Study On The Specific Classification And The Pathogenicity Differentiation Of Tobacco Brown Spot Pathogen In Chongqing

Tobacco brown spot, which is caused by Alter naria fungus and damages the tobacco leaves after their maturation, is one of the most baneful diseases in the world. Fifty five representative strains of Tobacco brown spot pathogen were isolated from more than 400 samples which came from different regions of Chongqing, such as Wushan, Fengjie, Pengshui, Qianjing, Xiushan, Shizhu, Youyang, during the tobacco harvest periods. The spore morphological characters of these 55 strains and their three-dimensional conidiation pattern were observed in order to identify their specific classification. And their pathogenicity, physiological and biochemical characters, and the germination and invasion of the spores were also measured in this study. The detailed experimental results are as follows.1. Fifty five representative strains of Tobacco brown spot pathogen were isolated from more than 400 samples which came from different regions of Chongqing, such as Wushan, Fengjie, Pengshui, Qianjiang, Xiushan, Shizhu, Youyang, during the tobacco harvest periods between August, 2005 and December, 2006. Six of the strains were isolated from Pengshui, 12 of the strains were from Wushan, 8 of the strains were from Fengjie, 4 of the strains were from Xiushan, 9 of the strains were from Youyang, 15 of the strains were from Shizhu, and only one was from Qianjiang.2. The spore morphological characters of these 55 strains and their three-dimensional conidiation pattern were observed under microscope, and the results indicated that all of the 55 strains isolated from different regions of Chongqing were Alternaria alternata (Fries) Keissler, Alternaria longipes (ELL.& Ev.) Mason and Alternaria nicotiana Cheng et An were not found in this research.3. These 55 trains were used to inoculate the detached leaves of K326 with spore suspension liquid. And the preparatory pathogenicity differentiation among the strains was found through the disease index of tobacco leaves infected by brown spots. According to the results of the preparatory pathogenicity test, all the 55 strains showed the pathogenicity differentiation on some extent. Four of the 55 strains were most virulent, 15 strains were relatively powerful virulent, 27 strains were moderately virulent, 9 strain showed weak virulence. The results indicated that the majority of the isolated strains were powerful virulent. It was also found that the ability for mycelium penetrating on detached leaves and its growth was correlated to its pathogenicity.4. According to the results of the preparatory pathogenicity test, the distribution of the different pathogenicity strains was not well-proportioned. As comparison among all the strains, the pathogenicity difference was showed in different tobacco-growing regions and in the same region, the same field, the same leaf and even in the same spot. And the means of the tobacco-growing regions disease indexes can be arranged in sequence as follow: Pengshui, 69.8; Shizhu, 58.4; Xiushan, 55.5; Qianjiang, 54.4; Wushan, 47.3; Fengjie, 45.7; Youyang, 39.8. Most of the disease indexes of strains from Pengshui were about 80, which indicated that these strains had a steady and relatively powerful virulent pathogenicity; The disease indexes of strains from Wushan were between 20～80, and the pathogenicity of these strains were not steady; Most of the strains from Fengjie and Youyang had a moderately pathogenicity. The two strains isolated from the same filed in Pengshui were both the most virulent, and their disease indexes were about 80; But the strains isolated from the same filed in Youyang or Shizhu showed quite difference in pathogenicity, and the discrepancy value of their disease indexes was about 30. The strains isolated from the same leaf in Wushan showed quite difference in pathogenicity, and the discrepancy value of their disease indexes was nearly 40; But The strains isolated from the same leaf in Fengjie or Youyang showed a steady pathogenicity. The strains isolated from the same spot in Wushan showed quite difference in pathogenicity, and the discrepancy value of their disease indexes was 54.2; And this value was 41.7 in Shizhu. The results suggested that the variation and the pathogenicity difference presented in all these occasions.5. The pathogenicity test also showed that the tobacco breeds K326 and Honghudajinyuan had a weak resistance against A. alternata, because the disease index of tobacco leaves infected by most virulent strain D7-2 was higher than 80; while the tobacco breeds MS87, MS85 and MS-K326 had a relatively powerful resistance against A. alternata, because the disease indexes of tobacco leaves infected by most virulent strain D7-2 were 48.5, 22.2, 18.8, respectively. It was also found that the same tobacco breed showed the different susceptibility to the different strains. For the tobacco breeds K326, Honghudajinyuan, MS87 and MS-K326, the tobacco brown spot strain D7-2 was most virulent, and the next was A1-1; while for the tobacco breed MS85, the tobacco brown spot strain A1-1 was most virulent, the next was B1-4, and D7-2 was least virulent.6. Physiological and biochemical measurement indicated that growning on the PDA solid culture medium under 25℃, for 7 days, all the diameters of the tested strains' colonies were more than 6cm, and all the number of spores were more than 10~5 individual/ml. But for the most virulent strain D7-2, the diameter of the colony was 6.05cm, and the number of spores was 3.03×10~5 individual/ml, which were a little less than others; for the moderately virulent strains B1-4 and C1-5, the diameter of the colony was 6.58cm, 6.50cm, respectively, and the number of spores was 4.23×10~5 individual/ml and 5.13×10~5 individual/ml, which were a littlle more than others. But in the PD liquid culture medium, these tested strains showed quite difference in their weight. The weight of the most virulent strain D7-2 was 0.623g, which was quite heavier than 0.371g (B1-4) and 0.362g (C1-5). And it was also found that the ability for the strains to produce toxin was positively correlated to its pathogenicity.7. The spore germination and invasion test indicated that all the strains tested had no significant difference in this process. The spores began to germinate after 6 hours and finished in 8 hours in the 1% glucose liquor. And when inoculating the detached leaves, the spores began to germinate after 24 hours, and intruded into the surface of the tobacco leaves in 48 hours. But after the invasion, the most virulent strains grew quite faster than the weak virulenct strains.