Pathogenicity Diferentiation And Identification Of Race Of Phytophthora Parasitica In Chongqing

The distribution of tobacco black shank was very extensive in China, and it was found in almost all the province which planting tobacco even in Taiwan except Heilongjiang. Phytophthora parasitica f. sp. nicotianae, the casual agent of tobacco black shank, infected all the tobacco cultivars including flue-cured tobacco, air-cured tobacco, sun-cured tobacco, burley tobacco, oriental tobacco and brought powerful destroy.The samples with typical symptom of tobacco black shank were collected from the area planting tobacco in Chongqing and were brought to the laboratory for isolation, purification and identification of the pathogen. The pathogenicity differentiation and race type were determined on strains which were identified to Phytophthora parasitica f. sp. nicotianae in this study. The highest virulence strains were mixed to inoculate the 10 main tobacco varieties in Chongqing to evaluate the disease-resistance. Expected to provide the prerequisite and scientific basis for the directed breeding and reasonable distribution of the disease-resistant variety. The results were as follows:1. Collection of the isolatesThe total number of 123 samples of tobacco black shank were obtained in different areas of Chongqing from July, 2005 to October, 2006. It was found 52 isolates belonged to Phytophthora parasitica f. sp. nicotianae after purification and identification in the study, in which 2, 2 , 7 ,12,12, 9 and 8 isolates were from Wulong, Wanzhou, Wushan, Pengshui, Fuling , Shizhu, and Youyang respectively.2. Identification of physiological raceBy means of injecting the stem, fourty strains that selected randomly were tested for physiological race using 4 differentials host and observed after ten days. NC1071 had resistance to all the strains with the disease index of 0 and N. nesophila also had resistance to all the strains with the disease index varying from 0 to 20; L8 were susceptive to all the strains with the disease index varying from 75 to 100; Xiaohuangjin 1025 were also susceptive all the strains with the disease index from 77.8 to 100.All the 40 strains incubated on the TTC medium agar and TTC liquid culture taked on red.Therefore, the 40 strains were identified to race 1 according to the differential host and TTC reaction.3. Pathogenicity differentiationTwenty-four representative strains were selected to inoculate three tobacco varieties (HD, MSK326, YY202) by irrigating root with zoospore suspension. The result showed that the strains had pathogenicity differentiation on single variety, on the combination of three varieties. The strains from different areas also showed pathogenicity differentiation. Fourty strains showed high virulence, 6 strains showed moderate virulence and 4 strains showed low virulence on HD. Three, eleven and ten strains showed high, moderate and low virulence respectively on MSK326. Two, six and sixteen strains showed high, moderate and low virulence respectively on YY202. Two, sixteen and six strains showed showed high, moderate and low virulence respectively on the combination of three varieties. Moreover, the area planting tobacco showed significant pathogenicity differentiation according to the average disease index, in which the strains from Wulong were the most virulent with disease index of 57.41 while the strains from Wanzhou were the weakest virulence with disease index of 18.50.4. The resistance of the main cultivated varieties in Chongqing to tobacco black shankTen main cultivated tobacco varieties in Chongqing were selected in this study. Mixed zoospores suspension with 7 highly virulent strains (F2-1, PL3-1, SL1-5, WH1-1, WL1-8, WX1-7 and YL1-2) selected from seven areas were inoculated to the 10 varieties. The results showed that the resistance to tobacco black shank had significant difference to the 10 varieties, which had no highly resistant, 4 of which were resistant (NC82, Yunyan201, Yunyan203, Guiyan4), 2 of which were moderate resistant (MSK326,MSYunyan85), 2 of which were susceptible (MSYunyan87,Yunyan202), 2 of which showed high susceptibility (Eyan4,Honghuadajinyuan).5. Preservation of the strainsThree strains (PL2-5, SS1-8, YL1-2) were selected to conserved with four different methods and were examined at regular intervals. The result showed that preserving in distilled water and paraffih liquid were both fit for preserving strains for long time, OA slant at normal temperature was fit for preserving for short time while OA slant at 4℃was not suitable for conserving the pathogen.