Winetech Technical Yearbook 2022

RESULTS Sentinel canes have a sensitivity (ability to correctly test those with the disease), specificity (ability to correctly identify those without the disease), and accuracy (percentage of true positives and true negatives identified within the population) of 82%, 100% and 87% respectively relative to ELISA (table 1). As expected from such a high sensitivity and specificity, very little differences were observed regarding the reliability of using either Pinot noir or Cabernet franc as sen tinel canes, and either can be used (table 2). The sensitivity and specificity of the three means of grafting the sentinel cane is presented in table 3, with all three meth ods providing a very high correlation with ELISA test for GLRaV-3. Chip-budding, however, proved to be the method with the least take, and comparative numbers with the other grafting methods were only obtained, because in the experiments many more chip buds had been conducted than the other two methods. Chip-budding could only be conducted in the field in the second season of growth as the stem diameters of the white cultivars were too thin in the first season. When done in the second growth season, take on the stems was low with less than 50% of the grafts sprouting. All chip-bud grafts (n = 151) that had not taken by the second season were re-grafted in the third season, but very few took, and it is clear this method of grafting onto established vines is problem atic and probably not practical once vine apical dominance has been established. Sandwich-grafting in the nursery was a very efficient and reliable means of grafting the sentinel cane with 80 of 105 (76,19%) successful graft take across all scion cultivars and rootstocks and with both Pinot noir and Cabernet franc utilised (figure 9). The position of the sentinel cane in the fully grown vine, however, was too close to the ground. It may be worth assessing whether longer inter stocks of the sentinel cultivar can be used to overcome this. The sentinel cane shoot derived from the sandwich grafts were also relatively vigorous and required considerable post-graft management. Amphi-grafts also yield very high take of the bud, but can only be performed while the stem is still not lignified. This would have to be done in the first or second season of growth in the vineyard, or stems would have to be cut back and allowed to re-sprout otherwise. This method of grafting of the sentinel cane once vines are already established is preferred to chip-budding. A comparison with the detection of GLRaV-3 in the upper canopy and sentinel canes (obtained through any graftingmeans) either on the same side of the stem as the inoculation site in the cordon or on the opposite site is provided below (table 4).

FIGURE 5. Three methods utilised for the grafting of the red cultivar sentinel buds onto the white cultivar stems. A) Chip-budding, B) omega sandwich graft of stem, and C) amphi-grafting (dormant winter bud grafted onto green stem).

FIGURE 6. Inoculation of white wine cultivar scion with grapevine leafroll-associated virus 3 (GLRaV-3) using A) amphi-grafting of GLRaV-3 infected stem pieces, or B) of Planococcus ficus mealybugs feed on GLRaV-3 infected vines and placed on the healthy plant in insect cages.

Sentinel cane:

Pinot noir Cabernet franc

Recipient vine:

Chardonnay Semillion Sauvignon blanc Chenin blanc Colombar

FIGURE 7. Chip-budding of either Pinot noir or Cabernet franc sentinel canes on various white wines to assess the graft compatibility and usefulness of these two cultivars as sentinel canes.

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WINETECH TECHNICAL YEARBOOK 2022