Biology, epidemiology

• Conservation, phytoplasma reservoirs

The blackwood phytoplasma multiplies in the phloem (sieve tubes) of plants of a large number of wild or cultivated plants. Wild reservoirs are bindweed, nettles or passerines, but also cultivated plants such as lavender and lavandin. The adults who fly from these infected plants transmit the phytoplasma during the search for food to a large number of crops such as vines, nightshades (tobacco, tomato, eggplant, peppers), strawberry, sugar beet, corn in Serbia and Hungary, endive, lavender and lavandins in France. Most of these species are “epidemic dead ends” because, apart from lavender, there is no acquisition of the pathogen from these plants. The acquisition is made by the larvae resulting from the summer laying on bindweed, nettles. These larvae, which spend the fall and winter on the roots of infected plants, acquire the phytoplasma by feeding on the infected sap. Only adults that can fly disseminate the phytoplasma from plant to plant.
 

• Transmission, dissemination

The blackwood phytoplasma is transmitted in summer by its vector insect in the persistent, circulating and multiplying mode. The vector is contaminated in the larval state by biting to feed in the phloem vessels of a diseased plant: this is acquisition. But the insect is not immediately infectious, it is only after a period of latency that the infection of the salivary glands occurs and therefore the insect becomes infectious. The phytoplasmas must therefore cycle through the latter before being transmitted again. They circulate in his body, first crossing the wall of the intestine, then reach the hemolymph and, from there, reach various organs (including the salivary glands) where they multiply. 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.

The major insect vector of black grapevine wood is the cixide Hyalesthes obsoletus , but other vectors of the same family such as Reptalus panzeri or Pentastiridius leporinus also transmit it to corn and beet. The habitat of H. obsoletus consists mainly of wild plants such as bindweed or nettle. The disease is also transmissible by grafting infected plants onto healthy plants.

• Factors favorable to its development

Several parameters influence the development of black wood:
-the climate, which influences the winter survival of the larvae;
- 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 susceptible and disease.