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Sclerotinia sclerotiorum is a cosmopolitan plant pathogen notable for its wide host range. The quinone outside inhibitor (QoI) fungicide pyraclostrobin has not been registered for control of S. sclerotiorum in China. In this study, baseline sensitivity of pyraclostrobin was established based on effective concentration for 50% inhibition of mycelial growth (EC 50 ) values of 153 isolates of S. sclerotiorum collected from five provinces of China and toxicity of alternative oxidase inhibitor salicylhydroxamic acid (SHAM) to S. sclerotiorum was determined. Results showed that the frequency distribution of EC 50 values of the 153 isolates was unimodal but with a right-hand tail. The mean EC 50 value was 0.1027 μg/ml and the range of EC 50 values was 0.0124 to 0.6324 μg/ml. Applied as a preventive fungicide in pot experiments, pyraclostrobin at 5, 15, and 45 μg/ml provided control efficacies of 61, 77, and 100%, respectively. There was no positive cross-resistance between pyraclostrobin and carbendazim or dimethachlon. EC 50 values for SHAM against four isolates of S. sclerotiorum were 44.4, 51.8, 54.4, and 68.7 μg/ml. SHAM at 20 μg/ml could significantly increase not only the inhibitory effect of pyraclostrobin on mycelial growth on potato dextrose agar media but also the control efficacy in planta. These results indicated that SHAM should not be added into artificial media in in vitro assay of S. sclerotiorum sensitivity to pyraclostrobin. This has broad implications for assay of sensitivity of fungal pathogen to QoI fungicides.

Growth and virulence stimulations of sublethal doses of fungicides on plant-pathogenic fungi and oomycetes have been reported and the stimulatory effects are potentially relevant to plant disease management. Sclerotinia sclerotiorum is one of the most devastating and economically important necrotrophic fungal phytopathogens, capable of infecting more than 400 species of plants worldwide. In order to study stimulatory effects of sublethal doses of fungicides on S. sclerotiorum, 55 dimethachlon-sensitive isolates and 3 dimethachlon-resistant isolates of S. sclerotiorum were assayed to determine effects of sublethal doses of dimethachlon on mycelial growth rate on potato dextrose agar (PDA) media and virulence on oilseed rape plants. Results showed that all 3 dimethachlon-resistant isolates and 13 of the 55 sensitive isolates exhibited stimulatory responses to sublethal doses of dimethachlon. Dimethachlon-resistant isolates grew significantly (P < 0.05) faster on PDA media amended with dimethachlon at 0.5 to 4 μg/ml than on fungicide-free PDA media. As for virulence on detached leaves of oilseed rape plants, lesion diameters of dimethachlon-resistant isolates after growth on PDA media amended with dimethachlon at 0.5 to 2 μg/ml were significantly larger (P < 0.05) than the control. The maximum stimulatory effects were 42.40 to 59.80%. In pot experiments, for both dimethachlon-sensitive and -resistant isolates, significant (P < 0.05) virulence stimulations were observed after spraying with dimethachlon at a concentration of 2 μg/ml. After growing on dimethachlon-amended PDA media, H 2 O 2 sensitivity of S. sclerotiorum decreased significantly (P < 0.05) compared with the nonamended PDA control.