Studies On Mechanism Of Virulence Variation Of Alternaria Alternata And Dynamics Of Tobacco Brown Lesion Area Expansion

Inoculation tests and RAPD marker were used to analyze virulence and DNA polymorphism of six isolates of A.alternata in different growth stage of tobacco plants, and systemic and quantitative studies to analyze the factors that affected the lesion area expansion. We assayed toxicities of Dimethachlon, Mancozeb and their mixtures to A.alternata. The results were summarized as follows:1.The virulence variation of six A.alternata isolates is significant in different tobacco growth stage ,the virulence of isolates (number four, five and six) in late stage was stronger than those (number one, two and three) in early stage. That was to say, the higher of the reproduction times were ,the stronger the virulence of the isolates was. The isolates could be differentiated into two groups on the basis of the virulence tests to three tobacco cultivars. 2.Of one hundred and five RAPD marker which amplified with twelve random decamer primers in six A.alternata isolates, 95.24% showed polymorphism. The results indicated that the genetic diversity existed among these isolates, the isolates could be differentiated into three groups on the basis of RAPD marker. Hierarchical cluster analysis indicated that the virulence and RAPD markers of A. alernata were related. Pathogenetic gene of the pathogen resulted in virulence variation.3.The lesion area expansion tendency of tobacco brown spot caused by A. alternata was analyzed in climatic chamber, the results showed that temperature and moisture time are two major factors for lesion area expansion .At constant temperature, lesion area marked expanded with moisture time up to sixty hours. From twelve to forty ℃,the lesion area can be expanded , and increased rate of lesion area increased with increased temperature up to about thirty and six癈, and then declined. The liner relation was among lesion area increased rate of two cultivars and temperature, moisture time.4.The resistance of six isolates to Dimethachlon was different, the sensitivity order was number four, one, two and three isolate .their EC50s were 193.3644 μ g/ ml ,102.3738 μ g/ ml ,42.0390 μ g/ ml and 58.4109 μ g/ ml, respectively. The number four isolate was the least sensitive. The results showed the time of collecting isolates and their sensitivity to Dimethachlon were not related. No matter in what stage the isolates all had sensitivity and nonsensitivity.5.The sensitivity of six isolates of A.alternata to Dimethachlon and Mancozeb all existed, but only their mixture in 70: 30 had obvious synergistic interaction .Four estimating methods ofsynergistic interaction obtained consistent results.