Physiological Differentiation And Resistance To Main Fungicides Of Tobacco Black Shank Pathogen In Chongqing

Tobacco black shank is one of the most destructive diseases of tobacco(Nicotiana tabacum L.)in southwestern China.The causal agent,Phytophthora parasitica,infects roots,stems and other organs,causes a serious impact on tobacco production and results in huge economic losses.So far,the most effective and efficient method for the control of this disease is the use of chemical fungicide.Because of the simplefunction modes of some chemical fungicides and theirirrational use inthe field,the development of fungicide resistance in the pathogen has been reported soon after use.In this study,we examined the black shank pathogen for its physiological differentiation and resistance to mainfungicides,performed field trials for the chemical control of the diseaseandfield survey for the incidence of the disease,in order for an improved understanding of the pathogen population and disease epidemics.The achieved main results are as follows:1.A total of 31 P.parasiticaisolates were examined for physiologicaldifferentiation.The results showed that all isolates were race 1.Race 1 was the dominant in Chongqing tobacco fields,and the population structure of tobacco black shank pathogen was simple in Chongqing,with no differences between different regions.2.The resistance of three main tobacco cultivars to four representative P.parasitica isolates was evaluated by zoospore inoculation.The results showed that Honghuadajinyuan and Yunyan 97 were highly susceptible,while Yunyan 87 wasmoderately resistant,indicating that Yunyan 87 can continuously be used as amain cultivar in Chongqing.3.The EC50valuesofmetalaxyl-mancozeb,mancozeb,metalaxyl and hymexazol to P.parasiticawere determined to be in the ranged of0.177~0.233?g/L,0.468 ~0.660?g/L,2.566 ~3.405?g/L,and 296.548 ~339.079?g/L,respectively,indicating that P.parasiticawassensitive to metalaxyl-mancozeb,metalaxyl,and mancozeb,while beingresistant tohymexazol.P.parasiticawasmost sensitive to metalaxyl-mancozeb.Field trialexperiments showed that metalaxyl-mancozeb was more effective than manganese zinc fluoride which was widely used in Pengshui.4.Field survey for the disease incidence showed that thedisease started from resettling stage and spreaded from squaring stage.The incidence and disease index peakedin late June and mid-July,and wereclosely coincided with sudden increase of temperature and increased rainfall.The diseasedeveloped most seriousbythe harvesting stage,and gradually stabilized to the end of the harvest.