Race shift in Xanthomonas campestris pv vesicatoria within a season in field-grown pepper

Race shifts in Xanthomonas campestris pv. vesicatoria, (proposed: X. axonopodis pv. vesicatoria), the pepper bacterial spot pathogen, were studied in the field during 1993 and 1994. In 1993, Early Calwonder (ECW) pepper plants inoculated with rifampicin-resistant race 1 strain Xcv 33(rif), which carries avirulence gene avrBs3, were planted in a plot of ECW-30R plants, which carry the Bs3 gene for resistance to race 1 of the pathogen. Disease was observed on plants adjacent to the inoculum plants 2 weeks after inoculation, and within 10 weeks, all plants were diseased. Rifampicin-resistant single colonies isolated from diseased ECW-30R plants were screened for race reaction on pepper differentials, and all were race 3 because they were compatible on ECW-30R. Total DNA from 25% of the single colonies hybridized to an avrBs3 gene probe, and the plasmid carrying avrBs3 was not detected in the colonies that did not hybridize to avrBs3. Similarly, in 1994 the population derived from a strain of race 2 that carries avrBs1 rapidly shifted to race 3 when inoculated plants were placed in a plot of ECW-10R plants carrying the single resistance gene Bs1. Total DNA from all isolated single colonies with the race 3 phenotype hybridized with avrBs1 and insertion element IS476. Thus, a race 3 pathogen population of X. campestris pv. vesicatoria rapidly developed from race 1 and 2 populations within a growing season in the field when exposed to a resistant pepper cultivar carrying a single resistance gene. The race shift from race 1 to 3 was due to loss of the plasmid carrying the avirulence gene or to inactivation of avrBs3. Shift from race 2 to 3 apparently resulted from insertion element IS476 inactivating avirulence gene avrBs1.