Ecology, epidemiology

• Conservation 

The watermelon ( mosaic virus Watermelon mosaic virus , WMV) infects quite a number of species that botanical do not belong to cucurbits, both cultivated (peas, beans, spinach, lamb's lettuce, vanilla, etc.) and spontaneous (groundsel, capselle, dead nettle, fumitory ). These plants play a very important role for the conservation of this virus during the winter by constituting " reservoir plants " in the absence of sensitive culture. In the spring, they will be the sources of viruses and sometimes also of vector aphids from which epidemics will start.

The diversity of reservoir plants identified throughout the world undoubtedly explains the ability of this virus to adapt to very different ecosystems ranging from sub-desert regions (California or Arizona desert) to Mediterranean or temperate zones (Europe , North Africa).

• Transmission 

WMV is transmitted non-persistent by more than 38 species aphid , including the melon aphid, Aphis gossypii , the green peach , aphid Myzus persicae , as well as A. citricola, A. crassivora and Macrosiphum euphorbiae . 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 ("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 capable of transmitting the disease generally for a few tens of minutes or even a few hours, but it quickly loses this capacity if it performs test bites or food bites. However, he may acquire the virus again, if he repeats a test bite on an infected plant. Transmission of WMV 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 would constitute a “bridge” between the end of the aphid stylet * on which it attaches and the viral particles.

The very high efficiency of this mode of transmission means that the disease can spread very quickly in a crop without having observed large populations of aphids. It is not uncommon to see all the plants on a plot contaminated with WMV within a few weeks.

So far, this virus has not been reported to be seed-borne in melon, zucchini, cucumber or in reservoir plants.

In the laboratory, WMV can be transmitted mechanically, but this mode of plant-to-plant transmission does not appear to occur in the field and play a significant role in the spread of the virus under natural field conditions.


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