Research On Biological Activity Of Two QoI Fungicide Azoxystrobin And Pyraclostrobin Against Ustilaginoidea Virens

Rice false smut(RFS)caused by Ustilaginoidea virens is one of the most severe rice diseases on rice.Chemical control is a significant measure to control RFS,but,at present,there are reports about resistance of U.virens to sterol demethylation inhibitor fungicides in China.Quinone outside inhibitor(QoI)fungicides have a wide spectrum and high efficiency,and have been widely used to control various fungal diseases.However,some of these products have not yet been registered for the control of RFS in China.This paper studied the biological activity and control effect of azoxystrobin and pyrazolate on U.virens,providing guidance for the control of RFS.In the sensitivity determination of QoI fungicides in vitro,salicylhydroxamic acid(SHAM)has been commonly added to artificial culture media in order to inhibit alternative oxidase of phytopathogenic fungi.However,some studies showed that artificial media should not include SHAM due to its toxicity.Whether SHAM should be added in the assay of U.virens sensitivity to QoI fungicides remains unknown.In this study,two media,potato sucrose agar(PSA)and minimal medium(MM),were selected to test SHAM toxicity and sensitivity of U.virens to azoxystrobin and pyraclostrobin.The mycelial growth and sensitivity of U.virens to azoxystrobin and pyraclostrobin had no significant difference between on PSA and MM.SHAM could significantly inhibit mycelial growth,conidial germination,peroxidase(POD)and esterase activity of U.virens.Average effective concentration for inhibiting 50%(EC50)values of SHAM against mycelial growth of ten U.virens were 27.41 and 12.75 μg/mL on PSA and MM,respectively.The EC50 values of SHAM against conidial germination of isolates HWD and JS60 were 70.36 and 44.69 μg/mL,respectively.SHAM at 30 μg/mL significantly inhibited POD and esterase activity of isolates HWD and JS60,and even SHAM at 10 μg/mL significantly inhibited POD activity of isolate HWD.In addition,SHAM significantly reduced EC50 values and EC90 values of azoxystrobin and pyraclostrobin on both PSA and MM.Even with the addition of 10 μg/mL SHAM,average EC50 values of ten U.virens isolates for azoxystrobin decreased 1.7-fold on PSA and 4.8-fold on MM,and for pyraclostrobin that decreased 2.8-fold on PSA and 4.8-fold on MM.Therefore,these results suggest that SHAM should not be added to artificial media in the assay of U.virens sensitivity to QoI fungicides.Without adding SHAM,this study has determined the sensitivities of 179 isolates of U.virens to the QoI fungicides azoxystrobin and pyraclostrobin.The distribution of the EC50 values of each fungicide was unimodal,and the frequency distribution of logarithmically transformed EC50 values fit or fit closer to a normal distribution.The ranges of EC50 values for azoxystrobin and pyraclostrobin were 0.001 to 0.864 and 0.001 to 0.569 μg/mL,with means and standard errors of the mean values of 0.203±0.012 and 0.079±0.006 μg/mL,respectively.There was a significant weak positive correlation(n=100,r=0.469,P=0.001)between the EC50 values of these two fungicides.No cross-resistance was found between azoxystrobin,pyraclostrobin and carbendazim or sterol demethylation inhibitor fungicides.Each fungicide had a significantly higher mean preventive efficacy compared with its curative efficacy.Field assays showed that the control efficacy of pyraclostrobin against RFS was greater than that of azoxystrobin.Pyraclostrobin had the best control of RFS in three rice varieties,with the control efficacy ranging from 81.5 to 95.5%,whereas azoxystrobin decreased the disease index by 64.1 to 69.2%under the same conditions.These results provide us a reference point in the management of U.virens and future QoI fungicides resistance monitoring programs.