GENETICS OF SPECIFIC RESISTANCE IN PEA (PISUM-SATIVUM) CULTIVARS TO 7 RACES 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 sequentially inoculated with races of P.s. pv. pisi provided evidence for four and possibly six putative resistance(R)/avirulence(A) gene pairs. R1, R2 and R3 are dominant resistance alleles at single loci, R4 is a dominant allele at a single locus which exhibits variable expression possibly dependent on genetic background. There is evidence that R3 and R4 are at linked loci. Homology tests provided proof of the occurrence of the 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 possibly A5 and A6.