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Mapping of Quantitative Trait Loci Controlling Resistance of Brassica oleracea to Xanthomonas campestris pv. campestris in the Field and Greenhouse. L. E. A. Camargo, Former graduate student, Department of Agronomy, University of Wisconsin, Madison 53706, Current address: Department de Fitopatologia, Universidade de São Paulo, Cx. P. 9, 13400, Piracicaba, SP, Brazil; P. H. Williams(2), and T. C. Osborn(3). (2)Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706; (3)Professor, Department of Agronomy, University of Wisconsin, Madison 53706. Phytopathology 85:1296-1300. Accepted for publication 17 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1296.

Despite the existence of resistant cultivars, little is known about the genetic control of resistance of Brassica oleracea to Xanthomonas campestris pv. campestris. Quantitative trait loci (QTL) controlling juvenile and adult plant reactions of B. oleracea to X. campestris pv. campestris were mapped using restriction fragment length polymorphism loci and disease ratings of F3 families from a resistant cabbage × susceptible broccoli cross. In the greenhouse, 2.5-week-old, plants were screened by inoculating the pathogen directly into the veins and measuring the diseased leaf area. In the field, 11-week-old plants were screened by inoculating intact plants and assessing the symptoms using an interaction phenotype scale. Two genomic regions on linkage groups 1 and 9 were associated with both young and adult plant resistance. These regions also were associated with variation for petiole length. Two additional QTL on linkage group 2 that were associated only with young plant resistance were found. For one of these, alleles from the resistant parent contributed to greater susceptibility. These results suggest that plants selected for resistance based on screening young plants in the greenhouse should exhibit adult plant resistance.