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

4.4

VIRUS AND VIRUS-LIKE DISEASES Shiraz disease

Spread Although acknowledged vectors such as mealybug (as with leafroll) are also involved in transmission and spread (Goszczynski & Jooste, 2003), the situation differs radically in Shiraz disease, where the spreading pattern and tempo are characteristic and unique (Engelbrecht & Kasdorf, 1990). Typical of this – as could be witnessed in a Merlot noir/ Richter 99-block (established as certified material) on Welgevallen experimental farm – was a sudden and unpredictable targeting of three adjacent grapevines in a particular row (Photo 224). Unlike leafroll, subsequent spread did not occur within the same row or across rows, but Shiraz disease gradually attacked adjacent grapevines (in groups of three to four, but no more than 10) in a widespread trend within the block. Despite the fact that these infected grapevines died prematurely and were not removed from the vineyard, it was not possible to detect any visual spreading of Shiraz disease to specifically adjacent grapevines within the same row(s) up to and including the eventual uprooting of the block (due to leafroll). Morphological reactions/changes Characteristic of grapevines associated with Shiraz disease (where the infection is visually detectable) is the floppy appearance which, combined with decreased vigour, makes the grapevines appear noticeably smaller (shorter shoots with fewer and smaller leaves) (Photos 225 & 226). The most obvious characteristic of such grapevines is the occurrence of green, rubbery shoots that are not subjected to normal lignification – resulting in a pendant format (Photos 227 & 228). In typical cases these shoots have numerous lenticels (from the bases); in most instances these occur in close proximity, which creates the impression that they run into each other – thereby showing close affinity which longitudinal cracks (Photo 229). As the season progresses, lignification is completely amiss in some canes – whereafter they change in colour from green to almost black, followed by dieback (as a result of winter chill). A characteristic of infected grapevines is regrowth (at the tips of unlignified, green shoots) at an advanced stage of leaf drop (Photos 230 & 231). In addition to the smaller format, leaves are subjected to typical discolouration patterns (from the edges to overall red/reddish brown discolouration) (Photo 232), while delayed leaf drop – as late as mid-July – is considered another characteristic trait of Shiraz disease (Photo 233). The incidence of smaller bunches with berry sizes ranging from small, green and underdeveloped to practically normal – as a result of inter alia poor set and small, poorly developed flower clusters (Photo 234) – can be considered one of the most important contributing factors, in combination with the absence of bunches on water shoots (which occur in reaction to the dieback of allocated bearers), to drastic crop reductions in infected grapevines (Photo 235).

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