1Scottish Agricultural Science Agency, East Craigs, Edinburgh EH12 8NJ, UK; 2Agriculture Canada (Now the Canadian Food Inspection Agency), 3851 Fallowfield Road, Nepean, Ontario K2H 8P9, Canada; 3University of Leicester, Leicester LE1 7RH, UK

Background and objectives
An unusual virus was found symptomlessly infecting the potato cultivar Red La Soda which had been imported from the USA as in vitro plants [1]. It was detected by local chlorotic lesions in Chenopodium murale and slight vein clearing in Nicotiana debneyi, but symptoms were not produced reliably. It was not detected using antibodies to 13 potato viruses [3]. The presence of carlavirus-like particles was confirmed by electron microscopy, and antibodies to potato viruses M and S (PVM and PVS) increased trapping of particles in immunosorbent electron microscopy. Chloroplast agglutination tests were positive using PVS antibodies and the virus had close homology with a PVS probe. Because of the apparent serological and molecular relationships with PVM and/or PVS, work was done to compare these viruses and other carlaviruses with the unusual virus.

Results and conclusions
The virus is a slightly curved, filamentous, rod-shaped particle, with modal lengths of 525 and 650 nm. It systemically infected a broader range of indicator plants than PVM or PVS, and symptoms were generally absent. Chenopodium amaranticolorand C. murale were infected systemically which is a characteristic of the Andean strain of PVS (PVSAN), although PVSAN usually produces severe symptoms. It also infected Nicotiana tabacum, which is unusual for a carlavirus. A monoclonal antibody produced to the virus did not detect other carlaviruses (campanula latent virus, carnation latent virus, chrysanthemum latent virus, cowpea mild mosaic virus, elderberry carlavirus, helenium virus S, hop latent virus, kalanchoe latent virus, narcissus latent virus, passiflora latent virus, pepino latent virus, poplar mosaic virus, PVM, PVS or shallot latent virus) and a polyclonal antibody to the virus did not detect PVM or PVS. The virus contains single-stranded RNA that has less than 62% sequence homology with seven other carlaviruses tested, the closest sequence being that of PVSAN. Experimentally, the virus is transmitted mechanically and by the aphid vector Myzus persicae.

In view of the biological, serological and molecular differences between the virus, PVM, PVS and other carlaviruses tested, the virus is proposed as a new member of the carlavirus group with the name potato latent virus (PotLV).

Recently two more carlaviruses have been discovered which infect potato. These are potato rough dwarf virus (PRDV) and potato virus P from Argentina and Brazil, respectively. They appear to be serologically similar if not identical to each other [2] and have a host range similar to PotLV. PRDV appears to be serologicaly different from PotLV (J Martin, personal communication). More detailed work is needed to clarify the exact relationship of these viruses with each other.

1. Brattey C, Jeffries CJ, Goodfellow S et al., 1996. In Proceedings of the 9th EAPR Virology Section Meeting, Bled, June 18-22, 1995, pp. 65-69.
2. Butzonitch IP, Nieto A,Truol GAM, Colavita ML, 1995. In V Congreso Argentino de Virologia. II Encuentro de virologos Lationoamericanos. Tandil, Buenos Aires 24-27 April 1996.
3. Jeffries CJ, Chard JM, Brattey C, 1993. In Plant Health and the European Single Market. British Crop Protection Council Monograph, No 54, pp. 145-156.