The University of Arizona, College of Agriculture and Life Sciences
Lettuce Bottom Rot (Rhizoctonia solani)
Life Cycle: Bottom rot prefers warm, wet conditions. It overwinters in the soil in decomposing organic matter, persisting mostly as mycelium. Bottom Rot can form very small sclerotia (tight masses of mycelium that can live for longer periods of time than mycelial strands). Bottom Rot in the soil is attracted to a crop when it is transplanted. It is possible that some strains of Rhizoctonia solani prefer certain crops.
Plants affected: A very wide host range to include, but not limited to, lettuce, crucifer (cole) crops, endive, escarole, pepper, eggplant, radish, turnip, cucumber and many other fleshy plants.
Symptoms: Lettuce seedlings are most susceptible at transplant time. The seedlings will show signs of damping off, coupled with brown lesions on stem and leaves, and rotting root systems. Older plants will rot from leaves touching the soil. From there the disease moves into the bottom leaves and crown causing brown lesions to appear. The lesions may appear to be furry brown spots with brown liquid seeping from the leaf tissue. Secondary pathogens can move in causing severe rot throughout the head.
Control: Keep greenhouses, flats, and benches where transplants are grown disinfected and clean throughout the year. Clean tools, hands, and field equipment to avoid spreading the disease. Crop rotation is very important since bottom rot can live in soil organic matter for long periods of time without a crop host. Cotton and alfalfa harbor Bottom Rot so avoid these crops in rotation where this disease is prevalent. Be certain that crop residues that were plowed under are fully decomposed before planting a new crop into the field. Plant lettuce crops in well-drained soil in raised beds. Plant seedling transplants high with about 1/4 in. of the root ball above the soil line. Do not over water the crop. If Bottom Rot is known in fields where planting, utilize erect cultivars of lettuce to avoid leaves touching the soil. Use Oxidate® (#9719) to disinfect greenhouses, benches, and plug flats. Where Bottom Rot is a problem, treat field soils with Actinovate® (#9804 2-oz, #9900 18-oz) in the off-season to build up beneficial bacterium colonies. In small plots and home gardens, Actinovate® can be used as a spray on the plants themselves as a preventative. Read all product labels in full and follow label instructions as specified for that particular product.