Reduced Sensitivity to Tebuconazole in Botryosphaeria dothidea Isolates Collected from Major Apple Production Areas of China.

China has the largest acreage and is the greatest producer of apples in the world. Apple ring rot (ARR) caused by Botryosphaeria dothidea is one of the major diseases affecting apple fruit production. Tebuconazole, a sterol demethylation-inhibiting (DMI) fungicide, has been increasingly applied to manage ARR in China. Resistance to tebuconazole in field populations of B. dothidea may be selected and accumulate to higher degrees over time. Establishment of tebuconazole sensitivity monitoring programs is critically important for resistance management and effective ARR control. However, the resistance level of B. dothidea to tebuconazole in China remains largely unknown. In this study, in vitro mycelial growth assays of B. dothidea in media amended with tebuconazole were conducted, and the sensitivity of B. dothidea to tebuconazole was determined with a set of 390 isolates collected from the major apple production provinces in China between 2006 and 2014. Results showed that the 50% effective concentration (EC 50 ) value ranged from 0.011 to 0.918, 0.040 to 1.621, and 0.052 to 1.925 μg ml -1 with a median value of 0.194, 0.386, and 0.782 μg ml -1 in the isolates collected in 2006, 2010, and 2014, respectively. The frequency distribution of EC 50 for tebuconazole was a nonnormal distribution ( P < 0.05), suggesting that subpopulations with reduced sensitivity to tebuconazole had emerged in these B. dothidea isolates. The frequency distribution of the B. dothidea isolates collected in 2006 fit a unimodal curve and could be regarded as the baseline sensitivity to tebuconazole. The resistance levels increased over time with the average occurrence frequency of 43.3% and resistance index of 0.38. Positive cross-resistance was observed between tebuconazole and metconazole, which is another DMI fungicide, but multiple resistance was not detected between tebuconazole and non-DMI fungicides. Our results demonstrated that regular long-term resistance monitoring combining with prudent fungicide use should be implemented to prolong the lifespan of tebuconazole in management of ARR in apples.