Summary: A mutant (PF24) of the race 1 strain, 299A, of pv. has been characterized in terms of its interactions with pea () cultivars. The mutant showed a changed reaction (avirulence to virulence) with a group of pea cultivars, including cvs. Belinda and Puget, previously thought to contain resistance genes R1 and R3. Avirulence towards cv. Puget was restored by transfer of any one of five cosmid clones from a race 3 (strain 870A) gene library to a rifampicin-resistant derivative of PF24. These observations were in agreement with a revised race-specific resistance genotype for Belinda and similar cultivars comprising a single resistance gene, R3. An incompatible interaction was observed between strain PF24 and cvs. Vinco (postulated to harbour race-specific resistance genes R1, R2, R3 and R5) and Hurst's Greenshaft (R4 and possibly R1), indicating that the mutant retains at least one avirulence gene (A1 or A1 and A4). Mutant PF24 showed loss of a cryptic plasmid (pA V212) compared with its progenitor, strain 299A. A subclone (pA V233) of one of the race 3 restoration clones showed strong hybridization with similar-sized digestion fragments in race 3 plasmid DNA, confirming the A3 gene to be plasmid-borne. Strong cross-hybridization was also observed with a single 3·27 kb Rl fragment of plasmid DNA present in strain 299A but absent from strain PF24. This is consistent with the corresponding A3 determinant being located on pAV212 in the race 1 strain 299A. The novel avirulence gene corresponding to A3 in strain 870A is provisionally designated . A spontaneous race-change variant (strain 1759, which expressed no avirulence phenotype toward the pea differential cultivars) was derived from the race 3 strain 870A. This race 6 strain and a wild isolate of a race 6 strain both lacked plasmid DNA sequences corresponding to the insert in pA V233.


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