How to Manage Pests
UC Pest Management Guidelines
Phoma Basal Rot
Pathogen: Phoma exigua
(Reviewed 4/17, updated 4/17)
In this Guideline:
Symptoms AND SIGNS
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 lettuce drop and Botrytis crown rot.
Examination of affected plants reveals distinct, black cavities (sunken areas) on the crown and upper taproot. The cavities extend 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.
Comments on the Disease
Phoma exigua is a soilborne fungus. Phoma basal rot was first documented in California in 2000 and caused significant losses in romaine plantings. The disease has been found in coastal counties, but has not yet been confirmed in San Joaquin Valley or desert regions. Little is known about the epidemiology of this disease.
Avoid planting romaine cultivars in fields having histories of this problem.
UC IPM Pest Management Guidelines: Lettuce
S. T. Koike, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to Diseases:R. M. Davis, Plant Pathology, UC Davis