by Sclerotinia species could resemble Verticillium wilt symptoms. The recent use of wider, 80-inch beds for lettuce production may cause lettuce drop from S. minor to be more severe because of increased bed moisture.