Grapevine Yellows: Flavescence dorée and Bois noir - Identification Guide

Background

Grapevine yellows diseases caused by phytoplasmas are found in many areas of the world where grapes are grown. Flavescence dorée (FD) and Bois noir (BN) are two grapevine yellows diseases that are considered quarantine pests in Canada because they can affect crop yields of affected grapevines. There are no human health risks associated with these pests.

BN and FD are commonly found in several European countries. The first detection of one of these pests in Canada was the confirmation of BN in British Columbia during the Canadian Food Inspection Agency's 2006 routine surveillance program.

Varietal Susceptibility

All Vitis vinifera varieties are susceptible to phytoplasmas. Some of the most important varieties such as Chardonnay, Pinot blanc, Pinot noir, Cabernet Sauvignon, Sauvignon blanc, Pinot gris and Sémillon are very sensitive, whereas Merlot appears to be relatively tolerant. Plants infected with FD and BN may recover, but symptoms may reoccur, sometimes years later. Vitis labrusca and American rootstocks develop very weak symptoms or no symptoms at all. However, if infected, they may act as phytoplasma reservoirs and facilitate disease spread.

Symptoms

Grapevine yellows diseases all produce similar symptoms and many plant parts are affected including fruit, growing points, leaves and canes. Plant growth may be stunted. Grafting is often not successful and young grafted plants often die quickly. Usually a combination of symptoms will develop on plants.

Disease Progress:

Symptoms may first appear in early summer and increase in incidence and severity until harvest. Early season symptoms can be observed on the flowers and as undeveloped flower/fruit bunches. Young shoots may have closely spaced branches and zig-zag growth (Figure 2) with some early leaf rolling. Subsequently, berries will shrivel as peduncles dry. Leaf symptoms become more pronounced as the summer progresses, followed by poor wood ripening that gives the vines a rubbery, "weeping" appearance. Symptoms on newly infected plants may be restricted to a few shoots with more of the vine becoming diseased with time.

Foliar Symptoms:
(Figure 1, 4)

  • Leaves are discoloured, edges roll downward, leaf blade becomes brittle.
  • Leaf rolling often results in an angular shape.
  • Spots form on sun-exposed surfaces of the leaves, which enlarge into bands along the veins. These will gradually cover the entire leaf surface.
  • Colour of the spots and bands varies with the cultivar: white-fruited. varieties usually have creamy-yellow spots and red-fruited cultivars have reddish spots.
  • Leaves may overlap each other down a cane.

Flower and Fruit Symptoms:
(Figure 5)

  • Flowers dry out and fruit will not set on initially infected plants.
  • Bunches on plants that become infected later in the season will become brown and shrivelled.
  • Fruit will drop very easily on some varieties.

Cane Symptoms:
(Figure 3)

  • Canes ripen poorly and unevenly, often showing green and brown sections along the cane.
  • Canes with poorly ripened wood will appear rubbery, often causing a "weeping" appearance.
  • Poorly ripened wood will often blacken and die during the winter.
  • Numerous small pustules may appear along diseased branches and longitudinal fissures may appear in the bark at the base of badly diseased branches.
  • Symptoms may be confused with Botrytis cinerea on canes and stems.

Comparison of Phytoplasmas with Other Grapevine Diseases

Leaf-roll Diseases: Phytoplasma symptoms may be confused with grapevine leaf-roll disease. Grapevine leaf-roll disease symptoms usually start on older leaves and develop up the cane. Phytoplasma diseases are often first seen nearer the top of the plant. Discoloration due to leaf-roll starts between the veins and the veins usually remain green. Also, grapevine leaf-roll disease does not affect cane ripening and fruit bunches do not shrivel.

Esca Disease: Withering of the bunches and leaf reddening may be confused with Esca disease. However in the case of Esca, the stock will collapse suddenly in July or August. Esca is caused by a complex of several fungi including Phellinus igniarius and Stereum hirsutum.

Photographs:
Dr. Michael Maixner
Federal Biological Centre for Agriculture and Forestry (BBA)
Institute for Plant Protection in Viticulture
Bernkastel-Kues, Germany

Examples of FD and BN Symptoms on Riesling

Figure 1: Foliar symptoms

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Examples of Flavescence dorée and Bois noir Symptoms on Riesling - Foliar symptoms

Figure 2: Early season short internodes

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Examples of Flavescence dorée and Bois noir Symptoms on Riesling - Early season short internodes

Figure 3: Uneven wood ripening

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Examples of Flavescence dorée and Bois noir Symptoms on Riesling - Uneven wood ripening

Examples of FD and BN Symptoms on Pinot Noir

Figure 4: Foliar symptoms

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Examples of Flavescence dorée and Bois noir Symptoms on Pinot Noir - Foliar symptoms

Figure 5: Affected fruit

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Examples of Flavescence dorée and Bois noir Symptoms on Pinot Noir - Affected fruit

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