A potexvirus related to Papaya mosaic virus isolated from moss rose (Portulaca grandiflora) in Italy

Moss rose (Portulaca grandiflora)is a common succulent, ornamental plant mainly used in hot dry areas, rock gardens, over walls or along paved surfaces. Moss rose is generally easy to grow and easily propagated through cuttings or seed. It is not commonly prone to major diseases but has been found to be susceptible to root-rot in poorly drained soils and to white rust (Horst, 2001). Portulaca grandiflora has also been found infected with Tomato spotted wilt virus in Pennsylvania (Hausbeck et al., 1992). In 2001, a few plants were received showing apical wilting, stunting and lack of flowers. These plants were part of a batch showing the same symptoms coming from a nursery in Albenga, Liguria Region (Italian Riviera) in northern Italy, where the plants were propagated and grown. Leaf dip extracts examined under an electron microscope showed long flexuous potexvirus-like particles. These could be mechanically transmitted to a range of test plants and to various commercial moss rose varieties, where the original symptoms were reproduced and where the same flexuous virus particles could be observed.

A general protocol for the RT-PCR amplification of genome fragments encoding for a conserved region of the polymerase protein belonging to the Potexvirus genus was performed on leaf extracts. A 700 bp band, not produced from symptomless control plants, was cloned and sequenced. A nucleotide sequence blast search showed high similarity to Papaya mosaic virus (PapMV). Further cloning and sequencing of the 3'end of the virus from moss rose was undertaken and a blast search showed 94% similarity at the nucleotide level with a proposed new viral species isolated in Australia from Alternanthera pungens (Geering & Thomas, 1999). This proposed new viral species had only 49% identity at the nucleotide level to PapMV. Sequence corresponding to the potexvirus isolated from moss rose in Italy has been deposited in the Genbank (Accession No. AY566288). Previously, un-characterised potexviruses have been detected on ornamental Portulaca spp. in the United States (Breman, 1999) but this is the first report from Italy or Europe.

Potexviruses are only spread through contact, or in propagation material and are not transmitted through seeds, so this virus would not seem to pose a major threat provided common preventive prophylactic measures are taken.