South Africa Wine Technical Yearbook 2025

OCTOBER

Sunburn in the Paarl and Swartland regions By Hanno van Schalkwyk

were relatively mild, with values below 35°C from 1 to 24 December. There was a sudden steep increase on 25 December, however, with the mercury rising to above 40°C. The increase was so sudden that the vineyards simply were not accustomed to the unexpected heat. Leaves were scorched (Photo 1), and significant grape losses were also observed (Photo 2). The grapevine produces various compounds which help to protect the leaves and grapes against sunburn. For instance, the concentration of certain flavanols in grape skins increases with more sunlight exposure. This is also the reason why canopy management actions should be performed at an early stage. Suckering should be completed before flowering, and if defoliation is required, it should be done right after set. Grapes are particularly susceptible to sunburn before véraison, and unlike most years, this heat spike occurred early enough, when virtually no grapes had yet entered véraison. Anthocyanins, which are responsible for the red colour of grapes, help to absorb ultraviolet rays and thus reduce the risk of sunburn.

The 2025 vintage will be remembered for the mild weather conditions during harvest, which resulted in even ripening and excellent grape and wine quality. There was, however, an outlier – a sweltering hot day on 25 December. Producers in the Paarl/Swartland are familiar with sunburn, but the timing of this particular event and preceding weather conditions led to significant losses in tonnage. Temperatures in excess of 40°C were measured at weather stations, but temperatures in the microclimate zones of the grapes are often even higher due to inter alia radiation from the soil surface. The stomata of leaves close at temperatures above 35 to 40°C, especially during conditions of intense sunlight radiation (afternoon sun), as well as low groundwater levels, which cause water stress in the vines. When the stomata close, the rate of transpiration (movement of water vapour through the stomata) decreases, and the plant’s ability to cool down is compromised. Overheating of the leaves and berries therefore occurs, resulting in subsequent sunburn damage to the leaf and berry tissues. As shown in Figure 1, the daily maximum temperatures

Daily max temperatures December 2024: Swartland

45

40

35

30

25

20

15

10

5

0

1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031

Degrees Celsius

FIGURE 1. Daily maximum temperatures for December 2024 in the Swartland region.

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TECHNICAL YEARBOOK 2025