Ecology, epidemiology

- Conservation, sources of viruses

The cucumber mosaic virus ( Cucumber mosaic virus , CMV) can infect over 700 different species representing 92 botanical families belonging to monocots such as the broadleaf weeds. Market gardening and ornamental crops (perennial and perennial) are particularly affected. In nightshades, CMV infects: chilli pepper, Capsicum frutescens, Cyphomandra betacea , 6 species of Datura and Physalis, Lycopersicon pimpinelifolium , several Nicotiana including tobacco, Petunia hybrida, eggplant, Solanum nigrum Other botanical families including vegetables host it: cucurbitaceae (melon, cucumber, squash, zucchini ), asteraceae (lettuce, escarole ), Apiaceae (celery, carrot, parsley ), chenopodiaceae (beetroot, spinach ). Ten families of monocots can also be affected such as Agavaceae, Amaryllidaceae, Iridaceae, Liliaceae, Musaceae, Orchidaceae and Poaceae.

Among the spontaneous plants, species as common as purslane, black nightshade, groundsel, sow thistle, wild lettuce, dead nettle, speedwell, madder, mercurial are infected. The most important are Portulaca oleracea, Senecio vulgaris, Solanum nigrum, and those which are perennial like madder, or capsella. Note that Stellaria media transmits the virus by seed. In Spain, a study of 51 weed species belonging to 19 botanical families revealed that 25 of them were infected with CMV. Among them, Convolvulus arvensis, Malva sylvestris and Sonchus tenerimus play an important epidemic role because they are perennial and often associated with tomato crops.

Let us remember that these various plants allow CMV to be maintained during the winter ; in spring, and throughout the production period, they will constitute particularly important reservoirs for viruses playing a major epidemic role.


- Transmission, dissemination

The first contaminations often occur when the first aphid flights arrive - CMV is transmitted by aphids in a non-persistent mode . Thus, the aphid vector can acquire the virus on an infected plant and transmit it to a healthy plant, in a few seconds, during very brief bites, called “test bites”. These allow him to ensure that the plant is a favorable host for its development. The aphid is able to transmit the virus immediately after its acquisition, and remains so for about ten minutes. It loses this capacity after 2 to 4 hours at most, all the more quickly if it performs other bites of tests or food bites. It can regain this ability by pricking a virus-infected plant again. One of the consequences of this mechanism is that aphids not attached to the crop, which only pass through it, are perfectly capable of transmitting the virus, which greatly complicates any fight with insecticides.

More than 90 species of aphids are likely to acquire and then transmit CMV. Their vection potentials are different; the best vectors appear to be Myzus persicae, Aphis gossypii, A. craccivora, A. fabae and Acyrthosiphon pisum . Let us add that certain species are only capable of transmitting a certain number of strains.

The proportion of plants that aphids are likely to contaminate is greatest in the vicinity of the source plant. Carried by the wind, they can be formidable vectors, effectively disseminating CMV over rather large distances. In addition, the very high efficiency of its mode of transmission makes the spread of this virus very rapid without significant outbreaks of aphids being observed. Several biotic and abiotic factors play an essential role in the biology of aphids, therefore in the dissemination of viruses and the development of viral epidemics:
- the wind conditions their distribution;
- temperature acts on plant growth, virus multiplication and the development of aphid colonies;
- the proximity of other sensitive crops and weeds contaminated particularly favor contamination.

To these factors, it is advisable to add regional parameters such as the arrangement of the plots, the protection of crops against the prevailing winds by hedges, the local climatic conditions and their influence on certain reservoir plants.

The development of CMV epidemics is therefore linked to climatic conditions . If the winter is severe, a majority of reservoir plants and aphids will be destroyed and the inoculum present at the start of the crop will be reduced.

As with almost all cultivated nightshades, CMV is not seed-transmitted in tomatoes . On the other hand, it is found in many plants belonging to different botanical families (fabaceae, caryophyllaceae) and, in particular, in several weeds (such as, for example, Stellaria media ). This virus could be transmitted mechanically during cultivation, pruning and leaf stripping operations (demonstrated for necrogenic strains and associated with satellite RNA). In this case, the distribution of diseased plants is more in line, unlike aphid transmissions which give rise to outbreaks or random distributions.