Baseline sensitivity and control efficacy of two QoI fungicides azoxystrobin and pyraclostrobin against Ustilaginoidea virens

Rice false smut caused by the filamentous fungus Ustilaginoidea virens is a devastating grain disease in rice. Fungicides have been an important measure for the control of this disease. In this study, baseline sensitivities of 179 isolates of U. virens to QoI fungicides azoxystrobin and pyraclostrobin were established. The distribution of the 50% effective concentration (EC50) values of each fungicide was unimodal. The frequency distribution of logarithmically transformed EC50 values fitted orand fitted closer to a normal distribution. The ranges of EC50 values for azoxystrobin and pyraclostrobin were 0.001 to 0.864 and 0.001 to 0.569 μg/mL, with means and standard errors of the mean values of 0.203 ± 0.012 and 0.079 ± 0.006 μg/mL, respectively. There was a statistically significant and moderate positive correlation (n = 100, r = 0.469, P = 0.001) in sensitivity between these two fungicides. No cross-resistance was found between azoxystrobin, pyraclostrobin and carbendazim or DMI fungicides. Each fungicide had a significantly higher mean preventive efficacy compared to its curative efficacy. Field assays showed that control efficacy of pyraclostrobin against rice false smut was greater compared tothan that of azoxystrobin. Pyraclostrobin had the best control of rice false smut in three rice varieties with the control efficacy ranging from 81.5% to 95.5%, whereas azoxystrobin decreased disease index by 64.1% to 69.2% under the same conditions. These results provide us a reference point in the management of U. virens and future QoI fungicide resistance monitoring programs.