Abstract: 【Objective】 The objective of this study is to evaluate the sensitivity of Sclerotinia sclerotiorum to fluopyram, a new succinic dehydrogenase inhibitor (SDHI), determine the virulence of fluopyram against S. sclerotiorum at different developmental stages, and the action mode and efficacy of fluopyram in managing Sclerotinia stem rot, and to guide the scientific application of fluopyram. 【Method】 A total of 173 S. sclerotiorum isolates were collected from 6 vegetables (including cucumber, tomato, eggplant, pumpkin, kidney bean and pepper). The sampling sites located in Changle, Shouguang, Qingzhou, Linqu and Taian of Shandong Province. The sensitivity of 173 S. sclerotiorum isolates to fluopyram was measured by mycelial growth rate method. The inhibitory activity of fluopyram on sclerotial production (number and dry weight), shape and myceliogenic germination of sclerotia was determined in laboratory. The action mode of fluopyram against S. sclerotiorum was tested on detached eggplant leaves. Field trials in two years were performed to evaluate the practical efficacy of fluopyram in controlling Sclerotinia stem rot. 【Result】Fluopyram had strong inhibitory activity on the mycelium growth of S. sclerotiorum and had little difference in EC₅₀ values against 173 S. sclerotiorum isolates, with the ranging from 0.02 to 0.30 μg·mL^-1, which suggested that these isolates could be used to analyze the sensitivity level of S. sclerotiorum to fluopyram. The frequency distribution of EC₅₀ showed a single peak skew normal curve, and the variation coefficient was small, indicating that the S. sclerotiorum isolates on vegetable in Shandong Province were sensitive to fluopyram. Additionally, fluopyram had high bioactivity in suppressing sclerotial production, sclerotial morphology and myceliogenic germination of sclerotia. After treated with fluopyram at 1.6 μg·mL^-1, the number and dry weight of sclerotia from three isolates significantly decreased, and the sclerotia became smaller, indicating that the fungicide could reduce the number of primary source of infection and suppress the activity of infection of this pathogen. After 3 consecutive days of observation, the inhibition rate of myceliogenic germination of sclerotia treated with fluopyram at 5 μg·mL^-1 was all above 95%, indicating that fluopyram could inhibit the infection approach of the fungus so as to protect the basal part of stem from infection. The inoculation test on the detached leaves of eggplant showed that fluopyram had protective and curative activities. Spray applications of fluopyram at 40 μg·mL^-1 provided protective efficacy of 100.00% and curative efficacy of 88.81%, respectively, which was significantly higher than that of carbendazim and dimethachlon at 40 μg·mL^-1. In addition, the protective efficacy of fluopyram was better than the curative efficacy, which suggested that fluopyram should be sprayed preventively or at the very early stage of disease occurrence to get a higher efficacy. In the field trials conducted in 2016 and 2017 under natural infection conditions, the control efficacy of 90.30% and 87.60% against Sclerotinia stem rot were given by application of fluopyram under the rate of 200 g a.i./hm², which was significantly higher than that of other treatments. 【Conclusion】 The novel SDHI fungicide fluopyram has high bioactivity in suppressing mycelial growth, sclerotial production, sclerotial morphology and myceliogenic germination of sclerotia and can effectively control the occurrence of Sclerotinia stem rot in the field, which means that the fungicide is an effective agent for preventing and controlling Sclerotinia stem rot, and can be served as an alternative fungicide to control the disease.

Key words: Sclerotinia stem rot, fluopyram, sensitivity, sclerotia, control efficacy