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The Genetics of Corky Root Resistance in Lettuce. P. R. Brown, Department of Vegetable Crops, University of California, Davis 95616; R. W. Michelmore, Department of Vegetable Crops, University of California, Davis 95616. Phytopathology 78:1145-1150. Accepted for publication 11 March 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1145.

Corky root is a serious disease of lettuce (Lactuca sativa) that causes deterioration in the root system of infected plants. A bacterium was confirmed to be a major etiological agent by fulfilling Koch’s postulates. Five hundred and fifty-five accessions of Lactuca spp. were screened for the lack of corky root symptoms after being challenged by this bacterium. Seedlings were grown in the greenhouse and inoculated at the second or third true-leaf stage with a bacterial suspension. Plants were evaluated for symptom expression 30 days after inoculation. Highly resistant plants exhibited minimal root discoloration with no cracking of the taproots. Susceptible plants showed dark brown discoloration with cracking extending into the cortical regions. High levels of resistance were identified in 19 accessions of L. sativa, L. serriola, L. saligna, L. dentata, L. virosa, and Lactuca sp. Data from F1, F2, F3, three-way crosses, and selfed backcross progenies demonstrated that this resistance was conferred by a recessive allele at a single locus (cor). Allelism tests indicated that the same locus determined resistance in most resistant accessions evaluated.

Additional keywords: disease screen, germ plasm, Lactuca.