| Strains with mutations in the avrBs2 locus of Xanthomonas axonopodis pv. vesicatoria, which causes bacterial spot in pepper, were detected and caused disease in pepper containing the Bs2 resistance gene. Three strains each of races 3, 4, and 6 were separately inoculated on susceptible pepper plants (cv. Camelot) in field plots. Inoculum naturally spread within plots to rows of plants of ECW (bs2/bs2) or ECW-20R (Bs2/Bs2). A specific strain within each race predominated on either ECW or ECW-20R. Races 3 (functional avrBs2), and 6 (non-functional avrBs2) had similar AUDPC values, and caused similar disease severity on ECW. Races 4 (non-functional avrBs2) and 6, which defeat the Bs2 gene caused severe disease on ECW-20R. Frequency of recovery of strains in field plots correlated with bacterial multiplication and lesion efficiency results. Although, gene expression of avrBs2 mutant strains was detected, AvrBs2 protein translation may not occur or be in an inactive form. Loss of avrBs2 activity apparently results in less fitness cost in race 6 than in race 4 strains; however, there is variability in aggressiveness among strains within races. It was hypothesized that if the avrBs2 gene is required for full virulence and fitness of Xav, in the absence of selection pressure by the Bs2 resistance gene, functional avrBs2 should be favored. Seven strains representing races 4, 5, and 6 with 5-bp mutations (5-bp addition and deletion) were tested for the potential to switch from non-functional to functional avrBs2. Although 3 of 1,961 colonies from the field experiment had functional avrBs2, these strains did not result in a detectable population shift. This suggests that the selection pressure for functional avrBs2 in the absence of resistance gene Bs2 is weak or absent. Results presents here indicated that pathogen strains with loss of avirulence gene functions (ie, no HR) can be less aggressive under field conditions and that competitiveness varies among strains. However, such strains retain the ability to cause significant, unacceptable disease loss if no other controls are used. |
