Ecology, epidemiology

- Conservation, sources of viruses

The mosaic virus lettuce ( Lettuce mosaic virus , LMV) is transmitted by seeds in lettuce and also in Lactuca serriola . Infection of the embryo can originate from that of the ovum, more rarely that of the pollen. The rate of transmission varies according to genotype, strain and environmental conditions; it can fluctuate from 3 to 15%. In varieties with a recessive resistance gene, the rate of transmission has been reported to be zero or less than 1 per thousand. However, certain strains capable of bypassing resistance and transmissible at rates of up to 10% have appeared in several countries.

When using uncontrolled and infected seed lots, young plants grown from seeds harboring the virus are the initial source of inoculum. In this case, the epidemics in the field are very serious. Furthermore, LMV reveals a fairly broad spectrum of hosts capable of being infected and of serving as sources of inoculum. Many weeds constitute reservoirs for potential viruses and act as source or relay plants. Among the asteraceae, we can cite: Lactuca serriola , Lactuca virosa , dandelion, Helminthia groundsel, sow thistles, sp .. Capsella, stellate, Lamium amplexiacaule , chenopods ... also harbor LMV. Sensitive cultivated plants also include peas, chickpeas and safflower; but their role in outbreaks of LMV on salads appears minor. In recent years, several ornamental species have been recognized as hosts of LMV: Osteospermum , gazania, petunia, queen-daisy, lysianthus. The favorable influence of the first two species in outbreaks of LMV on lettuce has been clearly demonstrated in California. Chicory crops, but also spinach, help to maintain the LMV during the winter.

Repeated cultivation of lettuce in the same location often leads to a gradual increase in the infection rate of the plots. This occurs especially when cultivated varieties are susceptible to native strains and when many virus-susceptible plants persist in the field environment and serve as virus reservoirs.


- Transmission, dissemination

LMV is transmitted by aphids in the non-persistent mode. The aphid vector can acquire the virus on an infected plant or transmit it to a healthy plant, in a few seconds, during very brief bites, called "test bites". These bites allow him to make sure 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. He loses this ability if he performs other test stings or longer food stings. The proportion of plants that it risks contaminating is greatest in the vicinity of the source plant. Carried by the wind, it can be a formidably efficient vector over relatively large distances.

Many aphid species including Myzus persicae , Macrosiphum euphorbiae , Aphis gossypii and Aphis craccivora , Hyperomyzus lactucae , Nasonovia ribis-nigri , Pemphigus bursarius ... are likely to transmit LMV more or less effectively.

The very high efficiency of the mode of transmission of this virus means that we can observe a very rapid dissemination of the disease in the plot without having noted significant outbreaks of aphids.