The never-ending story of cucurbits and viruses

Cucurbits are widely grown worldwide in diverse agroecosystems. Viruses represent one of the major threats for cucurbit production. More than 90 viruses have been described infecting cucurbits in natural conditions. Their prevalence and agronomic impact are highly contrasted, and the major problems at the worldwide level are related to circa 10 virus species, both "classic" - i.e., known for decades - or recently emerged on a large scale. Among the "old" mosaic-inducing viruses, aphid-transmitted viruses (potyviruses, Cucumber mosaic virus) remain agronomically important, particularly in temperate climates. Despite their long-term prevalence and apparent stability, introductions of new strains have been observed in several countries in the last years, probably via commercial exchanges of plant material. Some of these introduced strains have rapidly replaced pre-existing virus populations, sometimes resulting in an increased economic impact. In Mediterranean and tropical climates, as well as in heated greenhouses in temperate regions, whitefly-transmitted viruses (begomoviruses, ipomoviruses, criniviruses) now constitute major problems. Their rapid emergence can be explained by the combination of changes in climate and cultural practices that favor their natural vectors, and the increased exchanges of plant material that made possible their long-distance spread. Thrips-transmitted viruses are present in Asia and Latin America and can be locally very damaging. Viruses transmitted by seeds or by contact are also easily disseminated worldwide and represent important constraints for exchanges of plants or genetic material. Accurate and regular re-evaluation of the complex viral pathosystem infecting cucurbits in different countries, adaptation of cultural practices and development of resistant cultivars contribute to the continuous arm race against the introductions and local evolution of virus populations.