Ecology, epidemiology

The epidemiology of the zucchini ( yellow mosaic virus Zucchini yellow mosaic virus , ZYMV) in France is still poorly understood. In particular, it is not yet clear what are the sources of the virus from which the epidemics start in the spring. Does the virus overwinter in volunteer or ornamental plants? Is it introduced from more southern countries, by theft of aphids, or by the importation of contaminated fruits? Is it transmitted by seed in some species? The answers to these questions would make it possible to better adapt the control methods. 

• Conservation

Present irregularly in France, ZYMV is most often reported in the production areas of the Southeast and the Southwest. However, in some years it can reach the Paris region, or even Brittany. Under natural conditions, ZYMV infects mainly cultivated species belonging to cucurbits, but it has also been reported on ornamental plants (hollyhock, begonia). In laboratory conditions, it has been possible to show that ZYMV also infects spontaneous or ornamental plants belonging to other families (dead nettle, buttercup, delphinium, larkspur ).

In warmer regions (sub-tropical or tropical), where cucurbits   can be grown all year round, the virus is much more common as it can pass from one crop to another, and because various species of wild cucurbits can act as a reservoir.

• Transmission

ZYMV is transmitted non-persistent by more than 11 species aphid , including the melon aphid, Aphis gossypii , and the green peach , aphid Myzus persicae . The aphid vector is capable of acquiring the virus on an infected plant, or of transmitting it to a healthy plant, during very brief bites of the order of a few tens of seconds which are the "test" bites allowing the insect to recognize whether the plant on which it has landed is a favorable host for its development. The aphid remains able to transmit the disease generally for a few tens of minutes or even a few hours, but it quickly loses this ability if it performs test bites or feeding bites. However, he may acquire the virus again if he repeats test bites on an infected plant. The very high efficiency of this mode of transmission means that the disease can spread in a crop without significant outbreaks of aphids having been observed.

It has been shown that aphids could acquire the virus on the rind of melon fruit intended for consumption, thereby showing that the globalization of the fruit and vegetable trade could contribute to the spread of viruses between countries or continents .

The transmission of ZYMV by aphids involves, as with all potyviruses, a very sophisticated molecular mechanism. A viral protein, the helper factor , acts as a “double-sided” adhesive: it forms a bridge between the end of stylet ^* the aphids' and the viral particles.

Some observations suggest that ZYMV could be transmitted mechanically (during harvesting or pruning operations) or by leaf contact. It has not been reported to be seed-borne in cucumber or melon. Recent results confirm seed transmission of ZYMV in squash. However, this transmission by seeds would take place in a somewhat unusual way, since the young infected plants, resulting from contaminated seeds, initially show no or few symptoms. This makes the sanitary control of seed lots particularly difficult .

* The stylet is the organ that allows aphids to feed on plants