Sensitivity And Biological Fitness Of Rhizoctonia Solani To Fungicides

Rice sheath blight is a fungal disease caused by Rhizoctonia solani,which seriously affects yield and quality of rice.At present,chemical control are main measures for sheath blight.However,Long-term use of fungicides may lead to resistance risks.Therefore,sensitivity baselines of R.solani to hexaconazole,thifluzamide,azoxystrobin and difenoconazole were established,and resistance of R.solani were monitored during 2019～2020.The fitness of resistant isolates were evaluated,and it was related to expression of target genes.In this study,in order to provide reference for scientific control of rice sheath blight,we clarified that the resistance tisk of hexaconazole,thifluzamide and difenoconazole against R.solani were moderate resistance risk.However,the resistance tisk of azoxystrobin against R.solani was a low resistance risk.The main results are as below:1.The sensitivity baselines of R.solani to four fungicides were established,and three resistant isolates of difenoconazole were found.A number of 85 R.solani strains were collected from Guizhou,Hubei and Henan province in 2019,and their sensitivity of hexaconazole,thifluzamide,azoxystrobin and difenoconazole were measured using hyphal growth rate method.The EC₅₀ values 0.048,0.051,1.29,2.05μg/mL were used as sensitivity baseline,respectively.There were significant differences in sensitivity of R.solani to thifluzamide,difenoconazole and azoxystrobin.The field resistance monitoring distinguishing dose of R.solani to hexaconazole,thifluzamide,azoxystrobin,and difenoconazole were 10,18,170 and 150μg/mL,respectively.It was found that 3isolates were resistant to difenoconazole by monitored resistance from 204 isolates R.solani,and the resistance ratios were 3.69,3.57 and 4.37,respectively.This research had made clarified that rice sheath blight fungus strains in southern rice regions were still relatively sensitive to four fungicides,but the changes in sensitivity needed to be closely monitored.2.The mycelial growth rate was rapid type,and the pathogenicity of R.solani was different in different regions,and the sensitivity difference was related to mycelial growth rate or biomass dry weight.The mycelial growth rate of R.solani was 43.67～68.18 mm/d,and it was significantly different in different areas by determined the mycelial growth rate,biomass dry weight and pathogenicity,and the pathogenicity of R.solani was different in different areas.The sensitivity of R.solani to hexazolol,thiofuramide and azoxystrobin was significantly correlated with its growth rate.However,the sensitivity of R.solani to difenoconazole was significantly correlated with its biomass dry weight.The biological fitness of R.solani in different areas might be related to its location.3.The 27 resistant isolates were obtained by fungicides induction,and their biological fitness were better than or equal to parents.27 resistant isolates were obtained by exposing the parental isolates on PDA containg fungicides.The resistance level were between 18.80 and 79.06,which belonged to medium resistance level.After10th transfer culture on fungicides-free medium,one hexaconazole-resistant isolate and thifluzamide-resistant isolate were both decreased to sensitive level.The result showed that biological fitness of resistant isolates of hexaconazole,thifluzamide and difenoconazole were significantly stronger than their parental isolate.However,the biological fitness of azoxystrobin-resistant isolate was lower than that of their parental isolate.The cross resistance showed that hexaconazole-resistant isolate was showed positive cross resistance between the hexaconazole and propiconazole.However,resistant iaolates of thifluzamide,difenoconazole and azoxystrobin were no cross-resistance between six fungicides.4.The resistance of resistant isoletes related to target genes expression.The genes expression of resistant isolates were determined by RT-q PCR.The genes expression of CYP51B in HS2-JR-1 and HS2-JR-2 were 6.84 and 8.44 times higher than its parents,respectively.The genes expression levels of SDHA,SDHB,SDHC and SDHD in HS2-SR-1 were decreased by 71.73%,80.68%,74.42%and 77.27%compared with its parents,respectively.But the genes expression levels of SDHA,SDHB,SDHC and SDHD in HP1-SR-4 were decreased by 82.90%,83.91%,73.03%and 72.22%compared with its parents,respectively.The genes expression levels of CYP51A in difenoconazole resistant isolate HS2-BR-2 and WX4-BR-5 were 4.12 and 6.45 times higher than its parents,and CYP51B in difenoconazole resistant isolate HS2-BR-2 and WX4-BR-5were 11.99 and 8.27 times higher than its parents,respectively.However,there were no significant difference in the expression of Cytb between azoxystrobin resistant isolates and its parents.The resistance of R.solani to hexaconazole,difenoconazole and thiafluzamide might be related to the up-regulation or down-regulation of target genes expression.