Technical Yearbook 2024

restrictions highlighted the necessity for alternative sources of water for vineyard irrigation for the South African wine industry. Methods The field trial was carried out in full-bearing, commercial vineyards on a farm near Philadelphia in the Coastal region of the Western Cape from the 2006/07 until 2017/18 seasons. Three experiment sites were selected in different landscape positions. The first site was in a Sauvignon blanc vineyard on the shoulder of a hill. The second and third sites were in two Cabernet Sauvignon vineyards situated on a back- and a footslope, respectively. Details of the characteristics of the vineyards, irrigation treatments and application, and an assessment of the water quality and nutrient load were reported previously. 7,8,9 Grapevine water status and vegetative responses have also been reported. 10,11 At harvest, all bunches of the treatment plots were picked and weighed using a top loader mechanical balance. Grape mass per grapevine grapevines per treatment, which was then converted to yield (t/ha). Yield was determined from 2013/14 until 2017/18. The objective was to harvest the grapes at 24°B, but this was not always possible due to logistical constraints. In 2017/18, more detailed measurements were carried out to determine the effect of TMW irrigation on the various yield components. Ten randomly selected bunches were (kg/grapevine) was calculated by dividing the total grape mass per treatment by the number of

picked from each treatment plot and weighed using an electronic balance. A sample of 100 berries was obtained by picking 10 berries from each of the 10 bunches. The berry samples were weighed to determine the berry mass. The remaining bunches were picked by hand and counted using a mechanical counter to calculate the number of bunches per grapevine. A representative sample of bunches was selected from each plot at harvest. The grapes were gently crushed to extract juice from the berries. The juice was analysed for TSS, TTA and pH. Juice characteristics were only determined from 2013/14 until 2017/18. Data was statistically analysed as described previously. 10,11 Fisher’s least significant difference was calculated at the 5% level to compare treatment means. Results and discussion Over the 11-year period, irrigation with TMW increased yields significantly at all three experimental sites compared to the RF grapevines (Table 1). It must be noted that DLD only tended to increase yield compared to SLD, particularly at the back- and footslopes. The fact that twice the irrigation volume did not double the yield is important since it suggested that more water would probably cause over-irrigation. The latter can have negative effects on grape production, and more so where the irrigation water quality is questionable. Visual observation revealed the yield components generally responded positively to the TMW

irrigation (Figure 1). Results of the 2017/18 season showed that the increased bunch mass for the irrigated grapevines was associated with larger berries (Table 2). Furthermore, the SLD and DLD treatments tended to increase bunches per grapevine compared to the RF grapevines, except at the footslope (Table 2). The TMW irrigation increased bunch mass substantially at the shoulder and backslope sites compared to the RF plots. However, the additional irrigation water applied via the DLD did not result in a higher bunch mass compared to the SLD plots. In contrast, the bunch mass of the Cabernet Sauvignon grapes at the footslope increased with the amount of irrigation water applied (Figure 1B & Table 2). This could be explained by the amount of irrigation water applied at this site throughout the season 8 and the subsequent lower water constraints experienced at the DLD plot. 10 It was previously reported that the berry mass of Sauvignon blanc grapes was severely reduced when Ψ S became more negative. 12 Berries are most sensitive to water deficits during the beginning stages of berry development and berry size could be reduced. 13 Water constraints prior to véraison can result in smaller berries. High water constraints that were associated with low soil matric potential during the period from flowering to harvest reduced the berry size of Cabernet Sauvignon grapevines in the Swartland region. 15 In the Lower Olifants River region, berries of 0.78 g were measured for Cabernet Sauvignon grapevines growing on

TABLE 1. Yield of Sauvignon blanc grapevines on a shoulder and Cabernet Sauvignon grapevines on a backslope and footslope, respectively, under rainfed conditions (RF) and irrigated with treated municipal wastewater via single (SLD) or double dripper line (DLD). Data are means from 2006/07 until 2017/18. Cultivar Landscape position RF SLD DLD Yield (t/ha) Sauvignon blanc Shoulder 7.06 b (1) 12.34 a 12.48 a Cabernet Sauvignon Backslope 4.86 b 9.65 a 11.98 a Cabernet Sauvignon Footslope 6.63 b 11.11 a 14.30 a (1) Values designated by the same letters within a row do not differ significantly (p ≤ 0.05).

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TECHNICAL YEARBOOK 2024