Effects Of Prochloraz On The Growth And Virulence Of Sclerotinia Sclerotiorum

Sclerotinia stem rot?SSR?of oilseed rape is caused by Sclerotinia sclerotiorum,one of the most destructive and widespread fungal pathogens in the world.Prevention and control of SSR are still dominated by fungicides.It has been more than ten years since prochloraz was first registered in China for control of SSR around 2007,nevertheless,baseline sensitivity of S.sclerotiorum to prochloraz has not been established.In this study,baseline sensitivity of S.sclerotiorum to prochloraz,control efficacy of prochloraz and hormetic effects of the mixture of prochloraz and dimethachlone on S.sclerotiorum were investigated to provide a theoretical basis for scientific and judicious applications of prochloraz.Relative baseline sensitivity of S.sclerotiorum to prochloraz was established.The mean effective concentration causing 50%mycelial growth inhibition(EC₅₀)values for isolates collected in 2008?N=73?and 2014?N=76?were 0.0463?g/mL±0.0190?SD?and 0.0434?g/mL±0.0193?SD?,respectively.Both frequency distributions of EC₅₀values for 2008 and 2014 were unimodal.There was no significant difference?P=0.348?in EC₅₀ values between the two years.Preventive efficacy of prochloraz at 20?g/mL against SSR on potted rapeseed plants was 98.7%,and curative efficacy of prochloraz at90?g/mL was 78.97%.Cell membrane permeability decreased significantly?P<0.05?for mycelia grown on potato dextrose agar?PDA?amended with prochloraz at 0.01 and0.04?g/ml.Observations with a transmission electron microscopy showed that the cell wall of mycelia grown on PDA supplemented with prochloraz at 0.03?g/ml became thicker than the nontreated control.The observed percent inhibitions of mycelial growth on PDA amended both with prochloraz at 0.01?g/ml and the reagent Congo red at a series concentrations were lower than the expected percent inhibitions,indicating that prochloraz reduced sensitivity of mycelia to Congo red,thereby implying the content of chitin in cell wall declined for prochloraz-treated mycelia.Prochloraz at 0.03?g/mL in PDA significantly?P=0.041?increased the mannan content in mycelia.Prochloraz,dimethachlone and the mixture of the two fungicides had hormetic effects on mycelial growth on PDA and virulence on rapeseed leaves for dimethachlone-resistant strains of S.sclerotiorum.The mycelial growth stimulation ranges of prochloraz in PDA at 0.0002⁰.004?g/mL and dimethachlone at 1¹00?g/mL for isolate HLJ4 were 5.44%¹1.09%and 6.4%¹9.25%,respectively.The stimulation range of the mixture of prochloraz and dimethachlone?mix ratio,1:5000?at 0.0002?g/mL?prochloraz?+1?g/mL?dimethachlone?⁰.004?g/mL?prochloraz?+20?g/mL?dimethachlone?was 2.91%⁹.34%.The virulence stimulation ranges of spraying prochloraz at 0.0004⁰.04?g/mL and dimethachlone at 10²00?g/mL for isolate HLJ4on rapeseed leaves were 18.9%³0.19%and 23.03%⁴5.91%,respectively.The virulence stimulation ranges of spraying the mixture of prochloraz and dimethachlone?mix ratio,1:5000?at 0.0004?g/mL?prochloraz?+2?g/mL?dimethachlone?⁰.04?g/mL?prochloraz?+200?g/mL?dimethachlone?was 10.04%⁴8.33%.After the mycelia grown on PDA amended with prochloraz,dimethachlone or the mixture of the two fungicides were inoculated on PDA and oilseed rape leaves in the absence of any fungicide,mycelial growth rate and virulence were higher than those of nontreated control.Prochloraz significantly reduced?P<0.05?while dimethachlone significantly increased?P<0.05?mycelial cell membrane permeability,and the mixture of prochloraz and dimethachlone?mix ratio,1:5000?had no significant?P>0.05?effect on cell membrane permeability compared with the nontreated control.Low-doses of prochloraz,dimethachlone and the mixture of the two fungicides?mix ratio,1:5000?could increase tolerance of S.sclerotiorum to H₂O₂.