Parietaria mottle virus: a potential threat for tomato crops?

Several isolates of Parietaria mottle virus (PMoV), a member of the genus Ilarvirus, have been described that affect tomato plants. These isolates, named PMoV-T, cause rings and a bright necrotic mosaic on young leaves that progresses to necrosis of the leaves, stem and apex. Fruits of affected plants display corky rings and brown patches, which develop into ridges with necrotic scars. The virus has been described in tomato plants in several European countries, including Italy, France, Greece and Spain. In addition, an outbreak of a necrosis disease of tomato in California in 2008 was shown to be caused by a new ilarvirus species related to PMoV. The genome of PMoV-T is composed of three single-stranded positive-sense RNAs. Phylogenetic analysis of PMoV isolates from different hosts and geographical origins revealed important structural differences and the existence of two coat-protein types according to their size. Moreover, the PMoV isolates sequenced so far clustered in four different clades that were not related with host specificity. Non-isotopic molecular hybridization, tissue printing hybridization, indirect enzyme-linked immunosorbent assay (I-ELISA) and direct tissue-printing immunoassay using a polyclonal antiserum and one-step RT-PCR assays have been set up for routine detection of the virus. The virus was mechanically transmitted from Parietaria officinalis pollen extracts to other host species, including tomato and pepper. In addition, viral transmission can be facilitated by different insect species, some of them used for biological control (e.g., mirids), in a non-persistent manner. It has been suggested that elimination of P. officinalis plants surrounding tomato crops could help to restrain virus spread. However, further studies will be needed to prevent PMoV-T becoming a threat to tomato and pepper crops.