| Tobacco brown spot, which is caused by Alternaria alternata (Fries) Keissler and damages the tobacco leaves after maturation, is one of the most serious diseases worldwide. A total twenty-seven isolates were purified from different regions of Chongqing during the harvest seasons. The pathogenicity, cultural characteristics, and the DNA molecule diversity of these isolates are measured in this study.1. Twenty-seven isolates of Tobacco brown spot pathogen were purified from representative spot on old tobacco leaves in the harvest seasons. We found that different isolates showed differences in the colony characteristics, color and aerial hypha on PDA.2. The detached leaves of tobacco were inoculated with spore suspension of these twenty-seven isolates. The pathogenicity differentiation among the isolates was found through the disease index of tobacco leaves infected by the pathogen. According to the results of the pathogenicity test, the total twenty-seven isolates showed the pathogenicity differentiation in 3 degrees: 3 isolates were relatively powerful virulent, 20 of the isolates were moderately virulent, 4 isolates showed weak virulence and the proportion were 11.1%, 74.1%, 14.8%, respectively, and no sign indicated that powerful virulent or avirulent isolates existed in these 27 isolates. The results showed that the majority of the isolates belonged to moderately virulent pathogenicity. It was also found that the penetrating and extending ability of the hypha on detached leaves was positively correlated to the pathogenicity.3. According to the results of the preparatory pathogenicity test, the distribution of the different pathogenicity isolates was not well-proportioned. Comparing all the isolates, the pathogenicity difference was showed both in and out of the different tobacco-planting regions. The general pathogenicity of isolates came from Shizhu presented maximal among different regions, the mean of disease index reached about 62.0, which indicated that these isolates had a steady and relatively powerful virulent pathogenicity; the general disease indexes of isolates from Youyang were the lowest, 28.6. The isolates from the same filed in Wushan or Wulong also showed quite different in pathogenicity.4. Cultural characteristics measurement indicated that the diameters of the tested isolates' colonies were all over 4cm, and the amount of spores all reached 10~5 spores/mL on PDA under 25℃for 7 days. But the relatively powerful virulent strain D7-1 and J4-5-5 grew as slow as 4.27cm and 4.65cm in 7 days, and the amount of spores were respectively 4.0×l0~5 spores/mL and 2.2×10~5 spores/mL; For the moderately virulent isolates Al-1 and H2-2-3, the diameters of the colony were 6.14cm and 6.50cm, respectively, and the amount of spores reached the largest 5.0×10~5 spores/mL and 6.0×10~5 spores/mL. But in PD, these tested isolates showed quite a difference in hypha weight increasing. The hypha weights of the relatively powerful virulent isolates D7-1 and J4-5-5 were 1.098g and 0.886g which were quite heavier than 0.835g (Al-1) and 0.794g (H2-2-3). The result showed that those relatively virulent isolates expanded slower and produced less spores on PDA, while in PD, these isolates presented the largest hypha weight increasing. While the weak virulent isolates was opposite. Using the crude toxin extracted from relatively powerful virulent isolates to the tobacco seedling leaves, the spots' diameter reached 2.5cm in 3 days and finally the seedlings wilted or even died. While punctured with the weak virulence isolates toxin, the seedlings presented etiolating and growing slowly. The spots diameters were no more than 0.5cm in 3 days. And it was also found that the ability for the isolates to produce toxin was positively correlated to its pathogenicity.5. The analysis of RAPD of the different pathogenicity isolates from different areas showed: the PCR bands presented polymorphism among the different pathogenicity isolates, and we initially concluded that the differences were existed genetically among these isolates. Furthermore, the result indicated that the PCR bands of the isolates from different areas are also some different. In a word, the test showed the relatively rich genetic diversity in the Alternaria alternata of Chongqing.
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