Hormetic Effects of Mixtures of Dimethachlone and Prochloraz on Sclerotinia sclerotiorum

Abstract

Previous studies showed that dimethachlone has significant hormetic effects on phytopathogenic fungus Sclerotinia sclerotiorum. The present study investigated hormetic effects of mixtures of dimethachlone and prochloraz on mycelial growth and virulence of two dimethachlone-resistant isolates of S. sclerotiorum. The stimulatory dimethachlone dosage range was around 1 to 100 μg/ml in potato dextrose agar (PDA) medium for mycelial growth of the two isolates, and dimethachlone at 10 and 50 μg/ml had the maximum percent stimulations of 80.6 and 19.3% for isolates JMS14 and HLJ4, respectively. Prochloraz at 0.0003 and 0.002 μg/ml had the maximum percent stimulations of 9.3 and 11.1% for isolates JMS14 and HLJ4, respectively. However, dimethachlone and prochloraz mixed at their respective stimulatory concentrations had the maximum percent stimulations of 48.1 and 9.3% for isolates JMS14 and HLJ4, respectively. After the mycelia with increased and inhibited growth on fungicide-amended PDA were subcultured on PDA without fungicide, mycelial growth for the second generation increased compared with the nontreated control. After the mycelia grown on fungicide-amended PDA were inoculated on rapeseed leaves, the amplitude of virulence stimulation was much greater than that of mycelial growth on PDA amended with fungicide, and the inhibited mycelia also showed substantially increased virulence on leaves. The mixture of dimethachlone at 100 μg/ml and prochloraz at 0.03 μg/ml in PDA inhibited mycelial growth of isolate JMS14 by 59.4%; however, after the inhibited mycelia were inoculated on rapeseed leaves, virulence was stimulated by 69.0%. Spraying sublethal doses of dimethachlone and prochloraz on rapeseed leaves also exhibited significant stimulatory effects on virulence. For isolate JMS14, the stimulatory concentration ranges for dimethachlone and prochloraz were around 1 to 600 μg/ml and 0.0003 to 0.18 μg/ml, respectively. The fitted curve of virulence stimulation for the mixture of dimethachlone and prochloraz shifted to the left on the x axis, denoting dose-additive interactions between the two fungicides with regard to virulence stimulation. Spraying dimethachlone alone at 10 to 50 μg/ml had significant stimulations on virulence, whereas prochloraz alone at 10 to 50 μg/ml had significant inhibitory effects on virulence, and the mixture of dimethachlone and prochloraz at the concentration ratio of 1:1 had greater inhibitory effects than prochloraz alone, indicating dose-additive interactions for the inhibitory effects. Dimethachlone and prochloraz and their mixtures increased tolerance of mycelia to hydrogen peroxide. Dimethachlone significantly increased, whereas prochloraz reduced mycelial cell membrane permeability, and the mixture of the two fungicides had effect-additive interactions with respect to effects on cell membrane permeability. These studies will advance our understanding of hormesis of fungicide mixtures.