Bioactivity Of Four Fungicides Against Rhizoctonia Solani And Magnaporthe Oryzae

Rice sheath blight caused by Rhizoctonia solani and rice blast caused by Magnaporthe oryzae are two very harmful fungal diseases in rice.These diseases can affect the yield and quality of rice greatly,and threat the food security of the world seriously.Chemical control is the most effective ways,however,due to the resistance of the pathogen,the problem of the decline in control efficiency has become increasingly prominent.Therefore,it is urgent to screen new fungicides to the control of rice sheath blight and rice blast.In this paper,the sensitivity baselines of sedaxane and benzovindiflupyr to rice sheath blight,and the sensitivity baselines of fluazinam and metconazole to rice blast were established and their activities were determined.The risk of resistance to metconazole was assessed.The sensitivity of 107 strains of R.solani to sedaxane was determined by mycelia growth rate and the sensitivity baseline was established.The EC50 value ranged from 0.0035 to 0.0931 μg/mL,and the average EC50 value was 0.0325±0.018 μg/mL.After the treatment with sedaxane,the production of sclerotia of R.solani was seriously inhibited;mycelia branching were increased;respiration was inhibited;mycelia membrane damage caused electrolyte leakage increased;and mycelium was vacuolated seriously.However,sedaxane did not affect the distribution of mycelia nuclei and the formation of cell walls.200 μg/mL of sedaxane showed good control effect on rice sheath blight.In addition,sedaxane was safe for rice growth.The sensitivity of 92 strains of R.solani to benzovindiflupyr was determined by mycelia growth rate and the sensitivity baseline was established.The EC50 values ranged from 0.0002 to 0.0064μg/mL and the average EC50 value was 0.0025±0.0014μg/mL.After the treatment with benzovindiflupyr,there was no significant effect on the mycelia morphology,the distribution of nucleus and the formation of cell wall after the treatment with benzovindiflupyr.However,benzovindiflupyr inhibited the production of sclerotia,mycelial growth and respiration.The experimental results of the permeability of mycelia cell membrane and the ultrastructure of mycelia cells showed that benzoenflurazole destroyed the cell structure of mycelia,increased the extravasation of electrolytes in the cells,wrinkled the cells and degraded the internal tissues.When the concentration reached 200 μg/mL,benzovindiflupyr showed excellent protective and curative effects on rice sheath blight,and the protective activity and curative activity reached 92.90%and 70.41%respectively.In addition,benzovindiflupyr was safe for rice growth.The sensitivity of M.oryzae to fluazinam was determined by mycelia growth rate and conidia germination.The EC50 values of fluazinam on the mycelia of 102 M.oryzae strains ranged from 0.2453 to 0.8247 μg/mL,and the average EC50 value was 0.4733±0.1296μg/mL.The EC50 values of conidia germination were 0.0298～0.0966 μg/mL and the average EC50 value was 0.0567±0.014 μg/mL,respectively.Fluazinam could inhibit the conidia production and respiration of mycelia,increase the permeability of mycelia cell membrane,and degrade the internal tissue of mycelia.But fluazinam did not affect the distribution of mycelia nucleus and the formation of cell wall.The experimental results showed that fluazinam had excellent protective activity,but little curative activity.The safety test results indicated that fluazinam had no safety problem in rice growth.The sensitivity of 99 strains of M.oryzae to metconazole was determined by mycelia growth rate and the sensitivity baseline was established.The EC50 ranged from 0.2545 to 0.5758 μg/mL and the average EC50 value was 0.3638±0.0704μg/mL.After the treatment with metconazole,the structure of mycelia was destroyed,mycelia cells became wrinked,the permeability of cell membrane increased,but the formation of nucleus and the distribution of cell wall were not affected.Metconazole could also inhibit the conidia production of M.oryzae and provide good protective activity and curative activity.However,higher concentrations of metconazole could affect the growth of rice plants.Seven resistant mutants were obtained by fungicide domestication,and all of them were low resistant strains and could inherit stably.Compared with the parental strains,some metconazole-resistant mutants decreased the mycelial growth rate and cell membrane permeability,which showed different sensitivity changes to various stresses,and decreased resistance to temperature and PH.The pathogenicity of metconazole-resistant mutants increased or decreased,but the conidia production ability of all mutants decreased significantly.The above results showed that there was a low resistance risk for metconazole in M.oryzae.By assayed the cross resistance,there was positive cross-resistance betwee n epoxiconazole,prochloraz and metconazole,the sensitivity of metconazole-resistant mutants to azoxystrobin was decreased,but the sensitivity of metconazole-resistant mutants to isoprothiolane was higher than that of the parental strains.