Biology, epidemiology

• Conservation, sources d’inoculum

More than 200 woody or herbaceous plant species belonging to at least 50 different botanical families could harbor and therefore conserve this bacterium *. These species may or may not show symptoms, some of them behaving like healthy carriers. Thus, some of these plants, especially wild, serve as sources of primary inoculum via insect vectors. Vine plants do not seem to ensure the introduction of Xilella fastidiosa  subsp. fastidiosa in wineries in the USA. The situation could be different in Europe.

Examples of plants harboring: X. fastidiosa : almond, alfalfa, various wild grasses, peach, plum, apricot, red maple, red mulberry, plane tree, red oak, American elm, small periwinkle, several citrus fruits, etc.), etc. .

(* lists hosts of X. fastidiosa )

• Penetration, invasion

Contaminations are mainly carried out by vector insects. Many species of leafhopper and spittlebugs (Insecta: Hemiptera: Auchenorrhyncha) are known to transmit this bacterium; on vines we can cite Graphocephala atropunctata in California, Homalodisca vitripennis , etc. All these sucking insects feed from the xylem and acquire the bacteria quickly during nutrition (less than 2 h). The latter contaminates their mouthparts and also multiplies in the vector, without circulating in the hemolymph. So no latency period is needed before it can be transmitted, or it is very short, which makes it easier to transmit. The latter usually takes place from wild, usually symptomless host plants to cultivated host plants (grapevine, peach) rather than between cultivated hosts, although this can occur.

** Recall that its transmission in Italy would be ensured at by least Philaenus spumarius (see Figure 1).

• Multiplication and dissemination

Once the host plant is infected, the bacteria can be found in the xylem, both in the aerial organs (leaves, twigs, fruits) and in the roots, where it proliferates there, obstructing the vessels, because of tyloses and tyloses. gums generated by the plant. Subsequently, it is efficiently acquired by the various potential insect vectors, without a latency period; it remains indefinitely in infectious adults. Virtually all sucking insects that preferentially feed on the xylem are potential vectors. Leafhoppers **, as well as spittlebugs are by far the most frequent species among the known vectors.

On various plant species, pruning tools, or other tools causing injury are also believed to be responsible for the spread of the disease from plant to plant, although this mode of transmission has not been described as very effective. Root injuries sometimes induce autografting phenomena and also contribute to the transmission of the bacteria from plant to plant. The multiplication, export and planting of contaminated plants represent a significant risk of spread.

• Conditions favorable to its development

Pierce's disease is currently limited in the United States to hot producing areas, as temperatures and water availability can vary the time of onset of symptoms and their severity depending on the situation. Winter climatic conditions also seem to influence the distribution, and in particular the season-to-season conservation of X. fastidiosa . This bacterium is mainly maintained in production areas with mild winters, probably allowing it to survive in plants in vegetative rest. These climatic conditions also allow its various vectors to be sustainable in the environment **. Winter temperatures below 0 ° C would eliminate it from infected woody hosts. In vitro , its optimal growth oscillates between 25 and 32 ° C; it is reduced and even blocked when temperatures drop below 12 ° C.

The incubation period of the disease seems quite long, at least 3 months, or even much longer.

*** Vector biology

The biology of vectors is important for understanding the epidemiology of this bacteriosis. In California, for example, vector species such as Draeculacephala minerva and Carneocephala fulgida persist in meadows adjacent to vineyard plots, or on weeds in the latter. G. atropunctata also overwinters on other wild host plants. Irrigation and weeding practices, directly influencing the maintenance and development of potential host plants, more or less alter the levels of vector populations and the spread of the bacteria. These vector insects are able to disperse the bacteria regionally.