Identification Of The Pathogens Of Potato Scab In China And Cloning And Expression Of The Pathogenicity-related Gene Nec1

Potato scab is an ecomomically important disease caused by plant-pathogenic Streptomyces spp., and is world wide in its distribution. Scabby lesions were mainly formed on potato surface, which brings economic losses because of reduced marketability of table stock, the value of processing, and seed potato tubers. The same organism also attacks many other crops such as beet, radish, carrot etc. In recent years, potato scab has taken place in many potato-growing regions in China, and is very severe in some areas now. However, the pathogens and pathogenicity mechanism of the disease have not been reported well before in China.In this paper, the scabby tubers were collected from Heilongjiang, Neimenggu, Hebei, Shandong, Shanxi, Shaanxi, Gansu, Sichuan, Guizhou, and Yunnan provinces in China, and the scab symptoms were described correspondingly. The lesions were classified into three types, i.e., raised lesions, pitted lesions, and superficial lesions. Eighty Streptomyces strains were isolated from scab lesions. Pathogenicity tests, including pot test, potato slices and radish seedling assays, showed that 30 Streptomyces strains were pathogenic. Inoculation with pathogenic strains induced scab symptoms on potato tubers based on pot test, produced necrotic reaction on tuber slices, and significantly reduced shoot height of radish seedlings. The results show that the tuber slice bioassay and the radish seedling test were suitable for identifying pathogenic Streptomyces strains.Pathogenic strains were identified according to the biological characteristics, the 16S rDNA and ITS rDNA sequences. The results showed that they belonged to S. scabies, S. acidiscabies and a unidentified Streptomyces species, respectively. The S. scabies strain from China has the same 16S rDNA sequence to standard S. scabies strain ATCC 49173 from America, and the similarity of ITS rDNA sequence between them is 99.7%. But the biological characteristics of the two Streptomyces are somehow different. Chinese S. scabies strain did not use D-mannitol as single carbon sources, unsusceptible to penicillin G (10 IU ml~-1). The 16S rDNA and ITS rDNA sequences of Chinese S...