Selection of Mutant Japanese Pears Resistant to Black Spot Disease by Acute Irradiation of Gamma-rays

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  • ガンマ線の急照射によるナシ黒斑病耐病性突然変異体の選抜
  • ガンマセン ノ キュウ ショウシャ ニ ヨル ナシ コクハンビョウ タイビョウ

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Abstract

Black spot disease, caused by Alternaria alternata Japanese pear pathotype, is one of the most serious diseases of the Japanese pears, 'Nijisseiki', 'Shinsui' and 'Osanijisseiki' a self-compatible spontaneous mutant of 'Nijisseiki'. The susceptibility is controlled by a single dominant gene; the above susceptible cultivars are heterozygous, whereas the resistant ones are recessive homozygous. By chronic irradiation of gammarays at the Gamma Field of Institute of Radiation Breeding, NIAR, a mutant (γ-1-1) was produced resistant to the black spot disease from 'Nijisseiki'. The mutant was discovered in 1981, and was registered in 1991 as a new Japanese pear 'Gold Nijisseiki'.<BR>Between 1987 and 1990, dormant scions of 'Shinsui' and 'Osanijisseiki' were irradiated by acute irradiation, total exposure of 60 and 80 Gy at the dosage of 2.5 Gy per hour, to obtain more mutants resistant to the black spot disease. Irradiated dormant scions were top-grafted on mature tree of Japanese pear 'Yakumo' and 'Hakko' in the orchard of the Tottori Horticultural Experiment Station. Rate of graft-take and growth of grafted scion are varied between 'Shinsui' and 'Osanijisseiki'. A difference in the rate of graft-take between scions exposed to 60 and 80 Gy was observed, but no difference in the rate of subsequent growth of grafted scions between dosages in each cultivar was noted.<BR>After treatment with the crude host-specific toxin on the 4th leaf on each developed shoot, resistant mutants to black spot disease of 'Shinsui' (IRB 502-11T) and 'Osanijisseiki' (IRB 502-12T) were selected in 1989 and 1990, respectively. These induced mutants exhibited the same type of intermediate resistance to the black spot disease as 'Gold Nijisseiki'. We are examining the two mutants for additional horticultural characteristics, such as tree growth potential, fruit quality, and self-compatibilitiy.

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