Clonal Population Structure of Xanthomonas campestris and Genetic Diversity Among Citrus Canker Strains. D. W. Gabriel. Plant Pathology Department, University of Florida, Gainesville, FL 32611. J. E. Hunter(1,2), M. T. Kingsley(1), J. W. Miller(3), and G. R. Lazo(1,4). (1) Plant Pathology Department, University of Florida, Gainesville, FL 32611; (2) Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456; (3) Division of Plant Industry, Florida Department of Agriculture, Gainesville, FL 32611; and (4) Thimann Laboratories, University of California, Santa Cruz, CA 95064.. MPMI 1:59-65. Accepted 6 November 1987. Copyright 1988 The American Phytopathological Society.

Restriction fragment length polymorphism (RFLP) analyses of the bacterial plant pathogen Xanthomonas campestris reveal a generally clonal population structure, with each pathovar comprised of one or more clonal groups (Lazo, G. R., Roffey, R., and Gabriel, D. W. 1987, Int. J. Syst. Bacteriol. 37:214-221). Those pathovars with a narrow host range exhibit little polymorphism within clonal groups, and those with a wide host range exhibit moderate polymorphism. The population structure is likely the result of linkage disequilibrium between the RFLP marker loci and highly selected virulence factors. RFLP analyses of X. c. pv. citri strains revealed three clonal groups. One of the clonal groups corresponded to the previously described X. c. pv. citri “A” strains; another included the previously described “B,” “C,” and “Mexican bacteriosis” strains. The third group consisted of strains recently discovered in Florida citrus nurseries. These showed moderate polymorphism between strains of the group, a characteristic of strains included in pathovars with a wide host range. Furthermore, strains of this new group appeared to be related to X. c. pv. alfalfae by RFLP analyses, and strains of one subclone of the group had a host range similar to that of X. c. pv. alfalfae in limited pathogenicity tests. We propose that: 1) diverse X. campestris strains may have been selected on citrus because they independently carried genes with selective value on these hosts; 2) related strains may have different combinations of virulence factors conditioning host range specificity; and 3) RFLP analyses are a generally useful method to indicate wide versus narrow host range specificity of pathogenic populations without pathogenicity tests.