RESISTANCE OF Botrytis cinerea FROM STRAWBERRY (Fragaria x ananassa Duch.)TO FUNGICIDES IN MICHOACAN MEXICO.

Gray mold caused by Botrytis cinerea Pers. is one of the most important diseases of strawberry (Fragaria x ananassa Duch.), and fungicide resistance is a concern. The objective of this research was to evaluate the sensitivity of B. cinerea isolates to the fungicides iprodione and thiophanate-methyl in two strawberry-producing areas in Mexico. Sixty-two B. cinerea isolates collected from strawberry fruits from Maravatio Valley (MV) and Zamora-Jacona Valley (ZJ) in Michoacan, Mexico, were used to determine the sensitivity distribution based on mycelium growth (MG) and conidial germination (CG). Each monosporic isolate was grown in 4 plates containing media amended with 0, 0.1, 0.5, 1, 10, 50 and 100 mg mL-1 of iprodione and 0, 0.1, 10, 100, 1000 and 2000 mg mL-1 thiophanate-methyl, and the experiment was conducted twice. The effective concentration (EC50) that inhibits 50 % of mycelial growth and conidial germination was determined for each isolate-fungicide combination by fitting the log-logistic model to the data. Fungicide sensitivity distributions were illustrated by box plots and histograms, and compared by the Kolmogorov-Smirnov non-parametric test. For iprodione, there were significant differences between the sensitivity distributions (p£0.0001) of both of the sampling areas for mycelia. In MV, the median EC50 was 0.35 mg mL-1, while that of the ZJ isolates was 1.5 mg mL-1. There were no significant differences in the sensitivity distributions of CG (p=0.47) between producing areas for conidia (median EC50 = 3.73 mg mL-1). For thiophanate-methyl, the sensitivity distribution for mycelia was scattered and bimodal, with significant differences between the sampling areas (p£0.002). The median EC50 of MG was 3.11 mg mL-1 in MV, and 2000 mg mL-1 in ZJ. There were no significant differences between the sensitivity distributions from MV and ZJ in the conidial germination assay (p=0.13, Ks=1.17). The EC50 varied from 0.01 to >2000 mg mL-1 (median=2.01 mg mL-1). Resistance in B. cinerea to thiophanate-methyl and iprodione is widespread in strawberry-producing areas in Michoacan. [ABSTRACT FROM AUTHOR]