Identification and virulence test of a new pathogen that causes verticillium striping on rapeseed in northwestern China

Five stems of rapeseed with abundant black microsclerotia were collected from Huang Yuan County of Qinghai Province, China, and ten fungal isolates were obtained from the stems. They were identified based on morphology, molecular features and specific PCR detection. The results showed that the 10 fungal isolates belonged to Verticillium longisporum lineage A1/D3. One of the 10 isolates (HW7-1) was tested for virulence on three species of rapeseed, including B. napus Zhongshuang 9, B. rapa Qingyou 9 and B. juncea Tayou 2 by inoculation of conidia of HW7-1 on roots of young seedlings. Control seedlings were inoculated with V. dahliae conidia or water alone. The seedlings of these treatments were transplanted in culture mix and incubated in a growth chamber (20 °C). Results showed that while the control seedlings of three cultivars appeared quite healthy, the seedlings inoculated with HW7-1 showed leaf yellowing, seedling stunting and even death at 22 days post-inoculation. V. longisporum was re-isolated from yellow leaves, thus fulfilling Koch's postulates. Moreover, compared to the control treatments, inoculation with HW7-1 caused flowering delay and seed yield reduction on Tayou 2 with production of microsclerotia on the stems. To our knowledge, this is the first report of V. longisporum lineage A1/D3 on rapeseed in northwestern China.