Crop thinning of merlot in Queensland – influence of timing of thinning on grape quality

Crop thinning of merlot in Queensland – influence of timing of thinning on grape quality

Crop thinning is a common practice in wine grape vineyards, believed to improve fruit quality (Jackson and Lombard, 1993). This trial was carried out in a Queensland vineyard in 2007 and 2008 in order to investigate seasonal variation on effect of crop thinning on Merlot fruit quality.
Methods and Results: In both seasons half the crop was removed from randomly allocated panels at véraison. Vine measures were recorded at véraison and fruit samples from all treatments analysed at harvest for various fruit quality parameters.
In 2007 fruit from crop thinned vines ripened more rapidly, attained a higher pH and was significantly higher in anthocyanin concentration than fruit from unthinned control vines. Crop thinning in 2008 had no influence on TSS however this fruit had significantly lower anthocyanin and phenolic concentrations compared to fruit from unthinned controls. Leaf area to yield ratio (LA/Y) was significantly increased by all crop thinning treatments resulting in thinned treatments having LA/Y values greater than those recommended as optimal (Dry et al. 2004).
Conclusions: Crop thinning in 2007 improved Merlot TSS and anthocyanin accumulation. In 2008 no significant difference in TSS was observed in fruit from crop thinned vines, however this fruit was significantly lower in anthocyanin and phenolic concentration. Other authors have shown seasonal influences to override the effects of crop thinning (Keller et al. 2005), a factor believed to also impact on this study. A cool growing season in 2008 may have caused vines to be unable to accumulate high levels of secondary metabolites, thus masking positive effects of crop thinning. Crop thinned vines being out of balance may also have impacted on results.
Significance of the Study: This study indicates that the common practice of crop thinning may be beneficial to fruit quality in some seasons, however seasonal factors play a significant role in ripening of quality grapes.
References
Jackson, D.I. and Lombard, P.B. (1993) Environmental and management practices affecting grape composition and wine quality – a review. American Journal of Enology and Viticulture 44, 409–430.
Dry, P.R., Iland, P.G. and Ristic, R. (2004) What is vine balance? Proceedings of the 12th Australian Wine Industry Technical Conference, 24–29 July 2004, Melbourne. Editors Blair, R., Williams, P. and Pretorius, S. Australian Wine Industry Technical Conference Inc., Adelaide, pp. 68–74.
Keller, M, Mills, L.J., Wample, R.L. and Spayd S.E. (2005) Cluster thinning effects on three deficit-irrigated Vitis vinifera cultivars. American Journal of Enology and Viticulture 56, 91–103.

Abstract of unrefereed poster paper published in Australian Journal of Grape and Wine Research, no. 16, 2010, Special Issue: Selected Papers and Abstracts from the 8th International Symposium on Grapevine Physiology and Biotechnology, 23–28 November, 2008, Adelaide, Australia.