Comparison of the sensitivity of Monilinia fructicola isolates to tebuconazole in Brazil using three methods

The frequency of strains of Monilinia fructicola that are resistant to the demethylation inhibiting fungicide tebuconazole in Brazil was determined by three methods. The first involved testing 295 isolates for relative growth under a discriminatory dose of 0.3 mu g mL(-1). The second and the third methods involved testing 120 isolates for the effective concentration for 50% population inhibition (EC50); one method employed the discriminatory dose of 0.3 mu g mL(-1) and the other method utilized the baseline dose of 0.046 mu g mL(-1) to separate the strains based on sensitivity. The frequencies of resistance among the isolates based on the three methods were determined by measuring mycelial growth in fungicide-amended media. The shifts in M. fructicola sensitivities were determined by comparing the EC50 density distributions of the baseline (2000-2004, n=31), intermediate (2005-2008, n=88), and current (2009-2011, n=120) isolate populations. The frequency of resistance to tebuconazole varied according to the assessment method; the resistance frequencies were 18.3% for the relative growth method, 40% for the baseline EC50 method, and 8.33% for the EC50 method segregated by the dose of 0.3 mu g mL(-1). The application of the EC50 method involving the discriminatory dose of 0.3 mu g mL(-1) revealed a decrease in the frequency of resistant genotypes. However, the baseline EC50 method is recommended for future resistance surveys because this method was originally used to determine the mean sensitivity prior to the use of tebuconazole in Brazil. Additionally, a shift of the current population toward sensitivity was observed compared with the intermediate population. Finally, Sao Paulo was found to be the state with the highest EC50 values (0.186 mu g mL(-1)) compared with Parana (0.053 mu g mL(-1)) and Rio Grande do Sul (0.026 mu g mL(-1)). These results suggest that the build-up of fungicide resistance in Monilinia fructicola has been prevented by the use of anti-resistance strategies in the main peach production areas of Brazil; however, additional studies are required to demonstrate how resistance instability and fitness are affecting isolate sensitivity.