Tobacco Streak Virus Infecting Buxus Sempervirens

Box-tree (Buxus sempervirens) is an evergreen ornamental plant, appreciated for its small, dark-green glossy leaves attractive in all seasons, and mostly used as borders in the gardens. In the fall of 2001, three pot plants of box-tree coming from Tuscany region (central Italy) were sent to DiSTA-Plant Pathology of the Bologna University to be examined for virus presence. These plants showed in fact affected severe virus-like symptoms such as yellow mosaic and chlorotic stripes on the leaves; in addition, the symptomatic box-trees looked less vigorous and severely stunted. In initial experiments a small number of test plants were inoculated with sap from the symptomatic leaves. Only Vigna sinensis L. “Black eye” and Nicotiana benthamiana L. developed symptoms, in particular: the first plant showed local red-necrotic rings; the second one, necrotic local rings and systemic chlorotic mosaic. This strong reaction in only two of the tested species employed was possibly due to a low concentration of the pathogen in the box-tree. The local lesions of V. sinesins and the local and systemic symptoms of N. benthamiana were used as starting material for virus identification. In host range tests, plants of 26 species (belonging to 10 botanical families) were inoculated and 11 of these resulted infected. In particular, Solanaceae and Cucurbitaceae showed symptoms reminiscent of those described for the type strain of tobacco streak virus (TSV) (1, 2). Cucumis sativus L “Marketer” showed local necrotic rings; Nicotiana glutinosa L., N. tabacum L. “Samsun” and “White Burley”, displayed local whitish necrotic ringspots, line-patterns and oak-leaf patterns. The observations of ‘leaf-dip’ preparations from inoculated host plants and original box-tree material, carried out with a Philips CM 10 electron microscope, did not reveal the presence of elongated virus-like particles. Immuno-electron microscopy tests (ISEM) were applied to crude sap from infected herbaceous plants, by using the antisera to several bacilliform and spherical viruses, including that to TSV (PVAS-276) (from American Type Culture Collection, Manassas, VA, USA) and TSV-Gladiolus sp. (from the collection in Bologna). The use of these antisera revealed the presence of strongly decorated virus particles; no other viruses were found. In 2003, some symptomless box-trees (provided by a commercial grower and previously tested negative for TSV presence) were grafted with shoots from box-tree showing yellow mosaic on the leaves and maintained in a greenhouse under aphid proof-cage, to verify the aetiology of the disease observed. To our knowledge, this is the first report of TSV in box-tree.