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Virulence of Corynebacterium michiganense Isolates on Lycopersicon Accessions. B. D. Thyr, Plant Pathologist, Plant Science Research Division, ARS, USDA, Cheyenne Horticultural Field Station, Cheyenne, Wyoming 82001; Phytopathology 62:1982-1084. Accepted for publication 15 April 1972. DOI: 10.1094/Phyto-62-1082.

The virulence of seven isolates of Corynebacterium michiganense, collected from six geographical areas of the United States, was measured on five accessions of Lycopersicon esculentum, one of L. pimpinellifolium, and one of L. hirsutum. Data on stunting of host plants and on vascular discoloration were subjected to analysis of variance which showed significant differences among isolates and accessions. Highly significant interaction occurred when all three species of Lycopersicon were tested; but only borderline interaction occurred with internal discoloration, and none with stunting, when only L. esculentum accessions were subjected to analysis. Evidence supports the conclusion that resistance to bacterial canker in the Lycopersicon accessions tested is horizontal, and that isolates differ in degrees of aggressiveness. To obtain adequate levels of resistance in tomato lines, a backcross breeding program is suggested. Also, highly aggressive pathotypes should be employed in a breeding program to maintain an acceptable level of resistance.