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. Plant collapse usually occurs when lettuce is near maturity. Sclerotinia sclerotiorum can also infect lower leaves and stems, causing symptoms similar to those of S. minor. In addition, S. sclerotiorum has an aerial spore that 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. Wet soil conditions favor disease development of both species. In California, S. minor does not have a spore-producing stage. Symptoms caused by Sclerotinia species could resemble Verticillium wilt symptoms. For S. minor, crop rotations and deep inversion plowing may be helpful in reducing incidence of lettuce drop.