Biology, epidemiology

•  Conservation reservoirs phytoplasms

The phytoplasma of Flavescence dorée stricto sensu multiplies in the phloem (sieve tubes) of plants of the genus Vitis and is transmitted by the leafhopper Scaphoideus titanus (figure 1) which lives only on Vitis , unlike other leafhoppers which can multiply on different cultivated or wild hosts. The nature inferior of the leafhopper means that the disease is very epidemic in this crop.

Phytoplasmas genetically close to the Flavescence dorée phytoplasma have been described in various wild plants such as alder, clematis or broom. It has been shown that they can be transmitted occasionally to the vine by other leafhoppers living on these plants which could therefore act as a reservoir. However, the epidemic nature of these phytoplasmas on grapevine has not been demonstrated.

• Transmission, dissemination

The phytoplasma of Flavescence dorée is transmitted by its insect vector * in a persistent, circulating and multiplying mode. The vector is contaminated by biting to feed in the phloem vessels of a diseased plant: this is acquisition. But the insect is not immediately infectious, it is the latency period which lasts about 30 days. The phytoplasmas must cycle through the insect before being transmitted again. They circulate in his body by first crossing the wall of the intestine, then reach the hemolymph and from there, reach various organs where they multiply, including the salivary glands. When they reach the salivary secreting cells, they can be injected into a new plant during a meal of the insect. After this period of "incubation", the infectious insect remains so throughout its life but will not transmit the phytoplasma to its offspring. It will be able to disseminate the phytoplasma of Flavescence dorée during its flights and contaminate healthy plants. The organism, injected into the phloem of the plant, multiplies there and will perpetuate itself throughout the life of the vine.

In the absence of control, disease progression within a plot is rapid. The leafhopper's ampelophagous activity promotes gradual spreading and the disease spreads in the form of spots which enlarge from a primary focus from one year to the next.

The disease is also transmissible by transplanting infected plants to healthy plants.

* S. titanus only produces one generation per year and performs its entire cycle in the vineyard. Overwintering takes place in the egg state. After hibernating, the latter hatch between early May and early July and give rise to larvae which will evolve for 50 to 55 days through five larval stages. The first adults appear in early July and after 10 days of sexual maturity they mate. They die in the fall after egg laying.

• Factors favorable to its development

Several parameters influence the development of Flavescence dorée (via the host plant or the disease vector):
- the climate, which influences the cycle of S. titanus ;
- certain cultivation methods such as grafting. Indeed, the use of wood from contaminated mother vines was at the origin of the transmission of the disease to a more or less important number of plants in the nursery and subsequently to new farms;
- the cultivated grape variety. For example Cabernet Sauvignon, Sauvignon and Chardonnay are very sensitive and the disease spreads quickly in the plots. Conversely, Merlot, Sémillon and Syrah are more tolerant and the disease spreads more slowly.