Virus infections in zucchini: a story of tolerance and resistance to CMV and WMV

Virus infections with Cucumber mosaic virus (CMV), Watermelon mosaic virus (WMV) and Zucchini mosaic virus (ZYMV) are one of the major threats for the production of zucchini and other cucurbits in subtropical and temperate regions. They can cause symptoms like mottling, mosaic and leaf distortion as well as significant fruit malformations resulting in a important yield reduction and unmarketable fruits. These viruses are transmitted in a non-persistent way by several aphid species potentially leading to a rapid spread of the virus in the crop resulting in serious epidemics. In order to control introduction and eventual spread of viruses, several measures can be taken. This includes insecticide treatments, good hygiene practices and cultivation of resistance or tolerant varieties. In this project, we organized zucchini cultivar trials to determine the resistance or tolerance against virus infections with WMV or CMV and the difference in symptom development. We tested 6 cultivars in 5 rows and 7 cultivars in 4 rows with CMV and WMV, respectively. In each test, the first row (CMV) and the first and second row (WMV) were inoculated one week after planting and the second row (CMV) and third row (WMV) were inoculated three weeks after planting. The remaining rows were allowed to obtain the virus by means of cultivation activity to simulate introduction of the viruses in common practice. In the case of infection with CMV, the infection remained limited to the susceptible cultivars in the first row with a complete decline of the infected plants. The other rows remained healthy after inoculation or harvesting and no virus could be found with molecular detection. This indicates that the cultivars that remained healthy after CMV inoculation can be considered to be resistant to this virus. This was in contrast to the WMV trial were a gradual spread of the disease to all the cultivars in the different rows was recorded within one month after the final inoculation. In contrast to CMV infected plants, the WMV infected plants also remained more productive. Nevertheless, a clear difference in symptom development between the different cultivars was observed after WMV infection, with some cultivars remaining symptomless and not showing a yield reduction. In the case of WMV infection, no resistance was observed although some cultivars were highly tolerant to the virus. These results suggest that WMV is a more important threat for zucchini production than CMV, as no strict resistance to WMV was observed. As a consequence, WMV tolerant plants can serve as virus reservoirs from where less tolerant plants can become infected, while CMV infected plants are easily detected and decline after infection.
Keywords: Cucumber mosaic virus; Zucchini yellow mosaic virus; Watermelon mosaic virus; Epidemiology, cultivar trial