A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

VIRUS AND VIRUS-LIKE DISEASES Leafroll 4.2

Spread The occurrence of leafroll in newly established production blocks can be singled out as a direct result of reinfection of virus-eliminated material. Often the subsequent spread within specific blocks does not follow a set pattern and apart from the direct involvement of acknowledged vectors – the best known and most dominant of these being mealybug – the tempo is moreover determined by various factors. In this regard comprehensive investigations/research by Pietersen (2004, 2010) enabled the quantification of the most important spread patterns (Photos 188 - 193). This made it possible to draw up guidelines for application by the industry to delay, or even arrest the spread of leafroll. In addition, thanks to wide-ranging research on the grapevine virus complex (see Part 4.1 of this series), it has been possible to draw up specific guidelines and protocols with regard to the management of leafroll vineyards and the control/eradication of mealybug (Photos 194 & 195), resulting in particularly valuable applications for the industry, especially when integrated with biological control (Carstens, 2001; ARC Infruitec-Nietvoorbij, 2004/5; Addison et al ., 2011 and others). Morphological changes/reactions Although leafroll is associated, over an ongoing period of time, with smaller grapevines, shorter shoots, delayed bud burst, smaller leaves, later leaf drop, as well as smaller root systems (Photo 196), leaf discolourations and downward curling of leaf edges – which appear annually in midsummer/early autumn – may be singled out as the most characteristic, visually perceptible morphological reactions in infected material. Obviously such discolourations/warping patterns do not take place overnight, with the result that the full intensity thereof is visually best observed during late autumn/early winter. In addition to the downward curling of leaf edges such situations in red grape cultivars reflect intravenous reddening and in white grape cultivars intravenous yellowing of leaf tissue – in both instances, with normal, green-coloured primary and secondary veins (including a narrow strip of surrounding tissue which remains green) (Photos 197 - 202). Furthermore and typical of leafroll is the occurrence of smaller, fewer and looser bunches with the accompanying sub-optimal berry colouration, delayed ripening and lower sugar concentrations – a situation that is aggravated in due course and eventually necessitates premature uprootings (due to economic implications) (Photos 203 - 206).

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