How to Manage Pests
UC Pest Management Guidelines
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.
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.
Organically Acceptable Methods
UC IPM Pest Management Guidelines: Lettuce