GENETICS OF SPECIFIC RESISTANCE IN PEA (PISUM-SATIVUM) CULTIVARS TO 7RACES OF PSEUDOMONAS-SYRINGAE PV PISI

Seven races of Pseudomonas syringae pv. pisi were distinguished using eight differential cultivars of pea (Pisum sativum). Segregation among F-2 populations of crosses between differential cultivars sequentiall y inoculated with races of P.s. pv. pisi provided evidence for four an d possibly six putative resistance(R)/avirulence(A) gene pairs. R1, R2 and R3 are dominant resistance alleles at single loci, R4 is a domina nt allele at a single locus which exhibits variable expression possibl y dependent on genetic background. There is evidence that R3 and R4 ar e at linked loci. Homology tests provided proof of the occurrence of t he alleles R2, R3 and R4 in more than one cultivar. Two other alleles, R5 and R6, were postulated to explain the observed segregation ratios in certain crosses. It can be inferred that P.s. pv. pisi races 2, 3 and 4 each carry a different single avirulence gene, race 6 carries no apparent virulence genes, and race 7 carries at least A2, A3 and A4. Race 1 carries A1, A3, A4 and possibly A6; race 5 carries A2, A4 and p ossibly A5 and A6.