Baseline Sensitivity and Control Efficacy of Pyraclostrobin Against Botryosphaeria dothidea Isolates in China.

Botryosphaeria dothidea is an important fungal pathogen that causes apple ring rot, which can significantly reduce apple yield. Fungicide applications are the main control measure of apple ring rot worldwide. Pyraclostrobin is a quinone outside inhibitor (QoI) fungicide that has yet to be registered for control of B. dothidea in China. Baseline sensitivity of B. dothidea to pyraclostrobin (EC50 of mycelial growth inhibition) was assessed for 97 isolates collected in Shandong Province. The EC50 values ranged from 0.7010 to 7.1378 μg/ml with the mean value of 3.0870 μg/ml and displayed a unimodal frequency distribution. After cultured on fungicide-free PDA medium or on apples for multiple generations, the B. dothidea-resistant isolates (RST) remained resistant to pyraclostrobin, but exhibited similar virulence as the susceptible isolates (ST). Cross-resistance investigation revealed that pyraclostrobin was not cross-resistant to tebuconazole, flusilazole, carbendazim, and iprodione. Field evolution showed that pyraclostrobin at 200 and 250 g a.i./ha provided greater than 80% control efficacy against apple ring rot disease when applied as a therapeutic or preventive fungicide. The efficacy was similar to fungicides that have been registered for apple.