Specialisation of crown rust on highly and moderately resistant plants of perennial ryegrass

SUMMARY From two field samples of crown rust, isolates were selected for increased aggressiveness on four of 10 resistant perennial ryegrass clonal lines. Two lines which were formerly highly resistant became highly susceptible to the selected isolates but two which were formerly moderately resistant became only moderately susceptible. Thus, moderate resistance, although no less likely to be race‐specific, was more stable than a high level of resistance. Cultures from highly resistant lines did not show increased aggressiveness on the varieties from which the resistant lines had been selected; hence individual genes of major effect were unimportant in determining varietal resistance. In contrast, cultures from moderately resistant lines were substantially more aggressive on their corresponding varieties. Three varieties contained many genes which tended to buffer changes in virulence of the rust.