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Corky Root of Tomato in California Caused by Pyrenochaeta lycopersici and Control by Soil Fumigation. R. N. Campbell, Professor, Department of Plant Pathology, University of California, Davis 95616. V. H. Schweers, Farm Advisor, Tulare County Cooperative Extension, Visalia, CA 93277, and D. H. Hall, Extension Specialist, Department of Plant Pathology, University of California, Davis. Plant Dis. 66:657-661. Accepted for publication 19 June 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-657.

Corky root occurs on processing and fresh market tomatoes (Lycopersicon esculentum) in scattered fields in northern and central California. The primary pathogen was confirmed to be Pyrenochaeta lycopersici, which was also recovered from nightshade (Solanum nigrum) but not from other plants in infested fields. Colletotrichum coccodes was isolated infrequently. Corky root severity declined as the transplanting date was delayed and the soil became warmer in a trial involving plants inoculated in the greenhouse and transplanted to the field and noninoculated plants transplanted into infested soil. Yield loss estimates of up to 70% were made by growers of processing tomatoes, and losses of the same magnitude occurred in fumigation experiments with fresh market tomatoes. In field trials, chloropicrin alone or in combination with methyl bromide (117-168 kg/ha injected and covered with a tarp) gave good disease control and large yield responses. Methyl bromide (262 kg/ha), chloropicrin (168 kg/ha without a tarp), and a metham sodium drench (935 L of 33% a.i./ha) gave intermediate disease control and yield responses.