Symptoms and Signs
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.
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.
To prevent Rhizoctonia diseases:
- Avoid planting lettuce in fields having large amounts of undecomposed plant residues, since this probably enhances R. solani growth and survival in soils.
- Plant lettuce in fields that have no history of bottom rot problems.
No fungicides are effective.