Winetech Technical Yearbook 2022

MAY

Optimisation of grapevine leafroll detection by sentinel canes IN THIS PROJECT WE EVALUATED THE USE OF A BUD OF A RED CULTIVAR, GRAFTED ON INDIVIDUAL WHITE CULTIVAR VINE STEMS TO SERVE AS VISUAL INDICATORS OF LEAFROLL INFECTION IN THE WHITE CULTIVAR.

BY GERHARD PIETERSEN

INTRODUCTION To control leafroll in vineyards, it is important to plant certified leafroll-free vines, to control mealybug numbers and their spread, and to annually rogue infected plants. When applied stringently, these strategies have been found to be very effective in reducing the number of leafroll infected vines to very low levels (figure 1) and thereby prolonging the economic lifespan of the vineyard. Identifying infected vines in relatively healthy vineyards of most red wine cultivars is easy as symptoms are obvious in late summer and autumn allowing these infected vines to be marked annually for removal in winter (figure 2). For at least four of these cultivars (Cabernet Sauvignon, Shiraz, Merlot and Cabernet franc), the visual detection of the disease has been shown to correlate extremely well with ELISA tests for grapevine leafroll associated virus 3 (GLRaV-3), the major cause of leafroll in South Africa. White cultivars do not show clear leafroll symptoms (figure 3) and to control the disease by roguing, laboratory-based tests must be done annually for GLRaV-3. 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. In this project we evaluated the use of a bud of a red cultivar, grafted on individual white cultivar vine stems to serve as visual indicators of leafroll infection in the white cultivar. Should the vine become infected with GLRaV-3, the disease will manifest itself in the so-called “sentinel cane” (the red-grafted shoot) of the vine, making visual assessment for the disease possible (figure 4). The sentinel cane need only be grafted onto the vine once in its lifetime and can

FIGURE 1. View of red-cultivar vineyards in autumn on a wine estate where leafroll control is applied stringently. Notice the absence of any vines displaying leafroll symptoms.

FIGURE 2. Obvious symptoms of leafroll infection in autumn within a red cultivar vineyard where leafroll control is stringently applied. Infected vine marked with white PVA paint for removal (roguing) in winter.

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WINETECH TECHNICAL YEARBOOK 2022