Received for publication 2 Apr. 1985. This paper is contribution A-000 of the Fruit Tree Res. Sta. We thank all the members of Breeding Fourth Lab. of the Fruit Tree Res. Sta. in Ibaraki and Prof. K. Ryugo, Dept. of Pomology, Univ. of California, Davis, for their expert help and kind advice. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.