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A Technical Guide for Wine Producers
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The Relationship Between Vine Training Practices And Grape Quality
It is a well-known fact that the quality of any bunch of grapes is directly related to the physiological quality of the shoot on which it hangs. This is the reason why grapes from short shoots (either naturally short or topped too short) as well as from exceptionally long shoots taste different and are often lighter in colour than grapes from average shoot lengths. Depending on the cultivar, there should be between 16 and 26 leaves on a shoot for the grapes to ripen optimally. The exact number of leaves is affected by the efficiency with which the leaves are exposed to sunlight, as well as by the cultivar. White cultivars are usually at the bottom of the range (16 to 20 leaves), while red cultivars are usually at the top of the range (20 to 26 leaves). Homogeneous, even shoot strength should therefore make an important contribution to the production of homogeneous, top quality grapes and this aspect was investigated in a preliminary study in the course of the 2000 harvest.
INVESTIGATIVE PROCEDURE
Three commercial vineyards were selected, viz. Cabernet Sauvignon (3,2 ha), Sauvignon blanc (7,5 ha) and Merlot (4,7 ha). The vines in these vineyards were all grafted onto Richter 99 and are situated in the vicinity of Stellenbosch. In each vineyard three categories of shoots were identified, viz. ± 60 cm, ± 120 cm and > 200 cm shoot lengths. Several of these categories occurred on the same vines. A day before harvesting, a random sample of 20 kg grapes of each category shoot length was taken and standard grape analysis procedures were used to determine the chemical composition of the grapes.
RESULTS
The grape analysis data for Cabernet Sauvignon, Merlot and Sauvignon blanc are indicated in Tables 1, 2 and 3 respectively.
TABLE 1: Grape composition of Cabernet Sauvignon/R99 as affected by different shoot lengths, Stellenbosch, 2000
Shoot
length (cm) Sugar
concentration (°B) Acid
concentration (g/l) pH Skin
colour (520 nm) ±
60 23,4 5,2 3,8 1,203 ±
120 24,5 7,4 3,3 2,761 >
200 21,9 8,9 3,2 1,078
TABLE 2: Grape composition of Merlot/R99 as affected by different shoot lengths, Stellenbosch, 2000
Shoot
length (cm) Sugar
concentration (°B) Acid
concentration (g/l) pH Skin
colour (520 nm) ±
60 23,6 4,2 4,1 1,341 ±
120 24,9 7,1 3,4 2,043 >
200 21,3 10,3 3,9 0,981
TABLE 3: Grape concentration of Sauvignon blanc/R99 as affected by different shoot lengths, Stellenbosch 2000
Shoot
length (cm) Sugar
concentration (°B) Acid
concentration (g/l) pH Fruitiness
on taste ±
60 20,5 4,0 3,8 None ±
120 24,3 8,6 3,3 Outspoken >
200 19,1 14,0 3,7 Traces
only
DISCUSSION
There are large differences in the chemical compositions of grapes from different shoot lengths. The shorter shoots produce grapes that may be described as overripe, while the long shoots produce unripe grapes. In the case of the Merlot (Table 2) and Sauvignon blanc (Table 3), the high pH of grapes from the long shoots is hard to explain, but it should be mentioned that these grapes were encountered on horizontally growing shoots inside the foliage, where overshadowing played a significant role. In the case of Cabernet Sauvignon, the long shoots were positioned upright. In all three cultivars it is obvious that the best quality grapes come from the average shoot lengths - in all three cases it provides a better basis for making the highest quality wines than any of the other shoot lengths (see Fig. 1). The skin colour in both Cabernet Sauvignon and Merlot was noticeably better at this shoot length, while the Sauvignon blanc grapes from this shoot length category displayed an outspoken fig flavour on the palate. In each vineyard a rough count of shoots was done to determine how many shoots altogether occurred in each length category. In the case of Cabernet Sauvignon, ± 30% of all shoots fell either in the too short or too long category, while the Merlot showed ± 45% and the Sauvignon blanc ± 40%. In other words, the quality of the crop was diluted by 30% in Cabernet Sauvignon, by 45% in Merlot and by 40% in Sauvignon blanc. If this dilution of quality could be eliminated from these vineyards, an improvement of almost 50% in wine quality might be obtained.
THE CAUSE OF UNEVEN SHOOT LENGTH
In the three vineyards investigated, the main cause of varying shoot lengths was poor training practices of young vines. Vines with lopsided trunks occurred everywhere. The trunks were trellised askew, either to the left or the right, resulting in several vines developing with unbalanced cordons. Numerous vines were characterised by a short cordon arm on the one side, with an accompanying long cordon arm on the other side. Short shoots predominated in the middle of the long cordon arm, while long shoots occurred at the ends of the short cordon arms as well as close to the split. In addition insufficient suckering left a lot of short sucker shoots with grapes, thus aggravating the problem of short shoots.
SUMMARY
Poor vine training practices, resulting in unbalanced cordon arms, induce uneven shoot lengths. This variation in shoot strength is in turn responsible for big quality losses in the South African wine industry.
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