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Sensitivity of Monilinia fructicola from Brazil to Tebuconazole, Azoxystrobin, and Thiophanate-Methyl and Implications for Disease Management

The aim of this study was to investigate the sensitivity of Monilinia fructicola isolates to tebuconazole (demethylation inhibitor [DMI]), azoxystrobin (quinone outside inhibitor), and thiophanate-methyl (methyl benzimidazole carbamate) in 118 isolates collected from four states in Brazil from orchards with different histories of fungicide use. Sensitivity to fungicides was determined by inhibition of mycelial growth and spore germination on fungicide-amended media. Polymerase chain reaction was used to determine the frequencies of M. fructicola genotypes exhibiting high (HR) and low (LR) resistance to thiophanate-methyl among sampled populations. Resistance to tebuconazole was found in 15.8% of isolates collected from São Paulo State. The 50% effective concentration (EC₅₀) values varied from 0.01 to greater than 100 μg/ml. The EC₅₀ values for tebuconazole corresponded to its historic use frequency in the orchard; for instance, isolates from orchards with no DMI fungicide use had the lowest mean EC₅₀ value (0.04 μg/ml), while those collected from orchards where more than five DMI fungicide sprays were applied per season had a mean EC₅₀ value of 21.17 μg/ml. All isolates were sensitive to azoxystrobin but their EC₅₀, based on tests of inhibition of conidial germination, increased from 0.05 in 2002 to 0.44 μg/ml in 2008. The EC₅₀ values based on mycelial growth inhibition for thiophanate-methyl were >162 μg/ml, 1.99 to 12.5 μg/ml, and <1.0 μg/ml for HR, LR, and sensitive isolates, respectively.