Black Root Rot, Caused by Thielaviopsis basicola, on Tomato Transplants in California

In 1997, greenhouse-produced transplants of tomato (Lycopersicon esculentum) exhibited stunting, yellowing of leaves, and lack of vigorous growth. Roots of affected plants had numerous small (2 to 10 mm long), brown lesions. Isolations from symptomatic roots onto acidified potato dextrose agar and TBM-V8 medium (1) consistently resulted in gray fungal colonies that formed catenulate, cylindrical, hyaline endoconidia and catenulate, subrectangular, thick-walled, dark aleuriospores. The fungus was identified as Thielaviopsis basicola (Berk. & Broome) Ferraris based on colony characteristics and conidial morphology. Pathogenicity was tested by producing endoconidial suspensions of representative isolates and applying them as root drenches to 2-month-old tomato (cv. Early Girl) plants in soil-less, peat-based potting mix. Sterile, distilled water was applied to control plants. After 14 days in a greenhouse, symptoms similar to those originally observed developed and the pathogen was reisolated from lesions on the roots. Control plants developed no disease symptoms. The test was repeated and the results were similar. This is the first documentation of black root rot caused by T. basicola on tomato transplants in California. Disease incidence reached as high as 50 to 60% in certain plantings. Reference: (1) J. N. C. Maduewesi et al. Phytopathology 66:526, 1976.