Risk Assessment Of Resistance To Cyclobutrifluram In Fusarium Fujikuroi Causing Rice Bakanae Disease

Rice bankanae disease（RBD）,caused by Fusarium fujikuroi,is a seed-borne fungal disease with great harm and wide distribution.It has been found in the major rice cultivation areas of Asia,Africa and North America.And in China,this disease occurred commonly,especially in Jiangsu,Zhejiang,Shanghai,and Anhui Provinces.At present,the control of this disease mainly depended on fungicides.However,due to the long-term use of chemical fungicides,F.fujikuroi has developed resistance to some commonly used fungicides.Cyclobutrifluram,a novel succinate dehydrogenase inhibitor（SDHI）developed by Syngenta,has been reported to be effective in the control of soil-borne diseases.Previous studies have confirmed that cyclobutrifluram could inhibit the growth of F.fujikuroi significantly,while the inhibition mechanism is unclear.In the study,the biological activity of cyclobutrifluram against F.fujikuroi was determined and the resistance risk was assessed.The results were listed as follows:1.The inhibition effects of cyclobutrifluram against 13 pathogenic fungi were measured by mycelia growth rate method.The results showed that it had great inhibition activity against F.fujikuroi,F.graminearum and F.oxysporum and the EC₅₀was 0.0317、0.3747、0.7745μg/m L,respectively.Concerning the highest inhibition activity of cyclobutrifluram against F.fujikuroi,following experiments were carried out.2.The baseline sensitivity of F.fujikuroi to cyclobutrifluram was established by mycelial growth rate and spore germination method.The EC₅₀values varied from 0.0114to 0.1304μg/m L and 0.0012 to 0.016μg/m L,and the average EC₅₀values were 0.0410±0.0470μg/m L and 0.00378±0.00147μg/m L,respectively.Both of the two baseline sensitivity curves were unimodal over a sensitive range.3.Under the treatment of cyclobutrifluram,the morphology of mycelia and spores were altered with distortion,deformation,rupture and shriveled.And the membrane permeability increased,the ATP content and succinate dehydrogenase activity reduced.4.The results of greenhouse pot experiment cyclobutrifluram exhibited excellect protective and curative activities against F.fujikuroi.The protective and curative efficacies of 100、200、300μg/m L cyclobutrifluram were 60.00%,77.14%,88.58%and 48.56%,60.00%,71.42%,respectively,which were significantly higher than the treatment of prochloraz and phenamacril at the same dosage.5.Three cyclobutrifluram-resistant mutants were obtained through fungicide generation.Nucleotide sequences alignment results of Sdh gene of wild type and resistant mutants found a single point mutation:Ff Sdh C₂^A83V.6.The mycelial growth rate,sporulation ability and pathogenicity of all cyclobutrifluram-resistant mutants were increased compared with wild type,and the ATP content and succinate dehydrogenase activity were significantly increased,but the dry weight of mycelia appreared lower.7.The results of cross-resistance experiments showed that all resistant mutants had positive cross-resistance with pydiflumetofen,while no cross-resistance with phenamacril,carbendazim or prochloraz.In summary,cyclobutrifluram exhibited great activity against F.fujikuroi in vivo and in vitro,which indicated that it could be used as a candidate fungicide for controlling RBD.Besides,F.fujikuroi has moderate to high resistance risk to cyclobutrifluram and the resistance monitoring should be performed.All results could provide a theoretical basis for the use of cyclobutrifluram in field,and provide scientific guidance for monitoring resistance risk in the future.