Root Rot of Lettuce Incited by Pythium polymastum. D. L. Coplin, Assistant Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster 44691. A. F. Schmitthenner, and W. L. Bauerle, Professor, Department of Plant Pathology, and Associate Professor, Department of Horticulture, respectively, Ohio Agricultural Research and Development Center, Wooster 44691. Plant Dis. 64:63-66. Accepted for publication 12 March 1979. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-63.

A Pythium sp. with large spiny oogonia has been consistently associated with root rot and decline of lettuce cultivar Grand Rapids H5-4 in northern Ohio. The disease is characterized by severe stunting and rosetting followed by wilting and death of the plant; lateral roots and the cortical tissue of the tap root are severely rotted. The lettuce root isolates are morphologically similar to P. polymastum and can be identified by their large oogonia (average diameter 45 µm) with conical spines ranging from 3 to 7 µm in length. The fungus is slow growing and difficult to isolate. In pathogenicity tests on lettuce, the isolates with spiny oogonia were consistently more virulent than other Pythium spp. isolated from lettuce roots. In greenhouse experiments, escarole, endive, and the lettuce cultivars Salad King and Romaine were resistant to root rot and Grand Rapids H5-4, Slobolt, and Boston were susceptible.

Keyword(s): Lactuca sativa, soilborne pathogens.