Sensitivity of Ascochyta rabiei populations to prothioconazole and thiabendazole

摘要

Ascochyta rabiei causes Ascochyta blight, a yield-limiting disease of chickpea (Cicer arietinum) world-wide. In 2007, fungal populations of A. rabiei resistant to the Qol group of fungicides were detected in the Northern Great Plains of the United States. Assays were conducted to determine fungal sensitivity for two alternative fungicidal modes of action. A total of 78 isolates of A. rabiei collected between 1983 and 2007 were screened to determine baseline sensitivity to the demethylation-inhibiting foliar fungicide, prothioconazole, and 100 isolates collected between 1987 and 2007 were screened for sensitivity to the methyl benzimidazole carbamate (MBC) fungicide, thiabendazole. Isolates were tested using an in vitro mycelial growth assay to determine the effective fungicide concentration at which 50% of fungal growth was inhibited (EC50) for each isolate fungicide combination. Baseline EC50 values of prothioconazole ranged from 0.0526 to 0.2958 mu g/ml, with a mean of 0.1783 mu g/ml. Isolates of A. rabiei collected from 2007 to 2009 from North Dakota chickpea fields exposed to prothioconazole, were screened for prothioconazole sensitivity using the same assay. Mean EC50 values for these isolates were 0.3544 mu g/ml, 0.3746 mu g/ml, and 0.7820 mu g/ml, respectively. These values represent an approximate 2.0 (2007-2008) and 4.4-fold (2009) decrease in sensitivity from the baseline mean. EC50 values of thiabendazole ranged from 1.192 to 3.819 mu g/ml, with a mean of 2.459 mu g/ml. No significant decrease in fungicide sensitivity was observed for thiabendazole. To date, no loss of Ascochyta blight control has been observed with the use of either prothioconazole or thiabendazole