Control Effect Of Mefentrifluconazole On Sclerotium Rolfsii From Peanut And Toxicological Evaluation On Soil Microorganisms

Southern blight of peanut is a soil borne fungal disease caused by Sclerotium rolfsii.In recent years,due to the continuous cropping of peanut and the increase in planting area,the disease has become increasingly serious in China.Currently,the use of chemical agents is a commonly used measure to control southern blight of peanut,but there are few registered agents used to control southern blight of peanut in China.Considering that most registered agents have a single target of action,long-term use can easily lead to drug resistance issues,which further increases the difficulty of southern blight of peanut management.Therefore,there is an urgent need to find efficient and safe new fungicides as rotation agents to delay the development of resistance to southern blight of peanut.Mefentrifluconazole is a new type of isopropanol-triazole compound.Our laboratory has previously found that mefentrifluconazole has high inhibitory activity against S.rolfsii.Therefore,in this study,261 strains of southern blight of peanut were isolated from Henan,Shandong,and Hebei provinces.The sensitive baseline of southern blight of peanut strains to mefentrifluconazole was established using the mycelial growth rate method,and the sensitive correlation between mefentrifluconazole and seven fungicides was further evaluated,as well as its protective and curative activities against southern blight of peanut.In addition,in order to scientifically use mefentrifluconazole,this study also evaluated the toxicity of mefentrifluconazole to earthworms,soil microorganisms,and soil enzyme activities,and further determined its residual dynamics and leaching ability in soil.The main research results are as follows:1.The inhibitory effect of mefentrifluconazole on the growth of the mycelium of S.rolfsii was concentration dependent,and the number of mycelial branches significantly increased at the concentration of 1 mg/L and 5 mg/L of mefentrifluconazole.In the shake culture experiment,the dry weight of mycelium of ZM2 and XY17 strains could be inhibited under the treatment of 0.2 mg/L mefentrifluconazole,with inhibition rates of 55.5% and 57.1%,respectively,while the inhibition rates under the treatment of 2 mg/L mefentrifluconazole were 87.2% and 79.7%,respectively.The inhibitory activity of mefentrifluconazole on sclerotia germination of S.rolfsii was poor,but it could inhibit sclerotia formation.The research results showed that under the concentration of 1 mg/L,the sclerotia yield and dry weight could be significantly reduced,with inhibition rates of 66.45% and 46.61%,respectively.At the same time,compared with the control group,the germination rate of sclerotium formed after treatment with mefentrifluconazole was significantly decreased,and the germination rates of mefentrifluconazole at 0.5 mg/L and 1 mg/L concentrations were only 83.33% and 63.33%,respectively.After analyzing the nutrients in the sclerotium above,it was found that the protein,reducing sugar,and fat content in the sclerotium were lower than those in the control group,and the higher the concentration of mefentrifluconazole,the greater the decrease in the nutrient content.Due to the need for sufficient energy supply for sclerotia germination,the lack of three major nutrients inevitably leads to a decrease in its germination rate.The above results indicate that mefentrifluconazole has a high biological activity against the hyphae of S.rolfsii,and can significantly reduce the formation and germination of sclerotium of S.rolfsii,thereby effectively reducing the infection and prevalence of S.rolfsii.2.In view of the great application potential of mefentrifluconazole against S.rolfsii.In this study,the sensitivity of mefentrifluconazole to 261 strains of pathogenic fungi collected from Henan,Shandong,and Hebei was further determined.The average EC₅₀ value was 0.21± 0.11 mg/L,with a distribution range of 0.02 to 0.55 mg/L.The frequency distribution of sensitivity of all strains to mefentrifluconazole was a single peak curve,and the overall frequency distribution conformed to a normal distribution.Therefore,the average value can be used as a sensitive baseline value,Used for monitoring the field resistance of S.rolfsii to mefentrifluconazole.In addition,the average EC₅₀ values of mefentrifluconazole against strains from Henan,Shandong,and Hebei provinces were 0.20 ± 0.10 mg/L,0.21 ± 0.11mg/L,and 0.21 ± 0.12 mg/L,respectively,with no significant difference.By evaluating the sensitive correlation between mefentrifluconazole and seven fungicides,the research results found that there was no correlation between mefentrifluconazole and sterol demethylation inhibitors（DMIs,tebuconazole）,extraquinone inhibitors（Qo Is,picoxystrobin and pyraclostrobin）,and succinate dehydrogenase inhibitors（SDHIs,carboxin,thifluzamide,boscalid,and isopyrazam）,indicating that mefentrifluconazole can be used interchangeably with these fungicides,thereby delaying the development of its resistance.The indoor pot experiment was conducted to evaluate the protective,curative efficacy,and sustained effectiveness of mefentrifluconazole against southern blight of peanut.The results showed that the protective activity of mefentrifluconazole was superior to curative activity and had a longer duration of efficacy.After 9 days of inoculation with pathogenic fungi,the protective activity of mefentrifluconazole at 100 mg/L and 200 mg/L was 81.09% and 95.36%,respectively,while the curative activity of mefentrifluconazole at 200 mg/L was only 60.94%.3.Considering that southern blight of peanut is a soil borne fungal disease,mefentrifluconazole must inevitably enter the soil when applied.Therefore,this study further evaluated the impact of mefentrifluconazole on soil environmental organisms.The results showed that the LC₅₀ of mefentrifluconazole to eisenia foetida was 143 mg a.i./kg and 36.6mg a.i./kg at 7 and 14 days,respectively,indicating that mefentrifluconazole was low toxic to eisenia foetida.Two soils,clay loam soil and sandy loam soil,were treated at concentrations of 0.1,1,10,and 100 mg/kg of mefentrifluconazole.The results showed that mefentrifluconazole initially inhibited the activities of soil urease and acid phosphatase,and increased with the increase of concentration.With the extension of time,the inhibition gradually turned into an activation effect,and by the 60 th day of the experiment,the activities of the two soil enzymes returned to the control level.Sucrase activity could be activated by mefentrifluconazole,but the enzyme activity returned to the control level after 60 days of testing.There are also differences in the impact of mefentrifluconazole on the number of soil microorganisms,with greater impact on the number of fungi than on the number of bacteria and actinomycetes.The half-life of mefentrifluconazole at different concentrations in both soils are greater than 100 days,and they are not easily degradable pesticides.Through the leaching test,it was found that the Rf value of mefentrifluconazole in both tested soils was0.25,indicating that it was an immovable pesticide.The above results indicate that mefentrifluconazole should be used with caution.