South Africa Wine Technical Yearbook 2025

PHOTO 5. An early topping action on this bush vine led to branch shoot development, which protected the grapes against sunburn.

Bushvine and sprawl system vineyards In these vineyards, early topping actions and subsequent branch shoot development made a huge difference (Photo 5). Where shoots were not topped, they fell open and substantial sunburn damage was observed (Photo 6). PHOTO 6. Severe sunburn damage in sprawl systems where shoots fell open. Early topping actions and branch shoot development provided better protection against sunburn.

This can be attributed to a long morning and afternoon sun side. This led to longer exposure to direct sunlight on exposed bunches. Temperatures were already very high early on 25 December, which caused damage on both the morning and afternoon sun sides. Row orientations between east-west and southeast-northwest had significantly less sunburn damage.

Summary The sudden heat spike on 25 December 2024 caused widespread sunburn and tonnage losses in the Paarl and Swartland areas. The unexpected increase in temperature and the fact that grapes had not yet entered véraison, exacerbated the effect of this event. Instances of vineyards with relatively less damage were, however, observed. Vineyards with buffered root systems, which had higher groundwater contents at the onset of the extreme heat, showed less damage. It should also be noted that dryland vineyards with mulches handled the heat better. Hedge trellises with row orientations between east-west and southeast-northwest also offered greater shelter from the sun and had less damage. The early topping of bushvines and sprawl systems, and the subsequent development of branch shoots, resulted in better protection of the grapes. Other canopy management actions, such as suckering and defoliation, if applied, should be done early to allow the vine sufficient opportunity to form the necessary compounds to offer protection against sunburn. Grapes that are ‘opened up’ at a late stage in the growing season are extremely susceptible to sunburn damage. In sunburn-prone areas, practices like the suckering of bush vines can be adjusted by, for example, leaving three instead of two shoots per bearer for better shelter and protection. As climate change and extreme weather conditions advance, the danger of sunburn damage may increase. It could therefore be valuable to investigate the use of agents, such as those containing kaolin, to help manage this risk. 

For more information, contact Hanno van Schalkwyk at hanno@vinpro.co.za. Reference https://www.wineland.co.za/sunburn-in-the-paarl-and-swartland-regions/

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TECHNICAL YEARBOOK 2025