Ecology, epidemiology

- Conservation, sources of viruses

The virus des feuilles jaunes in re la tomate ( Tomato yellow leaf curl virus , TYLCV) is capable of if maintenir on a range of hosts diversifiée plus que ne laissaient entrevoir les premières études portant sur ce virus. It certainly infects especially tomatoes, as well as a few other cultivated or unripe nightshades: tobacco, more sporadically pepper, Datura stramonium and Solanum nigrum . D'autres espèces owned familles des différentes botaniques were répertoriées comme naturels potentiels Breakfast: bean Eustoma grandiflorum (syn. Lisianthus russelianus ), Malva parviflora, M. nicaensis, Cynanchum acutum, Euphorbia pulcherrima, E. heterophylla, Dittrichia viscose (syn . Innula viscose ), Nicandra physaloides, Achyranthes aspera

For example, in Lebanon, in addition to beans and eggplant (immune according to some authors), Amaranthus sp., Sonchus oleraceus, Plantago sp. and Mercurialis annua would play the role of reservoir in TYLCV. In southern Spain Datura stramonium, Solanum nigrum, S. luteum and Mercurialis ambigua show spoon-shaped leaves quite characteristic of TYLCV infection. In Spain, TYLCV gradually replaced TYLCSV during the years 1996-1998. It is more effectively transmitted there by local biotypes of Bemisia tabaci . Also in Spain, 4 new healthy carrier plants were more recently identified: Chenopodium mural, Convolvulus sp., Cuscuta sp. and Conyza sumatrensis .

Note that TYLCV can be transmitted experimentally, by grafting or via B. tabaci , to several dozen hosts belonging to very different botanical families.


- Transmission, dissemination

As previously reported, TYLCV is transmitted exclusively by Bemisia tabaci , a whitefly also known as the “whitefly” of cotton, tobacco, sweet potato or leaf silver.

Bemisia tabaci is a polyphagous insect widespread in all tropical and subtropical areas. During the last decade, it has progressed significantly towards more temperate zones. It is now reported in much more northern countries such as the Netherlands, Germany, Denmark and Sweden, but exclusively under cover. In Italy and Spain, it is also observed in the open field. It was in 1981 that B. tabaci was identified for the first time in France. It is currently rather well established on different plant species cultivated in greenhouses, even if it is found near crops under cover in the south of France during the summer. Generally, populations B. tabaci are reduced in spring and early summer; the incidence of TYLCV is therefore negligible on early field crops. Populations increase rapidly in late summer, and TYLCV attacks are very severe in fall.

Several biotypes of Bemisia tabaci have been described; they can only be identified in the laboratory, thanks to the characterization of their enzymatic profiles, but also now by molecular biology techniques. These biotypes also differ by some of their biological properties, in particular by their ability to transmit the different strains of TYLCV more or less well. The B biotype, also called “ Bemisia argentifolii ”, has largely contributed to the spread of the virus. It is characterized by fertility and polyphagia much higher than those of other biotypes. It is also often more efficient at transmitting TYLCV. Other biotypes perform this function, often less efficiently: the Q biotype, native to Mediterranean regions, the IC biotype of Ivory Coast Remember that biotype B is more suited to transmitting TYLCV than the Asian biotype, which is more suited to transmitting ToLCV. Recent work has shown that biotypes B and Q are found in France, as in many Mediterranean countries (Italy, Spain, Canary Islands, Morocco). In addition, the Biotype Q would be more frequent. Its predominance would be explained by a greater tolerance to insecticides and to extreme temperatures (cold or hot). Its biological properties should allow B. tabaci to maintain itself more easily in regions considered until now at the limit of its extension zone.

The TYLCV is transmitted according to the circulating persistent mode. The acquisition or transmission of viral particles takes place during prolonged feeding punctures localized in the phloem vessels. The acquisition period can vary from a minimum of 10 to 20 minutes to a few hours, or even 1 or 2 days. Nymphs are as effective as adults at acquiring TYLCV.

Once absorbed, the particles complete a cycle in the insect's body before they can be transmitted again: we speak of "circulating virus". They pass through the digestive tract, the general cavity, to concentrate in the salivary glands. The latency period lasts only a few hours (8 to 24 hours), the whitefly is then able to transmit the virus. The virus can be transmitted after a minimum inoculation time of 15 to 30 minutes; it is optimal if the whitefly's “meal” lasts 6 hours. Symptoms will appear on plants at least 2-3 weeks after the first infections.

On tomatoes, whiteflies cause various symptoms that you can see through this link . There you will also find other information on the biology of these insects.

Once viruliferous, whiteflies will remain so for several days (retention period), or even their entire life (35 to 40 days) according to the authors. Note that the viral particles are retained after the moult, and transmitted to the offspring. It was recently demonstrated that TYLCV was transmitted by the transovarian route to the offspring of its vector over at least two generations. This situation certainly has significant epidemiological repercussions.

Let us add that the larvae being almost immobile, it is essentially the adults who ensure the dissemination of TYLCV. In addition, females appear to be more efficient vectors than males.

Finally, TYLCV is not transmitted by contact between plants, nor by experimental mechanical inoculation. No seed transmission was observed either. It should be noted that tomato fruits heavily infected with TYLCV, imported from countries affected by yellow leaf spoon disease, are likely to allow Bemisia tabaci to acquire viral particles and transmit them to healthy plants.