Epidemiology and sustainable control of zucchini viruses in Flanders.

The cucurbit viruses Cucumber mosaic virus (CMV; Cucumoviridae), Watermelon mosaic virus (WMV; Potyviridae) and Zucchini yellow mosaic virus (ZYMV; Potyviridae) are one of the major threats for zucchini cultivation in temperate and tropical regions. Due to a more than 10-fold increase in zucchini production in Flanders during the final 20 years, zucchini viruses gained importance as well. These viruses are transmitted by aphids in a non-persistent way and cause comparable symptoms in zucchini including mottling, mosaic, chlorosis and distortion of the leafs or discoloration and distortion of the fruits. This leads to important yield reductions and unmarketable fruits. In order to gain knowledge about the epidemiology of these viruses in Flanders and to propose sustainable control measures, a 4 year project was conducted to address these points of interest. Based on an extensive survey between 2007 and 2013, we observed a clear shift in the virus populations. The level of ZYMV decreased, the level of CMV infections increased until a dropdown in 2013 while the number of WMV infections remained relatively constant until an important increase in 2013. As WMV is most related to the development of symptoms, and consequently a reduction of yield and fruit quality, the genetic variability of a selection of WMV isolates from different growing seasons was tested by amplification and sequencing a part of the coat protein (Desbiez et al 2007). This revealed the introduction of a new genotype in 2010 that was able to establish in the succeeding years. Besides zucchinis, also different weed species and the aphids were monitored for the presence of the studied viruses. Several weed species were identified to be an important reservoir plant for CMV and in a lesser extent for WMV. The number of aphids in the sampled fields were relatively low, although Aphis gossypii was most abundant in zucchini fields and can be considered the most important vector in regard to virus spread. Based on cultivar trials, it was clear that symptom development and yield reduction due to virus infection strongly depended on the cultivar. In the case of CMV we also noticed that symptomless plants in general didn’t contain the virus indicating resistance. This is in contrast to WMV plants were cultivars that didn’t develop symptoms upon infection appeared to be latently infected indicating tolerance rather than resistance. To avoid spread of the tested viruses during cultivation practices, we tested some disinfection agents for disinfection of the tools. This learned us that Virkon and Virocid with a 10 minutes contact time were usefull for disinfection, although they could not completely prevent spread in the field trial. In conclusion we can state that a sustainable management of zucchini viruses can be achieved by a combination of contol measures. These include a selection of resistance or tolerant cultivars, thouthful weed and aphid control and good cultivation hygiene including disinfection of harvest equipment.