WINETECH Technical Yearbook 2020

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.

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.

Affected shoots’ fruit set is reduced, as some bunches dry out and fall off early in the season. Later in the season, fully devel- oped 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. INCIDENCE AND SPREAD OF AY A survey conducted in the Vredendal area identified AY symptoms in a variety of

Detection of AY in grapevines can be tricky. AYwas 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

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