Regulating sugar accumulation through canopy management
Abstract
Leaf removal was investigated as a means of slowing the accumulation of sugars into grapes. This was undertaken in response to warmer seasons where grapes were reaching the optimum sugar maturity three to four weeks earlier than average and during a hotter part of the season.
Summary
Changes in climatic conditions, principally increased temperatures, have resulted in significant increases in the rate of sugar accumulation into the berries. This has led to increased wine alcohol concentrations and unbalanced wines that have elicited negative responses from some consumers and wine judges. The Bendigo Winegrowers Association obtained Wine Australia RITA funding to examine aspects of premature ripening. Part of the project looked at slowing down ripening in a field demonstration and the other part provided information back to the growers through seminars and industry journal articles.
In the first season in 2008-09 the field demonstration involved removing leaves to slow down sugar accumulation in the berries. The first year of the trial showed that there were no differences in ripening between the timing of leaf removal either at the end of rapid shoot growth or just before veraison. The degree of leaf removal had an impact on ripening with 50% leaf removal delaying ripening by around 7 days and 75% leaf removal producing a delay of up to 29 days to reach a target sugar concentration of 25° Brix (13.9 Baume). In the berries, leaf removal produced lower berry weights, higher phenolic and tannin concentrations and lower anthocyanin concentrations compared with control vines. These results and other related topics were discussed in a seminar in November 2009.
In the second season in 2009-10 only one leaf removal treatment was tested in the field (75% leaf removal just prior to veraison) and sufficient grapes were produced to harvest at three sugar concentrations to make small batches of wine. Leaf removal delayed ripening by 20 to 42 days depending on the target sugar concentration, although in the 2009-10 season substantial rainfall occurred from March onwards and this contributed to delaying sugar accumulation. Harvesting at successively higher sugar concentrations produced lower weight berries, lower titratable acidity and higher pH in the berries, along with greater colour density, total phenolics and tannin concentrations in the wines.