An apple chlorotic leaf spot virus isolate from ornamental dwarf flowering almond (Prunus glandulosa 'Sinensis') : Detection and characterization

An apple chlorotic leaf spot virus (ACLSV) isolate was detected by TAS-ELISA and RT-PCR in an ornamental dwarf fl owering almond (Prunus glandulosa Thunb.). This plant, maintained at the Centre for Plant Health, Sidney, B.C., Canada, has been showing transient leaf symptoms during the spring seasons. A 390-bp fragment and a 1,350-bp product, in the RNA polymerase and the coat protein viral coding regions, respectively, were amplifi ed by RT-PCR from the infected plant. A sequence comparison of the 390- bp fragment of this ACLSV isolate (designated as AL1292) with other published isolates, revealed a similarity of 81% to 84% at the nucleotide level and 88% to 100% at the amino acid level. In contrast to other ACLSV isolates, AL1292 has an exceptionally narrow range of experimental herbaceous and woody hosts, as determined by mechanical and graft inoculation assays. These standard bioassays may not be effective for the detection of the AL1292 isolate because of its limited host range. The results we report in this paper confi rm P. glandulosa as a natural host of this virus. Currently it is not known how ACLSV is spread, other than by bud-grafting and possibly by root grafts. The use of virus-tested source plants for the preparation of planting material will minimize its spread. Prunus glandulosa Thunb. is a deciduous, multistemmed shrub, known by the common name dwarf fl owering almond (U.S. Depart- ment of Agriculture, Agricultural Research Service, The Germplasm Resources Informa- tion Network). Native to China and Japan, it has been grown mainly for its fl owers coming into full bloom in early spring before new leaves develop. Plants of P. glandulosa 'Alba' bear single white fl owers while 'Sinensis' (also known as 'Rosea Plena') has double pink fl ow- ers. Dwarf fl owering almond was introduced to North America as an ornamental fl owering bush in 1683 (Adams, 2004). In western Europe, it is known in France since 1785 and later was cultivated in England mainly for cut fl owers (Flower Council of Holland, 2002). Propagation of planting material from seeds is possible but requires stratifi cation before sowing and may take up to 18 months to germinate. Vegetative propagation by rooting cuttings is less time-consuming and, therefore, has been applied routinely for production of planting material. As is the case for other vegetatively propagated plants, transmission of viruses in nursery stock of P. glandulosa is a potential problem. Prunus glandulosa, as well as other plants in the genus Prunus, grown for fruits and as ornamentals, are natural hosts of viruses like prunus necrotic ringspot virus (PNRSV), prune dwarf virus (PDV) and plum pox virus (PPV) (Nemeth, 1986). Virus-infected plants often express disease symptoms but may also be symptomless. Apple chlorotic leaf spot virus (ACLSV), the type member of the genus Trichovirus, was reported from the rosaceous species Prunus, Malus, and Pyrus (Martelli et al., 1994) and is also known to occur in one- seed hawthorn (Crataegus monogyna) and medlar (Mespilus germanica) (Nemeth, 1986). ACLSV isolates vary greatly in their biologi- cal and molecular properties. Some strains are latent and others may cause symptoms like bark split, dark green sunken mottle of leaves and even graft incompatibility (Nemeth, 1986). Symptoms in Prunus species are generally more severe than in Malus and Pyrus spp. (Dunez and Delbos, 1988). Based on bioassays results, Colin and Verhoyen (1975) suggested that leaf symptoms observed in P. glandulosa 'Sinensis' were caused by ACLSV. In this study, we applied biological, serological and molecular methods for the detection and characterization of an ACLSV isolate in the same host. The results we report herein confi rm P. glandulosa as a natural host of this virus.