Emergence of new strains of Watermelon mosaic virus in South-eastern France: evidence for limited spread but rapid local population shift.

Severe symptoms caused by Watermelon mosaic virus (WMV) in zucchini squash leaves and fruits have been observed since 1999 in South-eastern (SE) France. Their appearance correlates with the introduction of new, "emerging" (EM) isolates distant at the molecular level from the "classic" (CL) isolates present for more than 30 years. To understand the origin and spread of EM isolates, a survey was performed between 2004 and 2007. WMV isolates collected were characterized by sequencing part of the polymerase and coat protein coding regions. This revealed the presence of EM isolates in SE France only, whereas CL isolates were widespread throughout the country. Besides, four subgroups of EM isolates were observed in SE France, suggesting multiple introductions. Recombinants between CL and EM groups, which probably arose locally, were observed during the survey. A strong geographic structure that remained stable during the 4 years was observed between different EM isolates. Our results showed that EM isolates did not spread over long distances, but rapidly replaced the pre-existing CL isolates in all sites where both groups occurred.