Diseases of Lettuce

Lettuce Drop There are several diseases of lettuce in which the leaves turn brown and mushy at the base. One disease is called lettuce drop, caused by the fungus Sclerotinia sclerotiorum.   It commonly occurs during cool, moist weather.  Symptoms are seen on the lower leaves as the plant approaches maturity.  The lower leaves that are in contact with the soil will wilt. Then they will develop a slimy rot, and the plant collapses.  An interesting feature is the appearance of a white, cottony fungal growth.  Tiny, hard, black ‘seeds’ called sclerotia may also be seen among the white threads. Control is difficult: 1) keep the leaves as dry as possible,  and 2) use low levels of fertilizers since succulent growth is more favorable to the disease. 3) The fungus can remain in the soil for two or three years, so crop rotation is important. Rotating with corn and onions are two options. Other crops that are affected by this fungus include beans, carrots and celery, as well as cucurbit and solanaceous crops. Bottom Rot In addition to lettuce drop, a fungal disease called bottom rot (Rhizoctonia solani) also produces symptoms of a slimy rotting of the lower leaves. Rust colored spots on the leaf petioles and midribs can also be seen. The entire plant will eventually decay and die. The disease is more severe under moist, warm weather conditions. Again, control is difficult. Keep leaves as dry as possible and fertilize sparingly. Planting on raised beds can help to keep the plants free of disease. Gray Mold Another fungal disease called gray mold (Botrytis cineria) occurs during cool, moist weather. Symptoms include a brown, slimy decay on the undersides of leaves. A fuzzy gray growth can often be seen. In general, various bacteria also infect lettuce plants. They enter the plant tissue through stomata or wounds. Cool and moist conditions favor the disease. Symptoms include brown spots, a slimy rot and wilting.  These diseases can result in the decay of the entire head.  Applications of copper based bactericides are recommended.