Ecology and Epidemiology

- Survival and virus reservoirs 

The alfalfa mosaic virus (AMV) is a very polyphagous virus. It is able to infect a large number of hosts artificially, over 400 plant species belonging to 50 families. It has about 150 herbaceous or woody hosts. It can easily survive from one season to another in weeds, but also in several cultivated plants. Some of them can be sources of inoculum in winter or summer (tomato, pepper, potato, celery, lettuce, bean, pea, clover, alfalfa ...). In France, alfalfa crops (figures 1 and 2) are the most important sources of viruses. It is always near alfalfa crops  that the most severe outbreaks are recorded.


- Transmission and dissemination

AMV is transmitted from infected plants to other plants via aphids, in a non-persistent manner. The aphids, by short "test" bites, very quickly acquire adsorbed virus particles in their stilettos and teguments of their mouthparts. They are able to transmit the virus immediately, but only during a short period of time, that varies from a few minutes to a few  hours. More than ten species of aphids are likely to transmit AMV (Myzus persicae, Aphis gossypii, A. kondoi, Acyrthosiphon pisum..).

Transmission with seed occurs in some plant species, especially in alfalfa, red pepper and Nicandra physalodes but in any case not in tobacco.

AMV is mainly spread by aphids, and its dissemination depends on the nature of outbreaks of these insects. Several abiotic factors play a key role on the biology and effectiveness of aphid flights:
- The wind determines their distribution;
- The temperature affects the growth of tobacco, the multiplication of the virus and aphids in and on plants, respectively;
- The crop environment; proximity to other sensitive crops including infected alfalfa and many virus-infected weeds promote contamination;
- Finally, regional factors can be added to the above-mentioned conditions, such as the layout of the plots, orientation to the wind, possible protection by hedges, conservation of plants providing a virus reservoir ...