Resistance to anthracnose in common bean is conditioned primarily by nine major independent genes, Co-1 to Co-10 as the Co-3/Co-9 genes are allelic. With the exception of the recessive co-8 gene, all other nine are dominant genes and multiple alleles exist at the Co-1, Co-3 and Co-4 loci. A reverse of dominance at the Co-1 locus suggests that an order of dominance exists among individual alleles at this locus. The nine resistance genes Co-2 to Co-10 are Middle American in origin and Co-1 is the only locus from the Andean gene pool. Seven resistance loci have been mapped to the integrated bean linkage map and Co-1 resides on linkage group B1; Co-2 on B11, Co-3 on B4; Co-4 on B8; Co-6 on B7; and Co-9 and Co-10 are located on B4 but do not appear to be linked. Three Co-genes map to linkage groups B1, B4 and B11 where clusters with genes for rust resistance are located. In addition, there is co-localization with major resistance genes and QTL that condition partial resistance to anthracnose. Other QTL for resistance may provide putative map locations for the major resistance loci still to be mapped. Molecular markers linked to the majority of major Co-genes have been reported and these provide the opportunity to enhance disease resistance through marker-assisted selection and gene pyramiding. The 10 Co-genes are represented in the anthracnose differential cultivars, but are present as part of a multi-allelic series or in combination with other Co-genes, making the characterization of more complex races difficult. Although the Co-genes behave as major Mendelian factors, they most likely exist as resistance gene clusters as has been demonstrated on the molecular level at the Co-2 locus. Since the genes differ in their effectiveness in controlling the highly variable races of the anthracnose pathogen, the authors discuss the value of individual genes and alleles in resistance breeding and suggest the most effective gene pyramids to ensure long-term durable resistance to anthracnose in common bean.
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