Ecology, epidemiology

- Conservation, sources of viruses

The alfalfa ( mosaic virus Alfalfa mosaic virus , AMV) is very polyphagous , it can be artificially inoculated into a large number of hosts, ie more than 400 plant herbaceous or woody species belonging to 50 different botanical families. Some of them can harbor it without showing any symptoms. It can therefore be easily preserved from one season to another by means of a few weeds, but also of several cultivated plants. Alfalfa appears to be the most important reservoir; it expresses symptoms of mosaicism, mottling and malformation; however, no symptoms are visible during the summer. Many other plants can serve as a source of inoculum during winter or summer. A large number of solanaceae, both cultivated and spontaneous, can be contaminated: tomato, pepper, eggplant, potato, tobacco, Atropa belladona, Cyphomandra betacea, Datura stramonium, Hyoscyamus muticus, Nicandra physaloids, Physalis angulata, Sarachia edulis, Schizanthus pinnatus, Solanum dulcamara, S. nigrum, Withania frutescens, W. somnifera Other potential hosts are reported, such as lettuce, celery, beans, peas, chickpeas, alfalfa, clover, some woody plants (currant, lavandin, Cercis siliquastrum, Hibiscus cannabinus, Ruscus hypoglossum, Clematis vitalba, Glycine latifolia, Viburnum spp ., Buddleia davidii, Rhamnus angula ). In monocots, asparagus is the only susceptible species.


- Transmission, dissemination

From infected plants, AMV is transmitted to other plants via aphids , in a non-persistent fashion. These very quickly acquire viral particles adsorbed at the level of their stylets and integuments of their mouthparts, during brief “test” bites. They are able to transmit them immediately, but for a short period not exceeding a few minutes to a few hours. Twenty-six species of aphids are susceptible to transmit AMV, such as Myzus persicae, Aphis fabae, A. gossypii, A. craccivora, Acyrthosiphon pisum, A. kondoi However, their transmission rates vary depending on the aphid clone and species, AMV strain, and source and host plant.

The dissemination of AMV is mainly carried out by aphids ; it therefore depends on the nature of the epizootics of these insects. Several biotic and abiotic factors play an essential role in the biology of aphids, therefore in the dissemination of viruses and therefore in the development of viral epidemics:
* the wind conditions their distribution;
* the temperature acts on the growth of the tomato, the multiplication of the virus and the aphids respectively in and on the plants;
* the proximity of other sensitive contaminated crops (and in particular alfalfa) and numerous virus- weeds infected favors contaminations.

Finally, to these factors, we can add situations and regional cultivation practices such as the arrangement of the plots, their orientation in relation to the wind, their possible protection by hedges, the conservation of plants that are reservoirs for viruses.

Let us add that the development of AMV epidemics is also 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.

The transmission by the seed occurs in some plant species, especially in alfalfa, and to a lesser degree in pepper. In potatoes, seeds can occasionally be infected. Weeds such as Nicandra physaloides, Stellaria media, Senecio vulgaris, Chenopodium album and Holosteum umbellatum also transmit AMV by seed.