Japanese Pear Fruit Dimple Disease Caused by Apple Scar Skin Viroid (ASSVd).

Abnormal fruit disorders of Japanese pear cultivars, Niitaka and Yoshino, have been found in Japan since the 1980s. The characteristic symptoms consist of dimpling of mature fruit surface. A viroid was detected in association with this disorder. Healthy Japanese pear trees of cvs. Niitaka and Yoshino were back-inoculated with this viroid and fruit disorders subsequently occurred on these fruits. This viroid is similar in its properties to apple scar skin viroid (ASSVd). When healthy Japanese pear trees of cvs. Niitaka and Yoshino were inoculated with ASSVd, the same symptoms occurred on these fruits. Two primer pairs for RT-PCR amplification of ASSVd RNA were designed from the nucleotide sequence of ASSVd. RT-PCR produced cDNA products of the predicted size for ASSVd using electrophoretically purified viroid samples. Current shoots and dormant shoots were evaluated to be more useful for the detection of this viroid by RT-PCR with a simple extraction method. Amplified cDNA fragments were ligated to pCRTM II plasmid, and the cDNA inserts were sequenced. The sequence differs from the nucleotide sequence of ASSVd reported by Hashimoto and Koganezawa at only three sites: one nucleotide is deleted, one nucleotide is inserted and one nucleotide is mutated. Therefore the causal agent of this disease is ASSVd. We propose to call this new disease Japanese pear fruit dimple (JPFD). ASSVd was not transmitted from ASSVd-infected Japanese pear trees to pear seedlings by the direct knife-cut method.