Genetic interaction between Phaseolus vulgaris and bean common mosaic virus with implications for strain identification and breeding for resistance

Various strains of bean common mosaic virus (BCMV) occur in susceptible cultivars of bean. To compare these strains, a standard procedure for identification and a set of differential cultivars were established. The differentials are representatives of 11 resistance groups, determined by testing of about 450 bean cultivars with 8 to 10 strains. The virus strains and isolates were classified into 10 pathogenicity groups and subgroups, so that 10 strains were distinguished and the others considered as isolates of those strains.Twelve differentials were intercrossed and their F ₁ and F ₂ tested with most of the strains for genetical analysis of resistance in bean. Seven genes were distinguished: a necrosis gene I , already known from the literature, 5 strain-specific resistance genes be-1, be-1 ², bc-2, bc-2 ² and bc-3, and a strain-unspecific gene bc-u, complementary to the strain-specific ones. Genes be-1 and be-1 ² were allelic, as were bc-2 and bc-2 ². The 5 loci segregated independently or nearly so. The 4 strain-specific genes bc-1 to bc-2 ² had a gene-for-gene relationship with 4 pathogenicity genes, likely to be present in the virus strains. Gene bc-3 had not been overcome by a corresponding pathogenicity gene. Two bean genotypes were developed with resistance to all known strains. Some implications for resistance breeding are discussed.