Link: A guide to grapevine abnormalities in South Africa: Abiotic abnormalities – wind, heat, sunburn and frost damage (Part 6.1) | Wineland Magazine
By: PIET GOUSSARD

General and introductory

Abiotic abnormalities are associated with damage caused by environmental conditions. Harm may be inflicted in varying formats and levels of intensity – as reflected by the fact that serious conditions may result in permanent damage, or even the destruction of grapevines, while a lower profile impact will obviously be caused by more moderate conditions. Seeing that certain abnormalities in this category may resemble the damage caused by diseases, pests and even physiological problem situations – and furthermore contribute, to a greater or lesser extent, in almost all instances, to economic losses over the short and/or longer term – it remains important here too to remain abreast of identification procedures and it is recommended that expert advice be sought in confirmation. This may enable interested parties in the industry to not only act timeously and effectively in terms of control and/or implementation of remedial strategies, but also to apply and maintain possible preventative measures – based on specific guidelines proposed by expert advisors – on an on-going basis. In this part of the series, particulars about the incidence of the most common abiotic abnormalities are visually portrayed and elucidated by brief captions.

Wind damage

Depending on its speed/force, wind is considered to be one of the most important and best-known environmental conditions in terms of harmful impact on grapevines. In extreme instances leaves, shoots, berries and bunches are torn off by gusts and gales, while situations have occurred where serious/destructive damage has been caused to grapevines and even trellis systems (Photos 391 – 393). Although grapevines are susceptible to wind damage throughout the growing season and individual cultivars display varying degrees of susceptibility, grapevines are especially vulnerable during specific phenological stages and organs may be damaged to the extent that normal physiological activities are hampered – especially in areas where strong, but also more moderate prevailing winds occur. Typical examples include chafing of shoots, petioles and peduncles against foliage wires – in which case vascular tissue is damaged to the extent that it is unable to engage in optimal translocation processes (Photos 394 – 397). In addition wind during the flowering period may also contribute, inter alia, to poor(er) set of berries (Photo 398), while wilting and desiccation of young shoot tips on hot days is no exceptional phenomenon – especially in instances where soil water is limited. Furthermore indirect wind damage, as related to possible mealybug and leafroll distribution, should never be disregarded, but considered a high priority throughout.

Heat damage

Although heat and sunburn damage may occur jointly, the former is usually associated with situations where grapevine issue/organs are damaged by overheating without direct exposure to the sun. Typicalof this is the wilting and drying out of actively growing shoot tips either during heatwave periods, or simply in instances where sudden increases in temperature occur after relatively cool spring conditions – a phenomenon that is obviously exacerbated by dry winds. Apart from inter alia Muscat d’Alexandrie, where older leaves are often subjected to the development of uneven, yellow to brown heat spots (Photo 399), the incidence of heat damage/spots is especially noticeable in bunches – the berries of which are afflicted to varying degrees at any stage shortly before ripening. In such instances (due to overheating of fruit tissue) characteristic indentations are caused – in due course the tissue turns brown from the inside – on berry surfaces, resulting in misshapen development and eventual drying out thereof (Photos 400 & 401). Shrivelling and eventual drying out of berries in reaction to the browning/drying out of sections of the peduncle/ framework may be singled out as an additional characteristic example of heat damage – a phenomenon to which individual, separate groups or even all berries on a bunch may be subjected (Photo 402).

Sunburn damage

Sunburn by itself is usually associated with situations where serious damage may be inflicted on bunches of susceptible cultivars especially as a result of direct exposure. Abnormality phenomena associated with this are largely very characteristic and mostly related to wilting, shrivelling and eventual desiccation of berries on one or more parts of the bunch (Photo 403).

PHOTO 403. Severe sunburn damage in this bunch as related to wilting, shrivelling and eventual desiccation of berries due to direct exposure.

In addition light brown to orange-yellow discolouration in individual berries – which may be accompanied by varying degrees of shrivelling – may be considered characteristic of exposure to direct sunrays on very hot days (Photos 404 & 405).

PHOTO 405. As in Photo 404 this example is typical of situations where individual berries suffer from direct exposure to sunrays.

A common occurrence in bush vines is to fall apart in the centre, causing ripening grapes to be severely exposed to sunburn – a similar situation may occur when shoots of trellised grapevines hang free (Photo 406).

PHOTO 406. Severe sunburn damage in Muscat d’Alexandrie as a result of shoots hanging free from the trellis, thereby causing the bunches to be exposed to direct sun.

Although leaves may be affected/damaged by sunburn to a lesser degree, they are not immune and when affected, display characteristic traits. In typical examples one notices the emergence of pale brown spots between veins, encircled by a dark brown line – thus creating a distinct separation between the brown and green parts (Photos 407 & 408).

PHOTO 408. A further example of sunburn damage to leaves. In this specific case wind was a further factor contributing to the emergence of pale brown spots.

Frost damage

Although grapevines are usually not subject to frost during the dormant period (June to August), trunks, cordons and one-year-old canes may in extreme cases be affected to the extent that rifts/cracks develop – a situation that, in addition to creating convenient entry points for crowngall bacteria inter alia, may even contribute to dieback phenomena in grapevines. In contrast to the above, the incidence of spring frost is extremely harmful in September and October, when sensitive green shoots, flower clusters and even buds that are just starting to burst, may be destroyed (Photos 409 & 410).

Conclusion

Based on the considerable economic impact of damage that may be caused by the above-mentioned phenomena, the value of preventative, as well as remedial measures – based on comprehensive expert knowledge/advice – cannot be overemphasised. Obviously such guidelines – in addition to sensible terrain and cultivar choices – will play an important role when applying the total spectrum of practices in respect of sustainable/profitable viticulture in the various regions.

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.
Joubert, J., 2012. Heat damage to young grapevines. Wynboer Technical
Yearbook 2012, 151 (in WineLand, October 2012).
Marais, P.G., 1981. Wingerdsiektes en Abnormaliteite. In: Burger, J. & Deist,
J. (eds). Wingerdbou in Suid-Afrika. Trio-Rand/SA Litho, N’dabeni. pp. 384
– 432.

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