Stability and fitness of pyrimethanil-resistant phenotypes of Penicillium expansum from apple
R. CAIAZZO (1), C. L. Xiao (2). (1) Washington State University, Wenatchee, WA, U.S.A.; (2) USDA ARS, Parlier, CA, U.S.A.
Phenotype stability, fitness and competitive ability of pyrimethanil-resistant isolates of <i>P. expansum</i> were determined. The stability of pyrimethanil resistance (PR) was assessed after consecutive transfers on potato dextrose agar (PDA) or being cycled on apple fruit. Fitness components including mycelial growth, conidial germination, pathogenicity and virulence on apple fruit, and sporulation in vivo and in vitro were evaluated. PR was retained at the levels similar to that of the initial generation after 20 and 5 transfers on PDA and 4 and 3 cycles on apple fruit at 20 and 0°C, respectively. In general, there were no significant differences in the mean values of fitness parameters among the phenotype groups, though variability in individual fitness parameters was observed among the isolates within the same phenotype groups. After 4 disease cycles on apple fruit inoculated with a pair mixture of a sensitive isolate and one of the two resistant phenotypes at 75:25, 50:50 or 25:75 ratios, the final frequency of resistant individuals was significantly decreased compared to the initial generation except that when the mixture consisted of 75% highly pyrimethanil-resistant individuals, the frequency was increased. The results suggest that PR was stable and that PR did not significantly impair individual fitness parameters, but resistant phenotypes exhibited some competitive disadvantage when the pyrimethanil-resistant individuals were ≤ 50% in the population.
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