Phytoplasmas are microscopic plant pathogens similar to bacteria but much smaller and lacking cell walls. They live in the vascular system of plants and are spread by sap-feeding insects, including leafhoppers and planthoppers. There are several diseases of grapevine caused by phytoplasmas:
Flavescence dorée is the most important and destructive phytoplasma disease of grapevines and has been called “catastrophic” in France. It is known to occur only in France, Italy and Spain.
Impacts include reduced vitality of vines, yield reductions and reduced wine quality due to high acid and low sugar contents of fruit from infected plants. Without control measures, the disease spreads rapidly, affecting up to 80-100% of vines within a few years.
Flavescence dorée has destroyed large viticultural areas in France and Corsica. It is still spreading despite mandatory control programs. Popular cultivars such as Chardonnay, Cabernet Sauvignon, Pinot Noir, Riesling, Sauvignon Blanc and Sémillon are highly susceptible to flavescence dorée. Some cultivars such as Sangiovese and Garganega are extremely susceptible and are killed quickly.
In France, flavescence dorée is a regulated disease. Compulsory control measures for both the disease and the vector were strengthened in 2003. When an infected grapevine is found, a quarantine area is delimited around it for a minimum of two years. All infected plants must be destroyed, and control of the vector with several insecticide sprays is required in that zone.
Bois noir occurs in France and Germany but is less serious due to a slower rate of spread. Diseases similar to bois noir have also been reported in Switzerland, Italy, Greece, Hungary and Israel and are suspected to occur in Chile and South Africa. Bois noir was detected in B.C. and Ontario in plants imported from France. The plants were removed, and subsequent surveys have not detected new infections.
Phytoplasma diseases of grape elsewhere in the world are referred to as grapevine yellows, including an American grapevine yellows that occurs in New York and Virginia and an Australian grapevine yellows.
All grapevine yellows diseases have similar symptoms, including growth reduction, leaf discolouration, downward rolling of leaves and reduced quality and quantity of fruit. Symptoms are not uniform and may appear on some or all shoots of infected vines.
A few rootstock varieties are tolerant to flavescence dorée and can be symptomless carriers of the disease.
Leaf symptoms in white cultivars appear as small, yellow spots along the main veins. These spots enlarge to form yellow bands along the veins, which gradually extend over large parts of the leaf. Red cultivars develop a similar pattern on the leaves, but the discolorations are reddish.
Infected shoots often fail to lignify and appear thin and rubbery. They later become brittle, sometimes with bud necrosis.
Affected branches blacken and die during the winter. Numerous small black pustules form along the diseased branches of susceptible cultivars.
Fruit set is reduced on grapevines infected early in the season as the inflorescences dry out and fall off. In later infections, bunches become brown and shriveled. Premature berry drop occurs in some cultivars.
Flavescence dorée is spread by the leafhopper Scaphoideus titanus, which is native to eastern North America. It is known to occur in Canada from the Maritimes through to Saskatchewan, but it has not yet been detected in B.C.
S. titanus spends its whole life cycle on grapevines. It is a highly mobile and efficient vector that is largely responsible for the epidemic spread of flavescence dorée. Both nymphs and adults are able to acquire the phytoplasma while feeding. After a latent period, they are able to transmit the disease until they die. This leafhopper overwinters as eggs which are inserted (laid) into the bark of grapevines.
Bois noir is transmitted by a few planthopper vectors, the main one being Hyalesthes obsoletus. This insect is not known to be in Canada. It prefers herbaceous weeds over grapevines, and transmits the phytoplasma from these weeds to grapevines.
Bindweed (Convulvulus arvensis), nettle (Urtica dioica) and hedge bindweed (Calystegia sepium) are the main reservoirs in Europe. Since the vector only feeds incidentally on grape, bois noir is a much slower spreading disease than flavescence dorée. It is widespread in many parts of Europe. The recent identification of new bois noir vectors in Europe raises concerns regarding related insect species in Canada.
Both phytoplasmas and leafhoppers can be spread with propagation material. Flavescence dorée is symptomless in some cultivars, and it also has a long (up to three year) latent period before symptoms can be seen. Thus, even apparently healthy vines may be carrying grapevine yellows diseases. A few rootstock varieties are symptomless carriers that have been responsible for spread of flavescence dorée in Europe.
Control of phytoplasma diseases involves scouting and removing infected or symptomatic vines, as well as controlling the vector with insecticide sprays.
Use only planting material from reliable, approved sources and produced in areas where phytoplasma diseases are not present.
Canada has grapevine import regulations that help to protect grape growers from the introduction and spread of diseases and pests that are not established in B.C. Selected grapevine variety and rootstock clones from Canadian-approved nurseries in France and Germany are currently approved for importation into Canada.
Under this program, nursery plants are propagated from mother plants that were tested for regulated pests and diseases by the exporting country, then grown under conditions to reduce the risk of contamination. CFIA has tested samples of many of these grapevine varieties or rootstocks prior to approving their importation.
In addition, all plants or propagative material imported from France or Germany must be treated in a hot water bath at a minimum of 50°C for a minimum of 35 minutes.
Stricter import requirements for grapevines from France and Germany were implemented by the Canadian Food Inspection Agency (CFIA) in 2007 due to concerns about the increased incidence of flavescence dorée and bois noir in France. Under those requirements, plants and propagative material originating from, or propagated within, flavescence dorée areas of France — or any other infested area of a country where the disease occurs — are prohibited.
