How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Damping-off. This soil fungus, along with Pythium species and Thielaviopsis basicola, can sometimes cause damping-off of lettuce. Seedlings may be killed before or just after emergence. Infected seedlings exhibit decayed roots and brown lesions on stems. When seedlings are infected, the stem tissue collapses and plants fall over and die.

Bottom rot. In some regions, particularly the San Joaquin Valley and desert areas, R. solani infects lettuce plants as the heads begin to form. Brown, sunken lesions form on the midribs that are in contact with the soil. As the disease progresses, the fungus will infect leaves inside the head. Soft rots, due to secondary decay organisms, will often develop on bottom rot infection sites, resulting in collapse of the head.

COMMENTS ON THE DISEASE

Bottom rot is most damaging on early season lettuce (on crops that mature from late November through January). Rhizoctonia solani is a soilborne fungus that survives for indefinite periods of time. Warm, moist weather favors bottom rot development. No fungicides are effective.

MANAGEMENT

Cultural Control
Avoid planting lettuce in fields having large amounts of undecomposed plant residues, since this probably enhances R. solani growth and survival in soils. Avoid planting lettuce in fields that have a history of bottom rot problems. No other measures are recommended.

Organically Acceptable Methods
Cultural controls are organically acceptable.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Diseases
S. T. Koike, UC Cooperative Extension, Monterey County
R. M. Davis, Plant Pathology, UC Davis

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