SUMMARYViroids have been isolated from plum trees (Prunus salicina Lindley) affected with plum dapple fruit disease and from peach trees (Prunus persica Batsch) showing dapple fruit symptoms. The viroids were inoculated mechanically to cucurbitaceous plants, in which symptoms typical of hop stunt viroid (HSV) infection appeared. The complete nucleotide sequences of an isolate from plum and an isolate from peach (AF isolate) were shown to be identical, consisting of 297 nucleotides with a 93-6~o sequence homology to HSV-hop. Another isolate from peach (A9 isolate) also consists of 297 nucleotides, but the sequence homology to HSV-hop is 99"7~o, showing only one nucleotide replacement. These results indicate that these three viroids are strains of HSV, which we designate HSV-plum, HSV-peach (AF) and HSV-peach (A9), respectively. Comparative analysis of the nucleotide sequences of HSV strains from hop, grapevine, citrus, cucumber, plum and peach revealed variable and conserved regions in the HSV molecule. In Japan, these viroids are closely related not only to dapple fruit disease in plum cv. Taiyo, but also to dapple fruit symptoms on peach cv. Asama-Hakutou.

Sequential polyacrylamide gel electrophoresis analyses showed many viroid-like RNAs in samples collected from citrus trees in Japan. Reverse transcription polymerase chain reaction and sequencing analyses of the amplified fragments verified that they were derived from variants of six citrus viroids, Citrus exocortis viroid (CEVd), Citrus bent leaf viroid (CBLVd) including CVd-I-LSS (a distinct variant of CBLVd), Hop stunt viroid, Citrus viroid III, Citrus viroid IV, and Citrus viroid OS. The samples induced symptoms with variable severity in Arizona 861-S1 'Etrog' citrons (Citrus medica L.) likely due to the varying accumulation patterns produced by the different viroids. Some of the symptoms caused by the samples harboring the citrus viroids other than CEVd were as severe as those caused by CEVd. Some source citrus trees showing the severe bark scaling characteristic of exocortis disease in trifoliate orange (Poncirus trifoliata (L.) Raf.) rootstocks contained only citrus viroids other than CEVd in complex. This indicates that certain exocortis-like diseases in Japan were caused by some combination of citrus viroids not including CEVd.

Tobacco cultivar Virgin A Mutant (VAM) is reported to have the recessive potyvirus resistance gene va. Varied levels of resistance were observed in VAM plants inoculated with Japanese potato virus Y (PVY) isolates. VAM was highly resistant to most of the PVY isolates tested and tolerant to three necrotic strain isolates of PVY-T. Based on data obtained from tissue printing and press blotting, the resistance appeared to be mainly at the level of cell-to-cell movement. PVY replicated in VAM proto-plasts, but the replication was 30% lower than in susceptible tobacco, suggesting that impairment of replication also contributes to resistance. To identify the viral gene product or products involved in VAM resistance, we isolated spontaneous resistance-breaking mutants by passing vein-banding (O strain) isolates several times through VAM plants. By comparing the amino acid sequences of the mutants with their original isolates, we identified a single amino acid substitution in the viral genome-associated protein (VPg) domain that is correlated with VAM resistance breaking. Together, these results suggest that, in addition to its role in replication, VPg plays an important role in the cell-to-cell movement of PVY.

SUMMARYA viroid (GV) was isolated from grapevines recently introduced into Japan from France, West Germany, Austria, Hungary and U.S.A., as well as from those cultivated in Japan. It was detected in 28 out of 32 (88 ~o) grapevines tested. The isolates of GV had similar host ranges and induced symptoms in cucumber plants identical to those induced by hop stunt viroid (HSV). The nucleotide sequences of four GV isolates, from France, West Germany, Hungary and Japan, were identical and these formed covalently closed circular molecules 297 nucleotides in length. This sequence differed from that of HSV in one nucleotide only and from that of the cucumber isolate of HSV in being six nucleotides smaller and having 15 nucleotides different (95~o sequence homology). The result indicates that GV is a grapevine isolate of HSV and suggests that grapevines were the source of hop stunt disease in Japan.

The complete nucleotide sequence of a viroid isolated from citrus (Citrus medica L. USDCS 60-13), designated as citrus isolate of hop stunt viroid (HSV-cit)(1), was established. The single stranded circular molecules of citrus viroid RNA consists of 302 nucleotides, which can form a rod-like structure with extensive base-pairing characteristic to the other viroids. Heterogeneity was found in the sequence No. 207-209. The sequence was GAG when determined from cDNAs