A NEW JAPANESE PEAR CULTIVAR GOLD NIJISSE IKI, RESISTANT MUTANT TO BLACK SPOT DISEASE OF JAPANESE PEAR

A new Japanese pear cultivar named 'Gold Nijisseiki', registered as Pe ar Norin 15, was developed by the Institute of Radiation Breeding, Nat ional Institute of Agrobiological Resources in 1991. Black spot diseas e caused by the Japanese pear pathotype of Alternaria alternata (Fr.) Keissler is the most important and serious disease of Japanese pear (P yrus pyrifolia Nakai var. culta Nakai). 'Nijisseiki' which is one of t he most widely grown cultivars of Japanese pear is known to be suscept ible to the disease. Susceptibility to black spot is controlled by a s ingle dominant gene, and the majority of susceptible cultivars are het erozygous at this locus. 'Nijisseiki' was exposed to chronic irradiati on (30 x 10(-2)Gy is similar to 4 x 10(-2)Gy/day) in the Gamma Field, and six resistant mutant branches were selected from trees by both nat ural infection, and selection after applying a phytotoxin solution (AK -toxin I). The induced mutants exhibited intermediate resistance to bl ack spot disease, either by the inoculation test using a spore suspens ion or the phytotoxin method. However, black spot symptoms seldom deve loped under field condition. One mutant, 'gamma-1-1' was released as ' Gold Nijisseiki' after local adaptability tests at 27 prefectual horti cultural research stations. The fruit ripens a few days later than 'Ni jisseiki'. 'Gold Nijisseiki' is similar to the original 'Nijisseiki' i n such agronomical characters as tree habit, season of flowering, bear ing habit, fruit size, shape, attractiveness and colour. Fruit colour is yellow at maturity, but fruit skin has many russets if uncovered. I t is better to cover fruit with paper bags if the attractive fruit ski n is necessary for marketing.