Epidemiology of four cucurbit viruses in zucchini (Cucurbit pepo) in Flanders

In most temperate and tropical cucurbit growing regions in the world, some of the most important diseases on zucchini (Cucurbit pepo) are caused by Cucumber mosaic virus (CMV, Cucumovirus), Zucchini yellow mosaic virus (ZYMV, Potyvirus), Watermelon mosaic virus (WMV, Potyvirus) and Papaya ringspot virus (PRSV, Potyvirus). These viruses are transmitted by several aphid species in a non-persistent manner and the symptoms (mosaic, mottling, chlorosis and leaf distortion) caused on infected plants result in a significant decrease of production. In case of severe infections also fruit quality is seriously affected making the product unmarketable. Since 2005, a remarkable increase of virus infections was observed in the Flemish zucchini growing regions. Due to a 5 to 10 times increase of cultivated area and a similar increase in turnover of the zucchini production, the damage caused by these viruses have an important economic impact for the zucchini growers. Preliminary virus screenings were conducted during the growing seasons from 2007 to 2009, followed by more intensive surveys in 2010 and 2011. These surveys clearly reveals that there is a shift in the presence of viruses in the field between the growing seasons. In 2007 and 2008, damage was mainly caused by WMV and to a lesser extent by CMV and ZYMV. In 2009 CMV was the predominant virus while PRSV gained some importance. Additionally, the results for the last two growing seasons show that CMV remains the most important virus, followed by WMV, ZYMV and PRSV, which presence decreased again. Despite the importance of particular viruses in different growing seasons, more than half of the observed infections consisted of mixed infections. However, it is not clear whether mixed infections have an influence on crop and fruit damage, compared to single infections. Infections mostly occur in the field with only a few reports of a severe infection in greenhouse-grown zucchini in the 2007-2011 period. In the future, inoculation experiments will be performed to study the introduction pathway of the viruses as well as the influence of mixed infections and the choice cultivar on symptom development, yield and fruit quality. To allow a faster virus screening in the future a multiplex qPCR for the simultaneous detection of ZYMV and WMV is under development.