Genetic resistance to powdery mildew in common bean

Powdery mildew can cause severe yield losses in bean crops. Limited information about resistance sources, and nature and inheritance of resistance are available to bean breeders and plant pathologist. Sources of resistance were searched in seedling tests under controlled conditions in 44 well-known genotypes and in a Spanish germplasm core collection consisting on 201 accessions. A 0-4 scale was used to describe the infection types (IT) observed. Only six out of the 245 evaluated genotypes showed a complete resistance (IT0) without visible symptoms on the leaves: Amanda, Belneb, Cornell 49242, Negro San Luis, Porrillo Sintetico and the local accession BGE003161. Inheritance of resistance was studied in F and F segregating populations. Observed reactions in the five segregating populations fitted to Mendelian ratios with different modes of inheritance. Results revealed that cultivar Porrillo Sintetico carries two dominant and independent resistance genes: one gene conferring complete resistance (IT0), and another gene conferring IT3, characterized by a moderate mycelial development on the leaves. Both genes show a dominant epistatic relationship. Inheritance of response to powdery mildew in cv. Cornell 49242 was similar to cv. Porrillo Sintetico although the correspondence with the genes described in Porrillo Sintetico was not established. Line X2776 carries one dominant gene conferring IT3, and shares this gene with cv. Porrillo Sintetico. In cv. Amanda, two complementary genes appear to be involved in resistance to this fungus. This information will be relevant for the implementation of breeding programs focused on the development of cultivars carrying genetic resistance to powdery mildew

Markers linked to the bc-3 gene conditioning resistance to bean common mosaic potyviruses in common bean

Necrotic strains of bean common mosaic potyviruses are becoming increasingly problematic in bean growing areas of Africa and Europe. Pyramiding epistatic resistance genes provides the most effective long-term strategy for disease control against all known strains of the virus. Indirect selection using tightly linked markers should facilitate the breeding of desired epistatic resistance gene combinations. In common bean, the most effective strategy for broad spectrum control of the bean common mosaic potyviruses is to combine I and bc-3 genes. We describe the use of near-isogenic lines and segregating populations from different gene pools combined with bulked segregant analysis to identify markers tightly linked with the recessive bc-3 gene that conditions resistance to all strains of bean common mosaic necrosis virus. We identified a RAPD marker, OG6595, linked at 3.7 cM from the bc-3, and the marker was used to confirm the location of bc-3 gene on bean linkage group B6. A codominant AFLP marker, EACAMCGG-169/172was identified and linked at 3.5 cM from the bc-3 and the AFLP and OG6595 markers flanked the bc-3 gene. The AFLP marker was converted to the STS marker SEACAMCGG-134/137 which showed co-segregation with the original AFLP marker. The 134 bp fragment associated with resistance was linked with the bc-3 gene present in a diverse group of bean genotypes except in two kidney bean lines. The OG6595 marker mapped on B6 supported independence of bc-3 from the I gene located on B2, which provides the opportunity to readily combine both genes in a single bean cultivar for broad spectrum resistance to bean common mosaic potyviruses