Abstract
Osanijisseiki', a self-compatible, spontaneous bud sport of the Japanease pear 'Nijisseiki' is an excellent cultivar with a smooth skin. However, this cultivar is susceptible to Japnaese pear black spot disease caused by Alternaria alternata Japanese pear pathotype. To obtain resistants mutants from 'Osanijisseiki', nursery plants of 'Osanijisseiki' have been irradiated chronically with gamma-rays in the Gamma Field of the Institute of Radiation Breeding, NIAR, MAFF, since 1986. Screening tests using AK toxin, a host-specific toxin produced by A. alternata Japanese pear phathotype, were performed from 1988 to 1993.
Four branches of young trees planted at a distance of 40 m from the 60Co source were selected as being resistant mutants in 1991 (IRB 502-13T and IRB 502-14T) and 1993 (IRB 502-17T and IRB 502-18T). Sensitivity of the four resistant mutants to AK-toxin and susceptibility to the pathogen were compared with other of susceptible and resistant cultivars. The results showed that these four mutants possessed intermediate resistance. Furthermore, a mutant, IRB 502-13T, had the same characteristics as the original 'Osanijisseiki', except for the difference in toxin sensitivity. The characteristics of the other mutants, IRB 502-14T, IRB 502-17T, and IRB 502-18T, care beiing examined.
