Protection methods

- During cultivation

Unlike cryptogamic diseases, there is no curative control method that makes it possible to effectively control viruses during culture, and in particular the lettuce ( mosaic virus Lettuce mosaic virus , LMV). An infected plant will remain so for its entire life, although the symptoms sometimes tend to subside.

If attacks take place in the nursery and are detected early, the few plants showing symptoms of LMV will be quickly eliminated and in no case planted subsequently.

The Aphicides treatments are essential to control aphid populations on salads. Unfortunately, they are often ineffective in controlling virus epidemics. In fact, vector aphids frequently come from outside the plot and transmit the virus during brief bites, even before the aphicide has time to act. Furthermore, the difficulties currently encountered in controlling aphids on lettuce, sometimes linked to phenomena of resistance to insecticides, do not help to improve this situation.


- Next crop

The fight against lettuce mosaic relies mainly on the use of seeds tested without viruses and the use of resistant varieties .

Depending on the country, and depending on the seed and salad production conditions, the virus thresholds tolerated in seeds are different. For example, in many European countries seeds used by professionals are produced under conditions that minimize the risk of infection of seed carriers. The seed lots are still checked for absence of LMV. Also, the tolerance threshold, established a few years ago on the basis of epidemiological studies, is 1 per thousand. The ELISA tests, carried out by the selection establishments, make it possible to guarantee this threshold.

In the United States, where almost all cultivated varieties are sensitive to LMV but where the cultivation methods are different (large plots do not follow one another), the imposed threshold is lower, of the order of 0 infected seeds on 30,000. This policy has made it possible to greatly reduce the damage caused by this virus. Serious epidemics have nevertheless been observed locally and in some years. They were due, not to the poor quality of the seeds, but to the presence of unsuspected virus hosts ( Osteospermum , gazania) cultivated on the edge of the salad plots of certain farms.

Regarding the use of genotypes resistant resistant to LMV, a few varieties of iceberg and romaine (possessing the gene “ mo1² ” ) exist in the United States, but their use is still very modest. In France, and gradually in other European countries, the introgression of the gene “ mo1¹ resistance ” into most varieties of butter, batavia, romaine and cut lettuce provides a fairly complete range of resistant varieties.

Unfortunately, strains capable of bypassing the resistance conferred by the “ mo1² ” and “ ” mo1¹ genes have been identified in France and in a few European countries, more rarely in the United States. These strains were not seed transmissible. Over the past ten years or so, other strains, virulent on resistant varieties and capable of being transmitted by seeds at high rates on all varieties, have been isolated in France and South America (Chile, Brazil). . This situation makes it essential to control seeds on all varieties, whether sensitive or resistant. Knowledge of the resistance genes used LMV in salad has evolved. The “ gene mo1² ” , identified in the United States in a few accessions of Lactuca serriola from Egypt, and the “ gene mo1¹ ” , identified in France in the “Gallega de Invierno” variety, have long been considered identical. Recently, these genes have been shown to be allelic or very closely related. The gene mo1² appears to be effective against certain strains which bypass the gene mo1¹ . Another dominant resistance gene, “ Mo² ” has been demonstrated in certain varieties of lettuce. It is very effective against certain isolates identified in Greece and the Middle East, but this gene is bypassed by most of the strains characterized in Europe, the United States or Australia. Its use in breeding programs therefore appears to be of limited interest.

No new source of resistance to isolates bypassing the genes was found in lettuce mo1¹ and mo1² . On the other hand, a dominant gene Mo³ , effective with regard to all the identified pathotypes, was discovered in an accession of Lactuca virosa . The introduction of Mo³ in lettuce is underway from interspecific crosses. In fact, it would eventually be necessary to combine several genes controlling different resistance mechanisms in order to obtain lasting resistance.

It is also advisable to recall all the measures which will aim to prevent or, at least, to limit as much as possible the introduction of LMV and its extension in the lettuce plots. Also, in countries where contamination is very early, it will be necessary to protect nurseries and young plants ; for this, we can use non-woven sails . The mechanical barrier thus created will delay contamination. A weeding careful nurseries, plots and their surroundings (hedgerows and roads ...) will be carried out to eliminate the sources of viruses and / or vectors. It will be necessary to avoid setting up a nursery or a salad crop near productions already affected or sensitive to LMV, in particular peas, chickpeas, safflower in summer, curly chicory and spinach during the season. winter.

It should be noted that sprays of mineral oils on salad made it possible to reduce the percentage of infected plants and therefore that of the damage linked to LMV.


NB: Controlling insect populations on a crop implies, depending on the situation, the use of insecticides. As the number of insecticides available for a given use is constantly changing, we advise you to consult the e-phy site of the Ministry of Agriculture and Fisheries which is an online catalog of plant protection products and their uses, materials. fertilizers and growing media approved in France. This also applies to all biological products based on microorganisms or natural substances.