Ecology, epidemiology

For the moment, the squash mosaic virus ( SqMV ) does not seem to have established itself permanently in the various production areas in France where it has been reported in the past. However, we must remain vigilant, because this virus could very well adapt to our environment.

• Conservation

SqMV infects, under natural conditions, mainly cultivated or wild Cucurbits. Certain spontaneous perennial ( Ecballium elaterium ) or annual ( Cucumis melo var. Agrestis , the wild form of melon) species can constitute reservoirs of virus between two crops. But the most effective means of preserving the virus is undoubtedly the seed which sustains and disseminates it (see below).

Under experimental conditions, SqMV can infect several species not belonging to Cucurbitaceae: peas, Chenopodium album and C. mural . In addition, these last two species transmit the virus by seed, which could allow its maintenance in the environment in the absence of vector and sensitive culture.

• Transmission

SqMV is transmitted by many species of phytophagous beetles including Acalymma trivittata and Diabrotica undecimpunctata (Chrysomelid), Epilachna chrysomelina (Coccinelidae) or orthoptera ( Malanoplus differentialis ). The insects contaminate the mandibular parts by eating the leaves of diseased plants (an acquisition period of 5 minutes is sufficient), then transmit the virus by feeding on the leaves of healthy plants. Vectors may remain able to transmit the virus for 1 to 3 weeks.

The most efficient and dangerous mode of spread of SqMV is seed transmission (Figures 1 to 3). Indeed, this virus and some of its vectors are present in many seed production areas. It is the trade in these that has probably allowed its dissemination in many countries, including countries where insect vectors are not found, such as France or Canada. Seed transmission rates vary by strain and host. They can be very high and reach 94% in melons, however they are in the order of 1 to 10% in contaminated commercial batches. In zucchini, they are around 1 to 5%. Transmission rates can drop quite sharply depending on how long the seeds are stored.

Very stable, SqMV can also be transmitted mechanically during pruning or harvesting operations, or by simply rubbing the leaves together.

This virus has only been reported in France in connection with the use of batches of contaminated imported seeds, its secondary dissemination then taking place by mechanical transmission. None of the known vectors of the virus is present in France. However, a species close to one of them, Epilachna argus , is occasionally found in the South on E. elaterium . If SqMV were to become more frequent, and if E. argus were confirmed to be a vector, the latter could play a role in the epidemiology of the virus, in particular, by transmitting it to reservoir plants, such as E. elaterium , on which this insect grows.