Ecology, epidemiology

• Conservation

The melon screening virus ( Melon necrotic spot virus, MNSV) is present in many production areas both in France and abroad. The disease is found in very varied climates: Mediterranean (Greece, Israel, southern Spain - region of Almeria, South of France) or more northerly (Great Britain, Holland, West of France), the most often in cool periods and on short days. It has been detected in melon plants in summer, but not always associated with particular symptoms.

In nature, MNSV only infects Cucurbitaceae : melon, cucumber and more rarely watermelon. Conservation of MNSV therefore probably does not involve reservoir plants, and rather takes place either in the soil or in contaminated seeds.
 

• Transmission

MNSV is transmitted by a soil fungus , Olpidium bornovanus (synonym O. radical or O. cucurbitacearum ), whose motile zoospores can move in liquid media or in water films on colloidal soil particles. The virus can be acquired by the zoospore in the soil: the viral particles attach themselves to the zoospore membrane or to their flagellum and then are transmitted when the zoospore penetrates the root of a healthy plant.

The great stability of the virus and the form of preservation of the fungus (resting spores or chlamydospores) (figure 1) means that a contaminated soil can remain so for a very long time (several years).

Observations show that the virus is also very easily transmitted mechanically , in particular during pruning operations (Figure 2); it could also be by contact between sheets.

transmission Seed has been reported in melons where it sometimes occurs at very high rates: 1 to 22%. In fact, this transmission is indirect: it is not observed if we sow a batch of contaminated seeds in a sterile substrate. On the other hand, it is important if the soil contains the vector fungus. In this case, we speak of vector-assisted transmission of the virus by seed. It is indeed the contaminated seed which brings the virus, but this one would diffuse in the soil at the time of germination, and it is there that the zoospores of the vector> acquire the viral particles, before inoculating them with a rootlet of the seedling.

In the United States, transmission of the virus has also been reported by beetles phytophagous ( Diabrotica sp.), Not present in France, but this observation has not been confirmed.