Development of Boscalid Resistance in and an Efficient Strategy for Resistance Management.

Among succinate dehydrogenase inhibitors, only boscalid has been registered in China for controlling gray mold. In Shandong Province of China, it has been more than a decade since the first use of boscalid to control gray mold. In the current study, we monitored the resistance development process of to boscalid, identified the mutation types that occurred in boscalid-resistant isolates, and proposed an original application technique to delay resistance development. A total of 720 isolates collected from tomato and cucumber in Shandong Province from 2014 to 2019 were determined to be sensitive to boscalid. The results showed that the sensitivity of the isolates to boscalid declined gradually over time, with a mean half maximal effective concentration of 0.3 ± 0.02 mg/liter in 2014 and 6.39 ± 1.66 mg/liter in 2019. The proportion of resistant isolates quickly increased from 0.81% in 2014 to 28.97% in 2019. Mutations of P225F, N230I, H272Y, and H272R in the B subunit were responsible for boscalid resistance. Four concurrent mutations (G85A, I93V, M158V, and V168I) in the subunit were first discovered in Shandong Province, but they did not affect the level of boscalid resistance. Interestingly, this study found that the fruit dipping application, a precise topical application technique, could delay the development of boscalid resistance. Therefore, this application technique provides a new method for resistance management of .