A guide to grapevine abnormalities in South Africa: Abiotic abnormalities – hail, lightning, sulphur burn, bunch stem necrosis and growth arrestment (Part 6.3)

General and introductory Harmful occurrences such as hail, lightning, sulphur burn, bunch stem necrosis and growth arrestment phenomenon (hereafter GAP) – widely divergent as these may be – may be singled out as wellknown problem situations affecting grapevines in the South African wine and table grape industries. Unlike hail, lightning and sulphur burn, where visually noticeable abnormalities/damage in individual instances may be ascribed to specific causes, this is not the case with bunch stem necrosis and GAP – which occur mainly in table grapes, the latter almost exclusively restricted to the Lower Orange River area. Based on comprehensive research, investigations and observations over many years, it can be stated with certainty that both situations deserve acknowledgement as complex, physiological phenomena and specific/combined factors may be elucidated as important role players/drivers/causative agents (Van der Merwe, 1994; Van der Westhuizen et al., 2001; Myburgh, 2008; Laker & Conradie, unpublished; and others). Based on this research it has been possible to compile valuable, scientifically founded recommendations/guidelines, in order to reduce risks through improved management of problem situations such as bunch stem necrosis and GAP. Although above-ground abnormalities associated with the phenomena in question are sporadic occurrences, considerable economic damage/losses may be caused in instances of serious onslaughts. Consequently, it remains important here too to remain abreast of relevant identification procedures and it is recommended that expert  advice be sought to confirm a diagnosis. In addition it is strongly recommended that producers acquire a basic knowledge of contributing factors – either individually or in combined format – especially with regard to the incidence of bunch stem necrosis and GAP, namely: growth regulator imbalances; climate; nutrition; soil; rootstocks and scions, as well as vigour/crop load in the former; and dormancy physiology; climate; soil; irrigation; rootstocks and fungal diseases (early deleafing) in the latter. In this part of the series, particulars regarding the incidence of hail, lightning, sulphur burn, bunch stem necrosis and GAP are visually depicted and elucidated by means of brief captions.

PHOTO 429. An example of hail damage to this bunch of wine grapes in the Robertson area, which creates ideal ports of ingress for harmful organisms through induced wounds PHOTO 430. Serious hail damage to Sultanina grapevines in the Keimoes area, causing almost total deleafing, in conjunction with grotesque damage to bunches. PHOTO 431. A further example of the extent of hail damage as in Photo 430, to illustrate the coverage of the soil surface – practically overall – with carpet of leaves and bunches that had been knocked off. Also note the damage to the trunk sections only, as illustrated by the grapevine in the 10:00 position. PHOTO 432. A closer look at the hail damage illustrated in Photos 430 and 431. In addition to leaves and bunches, serious damage was simultaneously wrought to green, as well as partially lignified shoots. PHOTO 433. An example where lignified shoots (even with cordon and trunk sections) were so damaged by severe hail storms that sections of bark were knocked right off. In some cases such canes may die, while callus formation may sometimes occur around the damaged tissue. Obviously ideal ports of ingress are created for secondary wood rotting organisms – an aspect that emphasises the importance of timeous and correct actions/follow-up procedures PHOTO 434. An illustration of lightning damage representative of a situation where trellis wires are directly hit, resulting in the browning and drying out of leaves and shoots across the entire row. PHOTO 435. Very characteristic of lightning damage are instances where young, soft shoots come into direct contact with wires early in the growing season; these subsequently snap off and dry out, so too flower clusters in an adjacent position. PHOTO 436. In contrast with the example in Photo 435, thickening occurs in positions of direct wire contact with older, partially lignified shoots – a phenomenon associated with the healing/filling of cracks by means of callus formation. PHOTO 437. The drying out of pith sections of shoots damaged by lightning, thereby creating the impression that they are hollow inside, is considered very characteristic of this phenomenon. PHOTO 438. In reaction to sulphur burn in high temperature situations shortly after dusting, young leaves burn mostly from the edges, whereafter they undergo a typical reddish-brown discolouration followed by the curling up and hardening/drying out of damaged parts. PHOTO 439. In berries, sulphur burn is associated with the development of smooth, dark brown to black discolourations (within which cracks occur) – clearly illustrated by this example. PHOTO 440. A closer look at the cracks associated with dark brown discolourations (as in Photo 439) as a result of sulphur burn. Not to be confused with sunburn or heat damage. PHOTO 441. A further example of sulphur burn (as in Photos 439 and 440) occurring widely on berries and even pedicels. Not to be confused with sunburn or even anthracnose. PHOTO 442. A typical example of bunch stem necrosis in a bunch of table grapes, in which case the band formation occurs in positions where lateral branches develop out of the rachis – to be followed by browning and drying out, in conjunction with wilting, shrivelling and drying out of berries. PHOTO 443. An enlarged example of the situation seen in Photo 442. Note the typical band formation at the basal origin of the lateral branch in question, as well as in the pedicel in the 09:00 position. PHOTO 444. Typical of GAP is where flower clusters are choked off and dry out after the appearance of superficial, black lesions on the developing peduncles. Normally reduced shoot growth and shiny, dark green leaves will also be observed in affected grapevines

Hail
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).
Lightning
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).
Sulphur burn
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).
Conclusion
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 pgg@sun.ac.za.