Breeding of posterity crossing between cut rose ‘PEKcougel’ and tetraploid Rosa multiflora for disease resistance

The cut rose Rosa ‘PEKcougel’ is resistant to crown gall disease caused by Rhizobium radiobacter, and Rosa multiflora is resistant to root rot disease caused by Pythium helicoides. The resistance is heritable. We tried to combine these resistances by crossing ‘PEKcougel’ and R. multiflora ‘Matsushima No. 3’. Of plantlets raised from 121 torpedo-shaped hybrid embryos, 97 died from hypocotyl browning just after germination, another 18 died with browning of the shoot apex after true leaves expansion, and 6 survived and were grown on. Using the SCAK16583 marker, a DNA marker specific to R. multiflora, we identified three seedlings as progeny of ‘PEKcougel’ × ‘Matsushima No. 3’, and the others as selfed progeny of ‘PEKcougel’. These three progenies showed the tolerance to root rot and crown gall disease at the level intermediate between resistant and susceptible cultivars. In addition, the plants are self-compatible and are easily backcrossed with ‘PEKcougel’ and ‘Matsushima No. 3’. These progenies will contribute to the development of rootstocks with combined resistance to root rot and crown gall disease.