| Corynespora leaf spot caused by Corynespora cassiicola, an important plant pathogenic fungus with a wide a rang of hosts, has become one of the most widely distributed and devastating cucumber diseases in China in recent years. In this study, population differentiation of C.cassiicola isolated from different hosts derived from many provinces of China was studied by analyzing the morphological features, pathogenicity and genetic diversity. Besides, a method to quantify the pathogen in soil and seeds was established. The main results are as follows:1. We found that Corynespora leaf spot has become a main disease of vegetables in China based on the investigation data in 11 Provinces during 2008-2010. Corynespora leaf spot occurred in many vegetable hosts, including cucumber, tomato and eggplant. 130 Corynespora strains were isolated from 10 hosts.2. We confirmed that the pathogen, caused Corynespora leaf spot of eggplant in China, is Corynespora cassiicola by pathogenicity testing, morphological and molecular identification, instead of C. melongenae Takimoto and C. melongena Sharma & Srivastava reported in Japan and India, respectively.3. Pathogenicity differentiation among C.cassiicola strains from different host plants was demonstrated. Artifical inoculation indicated that there was host specialization in C.cassiicola and a distinct correlation existed between the pathogen pathogenicity and its host origin. The isolates from rubber, tomato and cowpea couldn't infect non-host plants or showed weak pathogenicity, while the strains from cucumber, eggplant and balsam pear could infect other hosts. Moreover, the isolates from the same host also exhibited different level of pathogenicity.4. 145 Corynespora strains in China were divided into two distinct clusters based on AFLP and ISSR data. Clusterâ… included 10 isolates derived from rubber, and clusterâ…¡included isolates derived from other host plants. All isolates from tomato belonged to a sub-cluster with higher similarity.5. ISSR analysis revealed that there was distinct correlation between the genetic clusters of Corynespora isolates, and their geographic origins. Cluster A included 7 isolates from Hainan, while cluster B was constituted by isolates from the other provinces. The genetic distance between the two clusters was up to 0.20, indicting that there was a significant genetic differation between the isolates from Hainan and the other provinces. The isolates collected from the fields in the North and Northeast regions had higher genetic similarity.6. Real time PCR-based specific detection method of C. cassiicola was established. The pathogen in cucumber seeds and soil could be detected by real-time PCR, and the sensitivity was down to C. cassiicola DNA of 1conidia / g, or 1 infected seed in 1 million seeds.7. Dot-ELISA was established to detect C.cassiicola by immunological method. Soluble protein of C.cassiicola was used as antigen, and the procedure of Dot-ELISA was established and optimized. Dot-ELISA could be effective to identify the pathogen in infected host tissues prior to the onset of visual symptoms in field.In this study, we studied the etiology of C. cassiicola from different hosts and geographic origins, including morphological characteristics, pathogenicity and genetic diversity. Our data provided evidence for the host specialization of the pathogen. These results provided useful information for the disease management. Moreover, real-time PCR and Dot-ELISA detection methods were established, and application of the procedures will contribute to a better understanding of the epidemiology of cucumber Corynespora leaf spot and help us to control this disease.
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