SOURCES OF RESISTANCE TO PSEUDOMONAS-SYRINGAE PV PHASEOLICOLA RACES IN PHASEOLUS-VULGARIS

One thousand and forty-eight Phaseolus bean accessions were evaluated for resistance to six races of Pseudomonas syringae pv. phaseolicola. The accessions originated from regions of the Americas and Africa wher e the disease is important and included wild type accessions and some known resistance sources. Resistance, graded on a five-point scale, wa s of two types: qualitative, which was shown to be race-specific, and quantitative. Race specific resistance genes (R-genes) were detected i n 49.4% of accessions with the following gene frequencies: R1 (10.3%), R2 (0.3%), R3 (25.0%), R4 (35.0%) and R5 (0.2%). Evidence for quantit ative variation in resistance, in the absence of specific R-genes, was shown by the distribution of infection scores, 76% of accessions show ing maximum susceptibility (grades 4-5), 23% showing intermediate resi stance (grades 2-4), and 1% showing high levels of quantitative resist ance (grades 1-2). The last 1% of accessions showed interactions which were not race-specific and it is suggested that they may possess race non-specific resistance. It is possible that several of the accession s in this category carry the recessive gene derived from PI 150414. Ot her accessions were of unknown parentage and may represent new sources of quantitative, potentially race non-specific, resistance. It is sug gested that the combination of race specific and race non-specific res istance could provide an effective strategy for establishing durable r esistance.