Variation in the response to ascochyta blight in common bean germplasm

Ascochyta blights are major diseases of many legumes and an emerging disease in many areas in which common bean (Phaseolus vulgaris L.) is grown. Ascochyta blight symptoms in bean can be caused by three fungus species. We investigated the species that caused ascochyta blight symptoms in bean crops from northern Spain using species-specific molecular markers and analysis of their nucleotide sequences. Results suggested that the two local isolates analyzed may correspond to the specie Phoma exigua var. diversispora. The variation in the reaction of common bean germplasm against a local isolate was also investigated in controlled conditions and was scored using a 1-9 severity scale. A total of 200 accessions from a core collection, 89 breeding lines and 11 accessions of Phaseolus coccineus L. were screened to identify potential sources of resistance. Most Phaseolus coccineus accessions showed higher levels of resistance (score < 5) while most Phaseolus vulgaris accessions and breeding lines had low levels (score > 5). However, five breeding lines and 17 accessions exhibited moderate resistance after five resistance tests, with variation in the response of seedlings within accessions. Thus, five lines were obtained by self-pollination from each of the 17 selected accessions. Evaluation of these lines revealed significant variation within lines derived from two accessions and allowed verification of the high level of resistance in lines derived from three accessions. Finally, the lines with higher levels of resistance identified in previous evaluations were tested in three additional tests. The lines UI465, BGE04435-22 and BGE04453-4 revealed resistance levels not significantly different from the best Phaseolus coccineus accession included in this study (score < 3.5). The identified resistance sources could be used in the in the short term to increase the level of resistance to ascochyta blight in specific bean genotypes or for development of new resistant genotypes by pyramiding of genes.