Identification Of Tobacco Root Rot Pathogen And Screening Of Control Agents

In recent years,tobacco Fusarium root rot has gradually increased from a secondary disease to a major disease in some tobacco areas,causing serious economic losses to tobacco production.At present,there are relatively few reports on the screening of field fungicides for tobacco Fusarium root rot,so it is of great significance to screen fungicides with good effect on the prevention and control of tobacco Fusarium root rot.In the past,the commonly used fungicides（metalaxyl,dimethomorph,thiram,zineb,etc.）not only had poor control effect on tobacco Fusarium root rot,but also showed certain resistance to other common tobacco rhizome diseases after years of use,so they should not be used more.Therefore,the author determined the indoor toxicity of six new and commonly used fungicides to tobacco Fusarium root rot,and then carried out the compound research.The main results are as follows:（1）The pathogen of tobacco root rot was isolated and identified by tissue isolation method and Koch’s rule.The strains G25-2 and X15-4 were identified as Fusarium commune,X7-2 and G29-3 were identified as Fusarium oxysporum by morphological observation and molecular identification.（2）Biological characteristics observation showed that:F.commune G25-2 could grow on PDA plates at p H 4-11,and the optimum p H was 8.It can use a variety of carbon and nitrogen sources,but its growth rate and mycelial morphology are different.The optimum carbon source is sorbitol,and the optimum nitrogen source is sodium nitrate.The suitable growth temperature range is 22～28°C,and the optimum temperature is 28°C.The lethal temperature of the pathogen was 55°C for 10 min.F.oxysporum X7-2 could grow on PDA plates at p H 4-11,and the optimum p H was 7.It can use a variety of carbon and nitrogen sources,but its growth rate and mycelial morphology are different.The optimum carbon source is mannitol,and the optimum nitrogen source is peptone.The suitable growth temperature range is 22～28°C,and the optimum temperature is 25°C.The lethal temperature of the pathogen was 65°C for 10 min.F.oxysporum G29-3 could grow on PDA plates at p H 4-11,and the optimum p H was 7.It can use a variety of carbon and nitrogen sources,but its growth rate and mycelial morphology are different.The optimum carbon source is sorbitol,and the optimum nitrogen source is peptone.The suitable growth temperature range is 22～28°C,and the optimum temperature is 28°C.The lethal temperature of the pathogen was 60°C for 10 min.F.commune X15-4 could grow on PDA plates at p H 4-11,and the optimum p H was 9.Itcan use a variety of carbon and nitrogen sources,but its growth rate and mycelial morphology are different.The optimum carbon source is sorbitol,and the optimum nitrogen source is sodium nitrate.The suitable growth temperature range is 22～33°C,and the optimum temperature is 28°C.The lethal temperature of the pathogen was 60°C for 10 min.（3）The toxicity of fungicides was determined by mycelial growth rate method.The results are as follows:The EC₅₀ values of the six fungicides to F.commune G25-2 were fludioxonil（0.0589mg/L）,fluazinam（0.0919 mg/L）,phenamacril（0.1593 mg/L）,difenoconazole（0.4740mg/L）,pydiflumetofen（1.0622 mg/L）,pyraclostrobin（1.2252 mg/L）.The EC₅₀ values of the six fungicides against F.oxysporum X7-2 were fludioxonil（0.0237mg/L）,difenoconazole（0.1767 mg/L）,fluazinam（0.1885 mg/L）,pyraclostrobin（0.2979mg/L）,pydiflumetofen（1.9075 mg/L）,phenamacril（2.2241 mg/L）.The EC₅₀ values of the six fungicides against F.oxysporum G29-3 were fludioxonil（0.0293 mg/L）,fluazinam（0.0567 mg/L）,difenoconazole（0.1381 mg/L）,pyraclostrobin（0.3013 mg/L）,pydiflumetofen（1.2259 mg/L）,phenamacril（2.8158 mg/L）.The EC₅₀ values of the six fungicides to F.commune X15-4 were fludioxonil（0.0495mg/L）,fluazinam（0.1503 mg/L）,phenamacril（0.2377 mg/L）,difenoconazole（0.4515mg/L）,pyraclostrobin（7.5686 mg/L）,pydiflumetofen（131.1416 mg/L）.（4）Sun Yunpei’s method was used to select the above two fungicides with strong toxicity to carry out the compound experiment.The results of the compounding experiment of difenoconazole and pyraclostrobin showed that:In the experiment of F.commune G25-2,the results showed that the 10 compoundingratios had synergistic effects,and the order of synergistic coefficient from small to large was1:10,8:1,10:1,6:1,4:1,1:2,2:1,1:8,1:6,1:4.The maximum synergistic coefficient of the compounding ratio of 1:4 was 2.47,and the EC₅₀ was 0.4041 mg/L.In the experiment of F.oxysporum X7-2,the results showed that two compounding ratios had synergistic effects.The order of synergistic coefficients from small to large was 1:2 and1:4.The maximum synergistic coefficient of the compounding ratio of 1:4 was 1.67,and the EC₅₀ was 0.1566 mg/L.In the experiment of F.oxysporum G29-3,the results showed that three compounding ratios had synergistic effects.The order of synergistic coefficients from small to large was10:1,4:1,1:4.The maximum synergistic coefficient of the compounding ratio of 1:4 was1.76,and the EC₅₀ was 0.1387 mg/L.In the experiment of F.commune X15-4,the results showed that three compounding ratios had synergistic effects.The order of synergistic coefficients from small to large was 1:8,1:4,and 1:10.The maximum synergistic coefficient of the compounding ratio of 1:10 was 2.01,and the EC₅₀ was 1.5471 mg/L.The synergistic coefficient of the compounding ratio of 1:4was 1.69,and the EC₅₀ was 1.0777 mg/L.（5）The mixture of difenoconazole and pyraclostrobin with a ratio of 1:4 was used to develop the emulsifiable concentrate.Finally,the formula of 40%difenoconazole+pyraclostrobin EC（mass fraction）was determined as follows:40%of the original drug,emulsifier（Nongru 500#:Nongru 600#=1:3）8%,solvent（ethyl acetate）52%.（6）The pot experiment showed that the control effect of 40%difenoconazole+pyraclostrobin EC was better than that of single dosage form,and the relative control effect was as high as 78.42%.