Selection and inheritance of resistance to Chrysanthemum stunt viroid

Stunting caused by the Chrysanthemum stunt viroid (CSVd) is one of the most damaging diseases in cultivated chrysanthemum (Chrysanthemum morifolium). Infected seedlings of asymptomatic cultivars may spread CSVd to other susceptible cultivars in nurseries. Thus, breeding of a CSVd-resistant cultivar would represent an important step in developing a protection strategy against CSVd. Our objectives were to (i) find CSVd-resistant cultivars and (ii) determine whether their resistance is heritable. We screened 35 chrysanthemum lines and cultivars, wild chrysanthemum species and interspecific hybrids for resistance to CSVd. Scions of screened cultivars were inoculated with CSVd by grafting them onto CSVd-infected plant roots. Every month after grafting inoculation, we used the reverse-transcription polymerase chain reaction to search for viroids in upper leaves of the scions. Viroids were not detected in ‘Okayamaheiwa’ 210 days after inoculation with CSVd, demonstrating strong resistance in this cultivar. F1 progeny were produced by crossing this resistant cultivar with the susceptible cultivars ‘Sei-elza’ and ‘Anri’. Among the 76 and 8 F1 progeny produced by these two crosses, 13 and 1, respectively, were not infected with CSVd following inoculation. Hence, CSVd resistance was expressed in the first hybrid generation. This is the first report demonstrating that resistance is heritable in crosses between a CSVd-resistant chrysanthemum cultivar and CSVd-susceptible cultivars. Chemical and cultural approaches for control of CSVd epidemics are difficult; therefore, breeding for CSVd resistance provides a promising alternative.

► We found CSVd-resistant chrysanthemum cultivars.
► The infection resistance in this cultivar appears not to be connected with phloem transport.
► CSVd-resistance is expressed in the first hybrid generation.