In general hail induced damage – as associated with the potential and sporadic occurrence of hail in all South African winegrowing areas – is easily identifiable. Although grapevines are especially vulnerable to hail damage early in the growing season, serious damage may be wrought throughout the entire spectrum of phenological stages – in
which case the economic impact will obviously be determined by the intensity of the storms and the size of the hailstones (Photos 429 – 433).
Although damage due to lightning is not a common occurrence in vineyards, considerable damage may be inflicted on grapevines, especially in instances where trellis wires are directly hit. In affected grapevines, browning and drying out of leaves and shoots will take place from the tips, while bunches are obviously affected too – a situation which can often be seen in totality across individual rows (Photo 434). In instances where young, soft shoots touch wires directly early in the growing season, shoots will snap and dry out, including flower clusters in adjacent positions – this is very characteristic of the radiation effect that may be caused by lightning (Photo 435). On the other hand thickening may occur in positions where older, partially lignified shoots come into direct contact with the wire. This phenomenon is associated with the healing/filling of cracks by means of callus formation (Photo 436). In addition the drying out of pith sections in such shoots – thereby creating the impression that the inside is hollow – is considered very characteristic of lightning damage (Photo 437).
Young leaves and bunches display a particular sensitivity to sulphur burn in high temperature situations shortly after dusting, and extreme care should be taken when dusting sulphur is used to combat applicable diseases/pests. Young leaves burn from the edges mostly and a typical reddish-brown discolouration takes place, whereafter the leaves curl up and damaged parts harden and dry out (Photo 438). In the case of older leaves burn damage can also occur on sections between the main veins, in addition to the edges – in which case leaves appear normal except for dry, reddish-brown spots between the veins. In berries, sulphur burn is associated with the development of smooth, dark brown to black discolourations (within which cracks occur) – in which case the texture of burnt areas looks hard and may result in asymmetrical berry development (Photos 439 – 441).
Bunch stem necrosis
Characteristic abnormalities associated with this complex physiological phenomenon manifest as small, dark spots of dead cells that usually develop on the narrowest parts of pedicels (during advanced berry development), whereafter they expand to develop a continuous/ girdling band or ring – the consequence being that individual berries are eventually subjected to wilting, shrivelling and drying out. In the case of table grapes especially a similar situation is very characteristic, the difference being that the formation of the band is often restricted to positions where lateral branches develop out of the rachis – whereafter it expands to contribute to the browning and drying out of the affected branch in its entirety. Obviously the same cycle is followed, namely wilting, shrivelling and drying out of berries on such branches (Photos 442 & 443).
Growth arrestment phenomenon
Although this phenomenon is mainly associated with temporary shoot growth/arrestment in the period between flowering and bud burst (when flower clusters are choked off at a very early stage), comprehensive research investigations revealed that several and even divergent abnormality phenomena may occur, either individually or in combination. Such situations complicate concrete identification procedures, to the extent that it is considered absolutely essential to obtain expert advice at all times. From the spectrum of abnormality phenomena, the characteristic trait of GAP is a delay/arrestment in the elongating tempo of shoots that emanate from successful bud burst, to be followed by active growth later in the season, either without or with smaller bunches only. Such situations therefore indicate that inflorescences, although initiated, may have been choked/ damaged in dormant buds already (even before bud swell). In cases where flower clusters are affected at a later stage – but before calyptra drop – superficial black lesions appear where pedicels are later choked. Usually reduced shoot growth and shiny, dark green leaves may be observed in affected grapevines (Photo 444).
Globally considered, based on the considerable economic impact of harm caused by the phenomena in this category, which may occur in widely divergent format, either singularly or in combined situations – the value of possible preventative and/or sensible actions cannot be overemphasised. Obviously the implementation of specific actions
will be fully dependent on expert advice, also taking into account, inter alia, the complexity of possible cause(s), with the emphasis on bunch stem necrosis and GAP.
References/additional reading and viewing material
Ferreira, J.H.S. & Venter, E., 1996. Wingerdsiektes en Plae in Suid-Afrika. ARC Infruitec-Nietvoorbij Institute for Viticulture and Oenology, Private Bag X5026, Stellenbosch.
Myburgh, P.A., 2008. The contribution of atmospheric humidity in winter to yield fluctuations of Sultanina in the Lower Orange River region. Wynboer Technical Yearbook 2008/9, 38 – 41. Smit, C.J., 1981. Droogdruifverbouing in Suid-Afrika. In: Burger, J. & Deist, J. (eds). Wingerdbou in Suid-Afrika. Trio-Rand/SA Litho, N’dabeni. pp. 514
– 537. Van der Merwe, G.G., 1994. Pedicel girdling in table grapes. In: Proc. Int. Symp. Table Grape Production. Davis, California. pp. 27 – 29.
Van der Westhuizen, J.H., Saayman, D., Knight, F., Myburgh, P.A., Volschenk, C.G., Malan, D., Burnett, J.J. & Steenkamp, J., 2001. Groeistilstandverskynsel
en terugsterwing van wingerde in die Benede-Oranjeriviergebied. Sagtevrugteboer, September, 23 – 25. Zeeman, A.S. & Archer, E., 1981. Stokontwikkeling, Wintersnoei en Somerbehandeling. In: Burger, J. & Deist, J. (eds). Wingerdbou in Suid-Afrika. Trio-Rand/SA Litho, N’dabeni. pp. 202 – 233.
For further information contact Piet Goussard at firstname.lastname@example.org.