Baseline sensitivity and efficacy of trifloxystrobin against Sclerotinia sclerotiorum

• Baseline sensitivity of Sclerotinia sclerotiorum to trifloxystrobin was established.
• Trifloxystrobin is highly efficacious to S. sclerotiorum.
• SHAM should not be included in in vitro assay of trifloxystrobin sensitivity.

The ascomycete fungus Sclerotinia sclerotiorum is a devastating plant pathogen notorious for its extremely broad host range. Trifloxystrobin has not been registered for control of S. sclerotiorum in China. In this study, baseline sensitivity of trifloxystrobin was established based on frequency distribution of 166 isolates' EC50 values and efficacy of trifloxystrobin was determined on potted oilseed rape plants. Trifloxystrobin EC50 values of the 166 isolates ranged from 0.01 to 0.80 μg/mL, with a mean value of 0.06 μg/mL. The frequency distribution of trifloxystrobin EC50 values was unimodal in shape, but with a long right-hand tail. After logarithmic transformation, the frequency distribution fitted closer to a normal distribution than did the original EC50 values. The preventive efficacies of trifloxystrobin at 5, 15, and 45 μg/mL were 71.4%, 96.5%, and 100.0%, respectively, while the curative efficacies were 40.6%, 48.7%, and 73.4%, respectively. Both preventive and curative efficacies of trifloxystrobin at 45 μg/mL were significantly higher (P ≤ 0.046) than those of the reference fungicide carbendazim. Assays with six arbitrarily selected isolates demonstrated that salicylhydroxamic acid (SHAM) at 20 μg/mL reduced trifloxystrobin EC50 values by 84.0% on average and greatly potentiated efficacy of trifloxystrobin. The average co-toxicity factor of SHAM and trifloxystrobin for preventive efficacy was 64.3, indicating considerable synergisms in planta. Hence, SHAM should not be included in in vitro assay of S. sclerotiorum sensitivity to trifloxystrobin.