Sensitivity of Sclerotinia sclerotiorum to bixafen and its mixtures in Henan Province

Sclerotinia stem rot is a destructive disease caused by the fungus Sclerotinia sclerotiorum, and it occurs in oilseed rape and many other broadleaf crops. The chemical control with fungicides is usually applied in fields. As a kind of succinate dehydrogenase inhibitors (SDHIs), bixafen showed higher efficiency in inhibiting mycelial growth. To establish the baseline sensitivity of S. sclerotiorum to bixafen in Henan Province, 119 isolates of S. sclerotiorum were obtained from diseased stems of oilseed rape collected from fields in different geographical regions in Henan Province, China, in 2015 and 2016, and their sensitivity to bixafen was determined based on mycelial growth rate method. The results showed that the 50% inhibition of mycelial growth (EC₅₀) values of S. sclerotiorum ranged from 0.0417 to 0.4732 µg/mL with an average EC₅₀ value (mean ± SD) of (0.1968 ± 0.1053) µg/mL. The frequency distribution range curve was narrow and unimodal, and the average EC₅₀ value obtained can be used as the baseline sensitivity of S. sclerotiorum to bixafen in Henan Province. In order to determine whether bixafen can be mixed with other fungicides with different modes of action, the EC₅₀ values of S. sclerotiorum to bixafen, carbendazim, fludioxonil, prothioconazole, dimetachlone, metconazole, and their mixtures were determined based on mycelial growth rate method. The results showed that the EC₅₀ values for bixafen, carbendazim, fludioxonil, prothioconazole, dimetachlone and metconazole were 0.1256, 0.1122, 0.0229, 0.0651, 0.8057, and 0.0278 µg/mL, respectively. For fungicide mixtures of bixafen and carbendazim, fludioxonil, prothioconazole, dimetachlone or metconazole (1 : 1, 1 : 3, 1 : 5, 3 : 1 and 5 : 1, V/V), the synergistic ratio (SR) ranges from 0.54 to 3.57, which indicates additive or synergistic inhibition. These results suggested that bixafen could be used alternately or in combination with carbendazim, fludioxonil, prothioconazole, dimetachlone and metconazole to prevent and delay the development of resistance in S. sclerotiorum. These results provide important and scientific information for the control of sclerotinia stem rot and for monitoring the sensitivity of S. sclerotiorum to bixafen in Henan Province.