The effect of fungicide resistance on the structure of Monilinia laxa populations in Spanish peach and nectarine orchards

Abstract

Fungicide resistance has been proposed as one of the causes for changes in the frequency of occurrence of different fungal pathogen species. The purpose of this investigation was to study the dynamics of fungicide sensitivity, resistance, fitness, and competitiveness of Monilinia laxa field isolates in peach and nectarine orchards. The investigation comprised 788 isolates, which were randomly collected from 121 peach orchards in the Ebro Valley during the 2006 to 2010 growing seasons. Resistance to thiophanate-methyl, iprodione, and cyproconazole was determined by plating on potato dextrose agar amended with discriminatory concentrations of fungicides and determining EC50 values. Significant (P = 0.0476) time-dependent differences in the number of thiophanate-methyl-resistant isolates in the M. laxa population were found. The frequency of thiophanate-methyl-resistant isolates (EC50 range 104.0–137.5 μg ml−1) progressively increased since 2007. Although the number of iprodione -resistant isolates in the population did not change over time, two subpopulations of isolates showed iprodione reduced sensitivity (EC50 2.62–4.19 μg ml−1). TMRIR or TMRIS have become the dominant strains in the M. laxa population since 2007, and isolates with the TMRIS phenotype were the fittest and the most competitive phenotype in this population. No cyproconazole-resistant isolates were detected. These findings suggest that the current brown rot management strategies in the Ebro Valley should be revised.