Disease potential of pepper bacterial spot pathogen races that overcome the Bs2 gene for resistance

Bacterial spot, caused by Xanthomonas campestris pv. vesicatoria, is a major limiting disease in bell pepper production. Gene Bs2 in pepper confers hypersensitive resistance to the three most common races (1, 2, and 3) and to race 0. Races 4, 5, and 6, which cause disease on pepper plants with the Bs2 resistance gene, recently were isolated from diseased pepper. Multiplication of races 1 to 6 did not significantly differ in 0.8% nutrient broth (NB), but strains of races 4 and 6 multiplied slower in 0.4 NB than did races 1, 2, and 3 after 32 h. Multiplication of races 4 to 6 did not differ from that of races 1 to 3 in susceptible cv. Early Calwonder (ECW). Races 4 and 6 multiplied similarly to race 1 in near-isogenic line ECW- 10R (contains resistance gene Bs1). The severity of disease caused by the races that overcome the Bs2 gene compared to the races that do not was not significantly different on three susceptible pepper genotypes. Races 4 to 6 caused significantly greater disease on plants carrying the Bs2 gene for resistance than did races 0 to 3, which caused a hypersensitive response. During 1995 when races 1 to 4 were used as inoculum in the field, both ECW and ECW-20R plants became severely diseased and had 83 and 74% lower yields, respectively, compared to noninoculated checks. Race 4 predominated on ECW-20R, whereas race 3 predominated on ECW. During 1996 when races 4, 5, and 6 were used as inoculum in the field, both cv. Camelot and cv. X3R Camelot (contains resistance gene Bs2) plants became severely diseased. Race 6 predominated on both Camelot and X3R Camelot. Avirulence gene avrBs2 was detected by hybridization in all the races without observed polymorphism. Our data indicate that races that overcome the Bs2 gene for resistance have the potential to cause severe disease and need to be taken into account in disease management strategies.