Screening of chrysanthemum plants with strong resistance to chrysanthemum stunt viroid

Chrysanthemum stunt viroid (CSVd) is one of the problematic pathogens known to infect chrysanthemum (Dendranthema grandiflorum) plants and causes various symptoms, such as a reduction in plant height, which is a serious problem in cut flower production. No natural sources with resistance to CSVd have been reported. By quantifying the CSVd titer within the plant, we identified the cultivar 'Utage' as a plant in which the increase of the CSVd titer was slowest among 6 cultivars tested. 'Utage' self-pollinated, and 67 resulting seedlings were screened for CSVd resistance using reverse transcription polymerase chain reaction (RT-PCR), nested-PCR, micro-tissue (MT) direct RT-PCR, and real-time RT-PCR. Of these 67 seedlings, 9 plants lacked the obvious CSVd band detected by RT-PCR. Five months after grafting to CSVd-infected plants, the CSVd titers of 3 plants (C7, A30, and A27) were about 1/240,1/41000, and 1/125000 compared to that of 'Utage', respectively. These 3 plants were comfirmed to have strong resistance to CSVd. In C7, local distribution of CSVd was observed in the youngest expanded leaf by micro-tissue direct RT-PCR and in situ hybridization. For A30 and A27, CSVd was hardly detected in the whole plant. These 3 plants will contribute to the elucidation of CSVd resistance mechanisms. JSHS