A new Japanese pear cultivar named 'Gold Nijisseiki', registered as Peer Norin 15, was developed by the Institute of Radiation Breeding, National Institute of Agrobiological Resources in 1991. Black spot disease caused by the Japanese pear pathotype of Alternaria alternata (Fr.) Keissler is the most important and serious disease of Japanese pear (Pyrus pyrlfolia Nakai var. culta Nakai). 'Nijisseiki' which is one of the most widely grown cultivars of Japanese pear is known to be susceptible to the disease. Susceptibility to black spot is controlled by a single dominant gene, and the majority of susceptible cultivars are heterozygous at this locus. 'Nijisseiki' was exposed to chronic irradiation (30 × 10-2Gy∼4×l0-2Gy/day) in the Gamma Field, and six resistant mutant branches were selected from trees by both natural infection, and selection after applying a phytotoxin solution (AK-toxin I). The induced mutants exhibited intermediate resistance to black spot disease, either by the inoculation test using a spore suspension or the phytotoxin method. However, black spot symptoms seldom developed under field condition.
