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.
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