How to Manage Pests
UC Pest Management Guidelines
Early symptoms of phoma basal rot on romaine lettuce consist of yellowing and wilting of lower leaves. Affected plants may later show an uneven appearance because one side of the foliage grew normally while the diseased side of the plant remained stunted and short. As plants mature, the entire plant becomes stunted and eventually wilts and collapses. The wilting and collapsing symptoms are somewhat similar to those caused by Sclerotinia minor (causal agent of lettuce drop) and Botrytis cinerea (causal agent of gray mold of lettuce).
Examination of affected plants reveals distinct, black cavities (sunken areas) on the crown and upper taproot. These cavities extended far into the crowns and roots, resulting in extensive weakening of the plant; these plants could be broken off easily at ground level. The black cavities are characteristically firm and lack any sign of fungal growth, sclerotia, or spores. This disease is primarily found on romaine; other lettuce types are susceptible but typically are infected at low incidences.
Phoma exigua is a soilborne fungus. Phoma basal rot was first documented in California in 2000 and can cause significant losses in romaine plantings. The disease has been found in coastal counties, but has not yet been confirmed in Huron or desert regions. Little is known about the epidemiology of this disease. A similar disease on greenhouse grown lettuce was reported in 1965 in the United Kingdom.
Avoid planting romaine cultivars in fields having histories of this problem. Protectant fungicides applied after thinning the crop can reduce Phoma basal rot problems. Treat before plants become too large and direct applications to the base of the young plants.
UC IPM Pest Management Guidelines: Lettuce