Bilogical Activities And Resistance Risk Assessment Of Four Triazole Fungicides Againist Rhizactonia Solani And Fusarium Fujikuroi

Rice sheath blight,caused by Rhizoctonia solani,is one of the three major diseases of rice.Rice bakanae disease is a fungal seed-borne disease caused by Fusarium fujikuroi.At present,chemical fungicides are mainly used to control these two diseases.However,due to the long-term use of single chemical agents and the unscientific use of fungicides,the pathogens have developed resistance,and the control effect is gradually declining.Therefore,screening new agents is very necessary to slow down the development of pathogen resistance and to prevent and control the disease.Four triazole fungicides epoxiconazole,mefentrifluconazole,propiconazole and prothioconazole were found to have strong inhibitory effects on Rhizoctonia solani and Fusarium fujikuroi in vitro.The method of mycelial growth rate was used to establish sensitivity baselines of R.solani to epoxiconazole,mefentrifluconazole,propiconazole and prothioconazole.The result showed that the distributions range of EC₅₀value of epoxiconazole,mefentrifluconazole,propiconazole and prothioconazole against 93 isolates,80 isolates,80isolates and 99 isolates of R.solani were 0.0069～0.1356μg/m L,0.0020～0.0440μg/m L,0.0101～0.3518μg/m L and 0.0667～0.6213μg/m L respectively.The average EC₅₀values were 0.0586±0.0330μg/m L,0.0180±0.0012μg/m L,0.1917±0.0113μg/m L and 0.2082±0.1003μg/m L respectively.The four sensitivity baselines were all unimodal curves.The effects of four fungicides on the biological activity of R.solani were determined.The results indicate that at two concentrations of EC_90,epoxiconazole,mefentrifluconazole,propiconazole and prothioconazole could reduce the yield of sclerotia and cause intracellular matrix edema of R.solani and resulted in large area of low electron density edema in mycelium cells,mitochondria swelling and rough endoplasmic reticulum expansion and increase the membrane permeability of mycelium and lead to the increase of relative electrical conductivity.All these four fungicides could increase the permeability of the membrane of mycelium and increase the electrolyte extravasation of mycelium cells.These four fungicides had excellent control effect on rice sheath blight.At the concentration of 200μg/m L,the protective effect of epoxiconazole,mefentrifluconazole,propiconazole and prothionazole were 94.07%,88.36%,95.44%and 89.06%respectively;the curative effect were 88.60%,88.60%,88.38%and 89.97%respectively.The method of mycelial growth rate was used to establish sensitivity baselines of F.fujikuroi to epoxiconazole,mefentrifluconazole,propiconazole and prothioconazole.The result showed that the distributions range of EC₅₀value of epoxiconazole,mefentrifluconazole,propiconazole and prothioconazole against 83 isolates,92 isolates,80isolates and 87 isolates were 0.012～0.1335μg/m L,0.0196～0.3570μg/m L,0.0298～0.2110μg/m L and 0.0782～0.7886μg/m L respectively.The average EC₅₀values were 0.0661±0.0038μg/m L,0.1551±0.0078μg/m L,0.1067±0.0043μg/m L and 0.3535±0.0184μg/m L respectively.The four sensitivity baselines were all unimodal curves,close to normal distribution.The effects of four fungicides on the biological activity of F.fujikuroi were determined.The results showed that EC₅₀or EC₉₀concentrations of epoxiconazole,mefentrifluconazole and prothioconazole had no effect on the mycelial morphology of F.fujikuroi;EC₅₀or EC₉₀concentrations of propiconazole could enlarge the hyphae tip of F.fujikuroi.The sporulation of F.fujikuroi was significantly decreased after treated with four fungicides respectively.EC₉₀concentration of the four fungicides did not affect the distribution and localization of mycelium nucleus.Analysis of the subcellular structure of F.fujikuroi after treated with fungicides showed that the four fungicides with EC₉₀concentration could cause the intracellular matrix edema of F.fujikuroi,make the cell membrane locally indent inward,the hyphae cells appear large area of low-electron density edema,and destroy the organelle structure of F.fujikuroi,and cause the mitochondria and rough endoplasmic reticulum to swell and expand.The four fungicides could increase the membrane permeability of mycelium,increase the electrolyte extravasation of mycelium cells,and increase the relative electrical conductivity.In the safety of fungicides to rice test,at the concentration of 1000μg/m L,epoxiconazole and prothionazole had no effect on the germination rate,plant height and fresh weight of rice;1000μg/m L mefentrifluconazole decreased the germination rate of rice seeds,but had no effect on plant height and fresh weight.When propiconazole was less than 500μg/m L,the germination rate and plant height of rice seeds were not affected,and the fresh weight of rice plants was significantly increased;When the concentration reached 1000μg/m L,the germination rate of rice seeds was significantly decreased,but the plant height and fresh weight were not affected.All the four fungicides could shorten the germination length of rice seeds,and the germination length decreased gradually with the increase of the concentration of fungicide.Five propiconazole-resistant mutants were obtained by means of fungicide acclimation.Resistance frequency was 9.1×10^-4.The mutants were all low resistant strains and the resistance could be inherited stably.Compared with the parent strain,the colony diameter of the propiconazole resistant mutants were smaller,and the dry weight of the mycelium was larger.There was no uniform trend of sporulation.Propiconazole resistant mutants were less sensitive to the same type of fungicides tebuconazole and different types of fungicides fludioxonil,and the membrane permeability of mycelium was reduced in resistant mutants.The mycelial morphology of the mutant was denser.The mutants were less resistant to low temperature and more sensitive to metal ions.The pathogenicity of most propiconazole resistant mutants was lower than that of their parents.Based on the above results and the inherent resistance risk of F.fujikuroi,we think that the resistance risk of F.fujikuroi to propiconazole is low to medium.Three mefentrifluconazole-resistant mutants were obtained by means of fungicide acclimation.Resistance frequency was 5.7×10^-4.The mutants were all low resistant strains.And the resistance could be inherited stably.The dry weight and sporulation of the mefentrifluconazole resistant mutants were significantly lower than that of their parents,and the colony diameter showed no uniform trend.Mefentrifluconazole resistant mutants were less sensitive to tebuconazole,fluazinam,prochloraz,fludioxonil and phenamacril,and the membrane permeability of mycelium was reduced.The mycelial morphology of the mutant was denser.The mutants were less resistant to low temperature and more sensitive to metal ions.The pathogenicity of resistant mutants was significantly lower than that of their parents.Based on the above results and the inherent resistance risk of F.fujikuroi,we think that the resistance risk of F.fujikuroi to mefentrifluconazole is low to medium.