Baseline sensitivity and toxic actions of boscalid against Sclerotinia sclerotiorum

Baseline sensitivity of Sclerotinia sclerotiorum to the succinate dehydrogenase inhibitor (SDHI) fungicide boscalid was established by determining the effective concentration causing mycelial growth inhibition by 50% (EC50) values. The mean EC50 values of 76 isolates collected in 2008 and 77 isolates collected in 2014 were 0.0383 and 0.0395 μg/mL, respectively. There was no significant difference (P = 0.695) in boscalid sensitivity between the two years, and the frequency distribution of EC50 values was unimodal, but right-skewed. Correlation analysis showed that there was a statistically significant positive correlation (R2 = 0.856, P<0.001) of sensitivity between boscalid and thifluzamide, but no significant correlation (R2 = 0.01, P = 0.650) was detected between boscalid and the demethylation inhibitor (DMI) fungicide prochloraz. Both preventive and curative efficacies of boscalid were significantly higher (P<0.05) than those of the reference fungicide carbendazim. Boscalid in potato dextrose agar (PDA) at 0.04-0.16 μg/mL significantly reduced the number of sclerotia per plate, but the weight of sclerotia increased slightly. Light microscopic observations showed that boscalid in PDA caused hyphae to branch more at the tip. Transmission electron microscopy (TEM) observations showed that mitochondria were deformed for the mycelia grown on boscalid-amended PDA. Boscalid at 0.01 or 0.05 μg/mL had no significant effect on mycelial cell membrane permeability of S. sclerotiorum. These results will deepen our understanding of the toxic action of boscalid, and the baseline sensitivity can be used as a reference point in future boscalid resistance monitoring programs.