Optimization Of Grapevine Leafroll Detection On White Cultivars By Sentinel Canes
ARC Plant Protection Research Institute University Of Pretoria. Faculty Of Natural And Agricultural Sciences. Department Of Microbiology And Plant Pathology
Objectives & Rationale
Effective control of leafroll in commercial vineyards of red cultivars relies very heavily on visual detection of the disease in individual vines in autumn, prior to roguing of those recorded as infected. To achieve the same level of control in white cultivars, which do not show obvious symptoms of leafroll, each vine must be tested annually for GLRaV-3 using a laboratory-based technique prior to roguing infected vines. This makes control of the disease on white cultivars extremely expensive and difficult to achieve with the large numbers of vineyards planted to white cultivars. We propose to evaluate the use of a bud of a red cultivar, grafted on individual white cultivar vine stems to serve as visual indicators of leafroll infection.
Using buds of red cultivars (Cabernet franc and Pinot noir) inoculated onto white cultivars we evaluate various graft mechanisms, inoculation by mealybug, different cultivars as indicators, recipient cultivar compatibilities, the effect of the rootstock and relative sensitivity of the indicator bud to detect GLRaV-3 compared to ELISA.
Sentinel canes have a sensitivity (ability to correctly test those vines with the disease), specificity (ability to correctly identify those vines without the disease), and accuracy (percentage of true positives and true negatives identified within the vine population) of 82%, 100% and 87% respectively relative to ELISA. As expected from such a high sensitivity and specificity very little differences were observed with regards the reliability of using either Pinot noir or Cabernet franc as sentinel canes, or whether the sentinel cane had been established through sandwich grafts, chip-budding or amphi-grafts. However, poor graft take using chip-budding would render this means of grafting unviable in commercial settings. While the sandwich grafts have a very good take and can be prepared in the nursery, they result in sentinel canes that are too low on the stem, are too vigorous and possibly also affect the vigour of the canopy. Amphi- grafts by contrast, are easy to perform and have an excellent rate of take, but would require in-field grafting, preferably within the first season while the stem is still a green shoot. To achieve this practically buds would have to be taken from thinner dormant canes. This modification was not tested. Due to the poor take of chip-budding the experiment to determine whether any genetic compatibility of Pinot noir and Cabernet franc existed when grafted onto the most prevalent white cultivars in South Africa did not yield useful information. As rootstocks do not display leafroll symptoms and GLRaV-3 is poorly detected in them, it is possible that rootstocks may affect the systemic spread of the virus in the vine itself. However very little difference was observed in detection of GLRaV-3 by sentinel canes relative to ELISA when the sentinel cane was grafted on the same side of the stem or on opposite sides to the cordon to the site of inoculation, suggesting that GLRaV-3 translocation through various rootstock crosses was not altered.
Key Conclusion of Discussion
The project suggests that sentinel canes are a feasible means of detecting GLRaV-3 infection in white cultivars and that it has potential for use in the certification scheme to identify infected vines for annual roguing. The technique also has potential use for commercial wine estates, serious about controlling leafroll disease.
Take Home message for Industry.
Sentinel canes are a viable alternative to the use of ELISA for the detection and roguing of white cultivar vines infected with GLRaV-3.
Pietersen, G, Bell, V A, Krüger, K. Management Of Grapevine Leafroll Disease And Associated Vectors In Vineyards, IN: Meng, B, Martelli, G P, Gollino, D A, Fuchs, M. (Eds). 2017. Grapevine Viruses: Molecular Biology, Diagnostics And Management. (P. 531-560) Springer,