Symptoms associated with aster yellows (AY) vary during the season and between cultivars.
AY is caused by a phytoplasma, Candidatus Phytoplasma asteris. Phytoplasmas are very primitive bacteria without cell walls, which occur in the vascular tissue of the plant. The phytoplasma causes a decline in vine growth and enormous crop losses in some cultivars. The disease is spread by an indigenous leafhopper.
Symptoms appear during summer, but infected vines can be identified from spring onwards by delayed bud break and reduced growth. Usually a combination of symptoms will develop on plants.
Early in the season, affected leaves have a wrinkled appearance (Photo 1). These leaves gradually become crisp and thicker than normal leaves, and roll downwards. Some leaves exhibit a general yellowing/reddening and turn to a golden yellow in white cultivars (Photo 2) or red in red cultivars (Photo 3). Pinotage sometimes shows a sectorial discoloration of leaves (Photo 4). Leaf rolling can result in a typical triangular shape, as recorded on Chardonnay (Photo 5). Symptoms of AY infection can be localised on a few shoots or on one cordon of the vine. In other cases, the entire grapevine can be affected. These affected shoots are thin and rubbery, can have shortened internodes with a zigzag growth pattern and later show partial or total lack of lignification (Photo 6). Affected shoots display tip death, followed by node-by-node dieback of the shoots (Photo 7). Affected shoots fruit set is reduced, as some bunches dry out and fall off early in the season. Later in the season, fully developed bunches become dry and shrivelled before the berries can ripen (Photo 8). Symptom expression differs from season to season, which can be ascribed to climatic differences between the seasons.
PHOTO 1. Leaves of infected vines have a wrinkled appearance early in the season.
PHOTO 2. Leaves of infected white cultivars turn to bronze yellow and roll downwards.
PHOTO 3. Infected vines of a red cultivar show red discoloration of leaves.
PHOTO 4. Sectorial reddening of leaves is a typical symptom of phytoplasma infection on some red cultivars.
PHOTO 5. Chardonnay shoots show a typical grey waxy substance and leaves have a triangular shape.
PHOTO 6. Thin and stunted shoots do not lignify properly.
PHOTO 7. Growth tip death followed by die back of shoot node-by-node.
PHOTO 8. Infected bunches shrivel and dry out before ripening.
Incidence and spread of AY
A survey conducted in the Vredendal area identified AY symptoms in a variety of wine grape cultivars, including Shiraz, Sauvignon blanc, Colombar, Cabernet franc, Chardonnay, Chenin blanc and Pinotage.
Chardonnay vines infected with AY declined rapidly and eventually died. This indicates that Chardonnay is very sensitive to AY infection. Cultivars, such as Colombar, were not affected so severely and only showed a few affected bunches and yellow leaves without much effect on vine growth and yield.
Detection of AY in grapevines can be tricky. AY was not detected in all symptomatic vines, which indicates uneven distribution of the AY phytoplasma in vines. On the other hand, some vines without symptoms tested positive for AY, indicating latent infections.
The incidence of AY was studied in two Chardonnay, two Chenin blanc and three Pinotage vineyards over four years. Disease incidence varied between cultivars and between the different sites, but there was no statistically significant difference in disease incidence between the three cultivars. The overall incidence of AY in all vineyards increased from a mean yearly disease incidence of 7.24 – 19.47% over the four-year survey period. The study also showed that a vineyard with a sensitive cultivar, such as Chardonnay, could be 100% infected with AY in a couple of years. Diseased vines in each vineyard mostly occurred in clusters on the side of the vineyards adjacent to infected vineyards and the disease did not spread randomly through the vineyard. This is typical of disease transmission by an insect vector.
The high incidence and rapid progression of AY disease in some vineyards indicate a need for control of the disease in the form of an integrated management strategy. This includes the use of disease-free planting material, chemical control of the insect vector and sanitation practices, such as the removal of infected young vines in newly planted vineyards to reduce disease inoculum. Weed control is important, as the weeds can serve as an overwintering host for the leafhopper vector or as a source of the phytoplasma.
Chardonnay is very sensitive to AY infection, showing rapid decline and death of vines. The disease does not spread randomly, but from cluster of infected vines on the side of the vineyard adjacent to an infected vineyard. This is typical of disease transmission by an insect vector. An integrated management strategy to control AY includes the use of disease-free planting material, chemical control of the insect vector, sanitation practices, such as the removal of infected young vines in newly planted vineyards, and weed control.
This research was funded by Winetech, the ARC and Technology and Human Resources for Industry Programme (THRIP) – project WW 06/40.
– For more information, contact Roleen Carstens at email@example.com.