Winetech Technical Yearbook 2022

be pruned back annually. This would cir cumvent the need to do laboratory tests yearly and makes roguing of infected vines cheaper and more practical. We foresee that this technique could be used primarily in white cultivar foundation and moth er-blocks of the One Vitis certification scheme, but also at growers who wish to achieve low or zero leafroll infections on their estates and therefore also control leafroll in the white cultivars on the estate. METHODS In a field trial conducted from 2015 to 2019, we tested the relative sensitivity of Pinot noir or Cabernet franc as sentinel cane indicators on various white cultivars (mainly Chenin blanc) to detect leafroll compared to ELISA detection of GLRaV-3 in green-berried vines. We tested grafting of the sentinel cane bud onto the stem through three grafting techniques (chip budding, omega sandwich graft and amphi-grafting (figure 5). We tested the ability of the sentinel cane to detect grapevine leafroll-associated virus (GLRaV-3), the most prevalent cause of leafroll disease in South Africa, by artificially inoculating the sentinel cane grafted vines with GLRaV-3 through either graft inoculation of GLRaV-3 infected stem pieces (figure 6A), or by placing mealybugs which had been feeding on GLRaV-3 infected vines onto leaves of the sentinel cane grafted vines (figure 6B). Using only chip-budding, we attempted to test the graft compatibility of Pinot noir or Cabernet franc onto the most common wine grape cultivars, as well as their rel

ative usefulness to manifest GLRaV-3 symptoms as sentinel canes (figure 7). We also tested the placement of the sentinel cane relative to the GLRaV-3

inoculation site on the ability of the sentinel cane to detect GLRaV-3, and the potential effect of the systemic spread through various rootstocks (figure 8).

FIGURE 3. Leafroll symptoms in autumn on Chardonnay, one of the few white cultivars that shows some leafroll symptoms, albeit these are only expressed following a relatively long latent phase in which no symptoms are observed. Some cultivars, such as Sauvignon blanc, never show any obvious symptoms when infected by grapevine leafroll-associated virus 3 (GLRav-3) infection, but are negatively affected by the disease.

FIGURE 4. Use of a sentinel cane to express leafroll disease on a white-wine cultivar. Berries retained on the sentinel cane are purely for illustrative value and would have been removed in practice.