Aspects of Vitis vinifera L. cv Pinot Noir fruitfulness in Tasmania

The issue of yield variability and the physiological factors that determine its severity and patterns of year to year fluctuation remain much of a mystery in the cool climate wine-producing region of Tasmania. Research into causes and management has generally focused on the influence of one specific factor, or issues within any one growing season, with little attention to the full cropping cycle. Yields of both Pinot Noir and Chardonnay over an eleven-year period were analysed for correlations between vineyards and for consistent responses to seasonal weather. Fluctuations in yield across the industry showed no consistent patterns, possibly because available data corresponded with a period of rapid growth and change throughout the region. However, yields from two vineyards with established and uniform vine age and management, were correlated with each other and with weather events corresponding with previously identified periods of sensitivity in the reproductive growth cycle. At one site a multiple regression model of yield against mean maximum daily temperature and mean sunshine hours at flowering accounted for 90% of the variation in yield. At the second site, sunshine hours and rainfall at flowering accounted for 76%, with sunshine hours alone accounting for 63%. Due to the effects of weather at flowering on yield and uncertainty surrounding the method of pollen transfer and cross and self-pollination in Vitis vinifera, a detailed study into the mode of pollination in Pinot Noir was conducted. The stigmas of both open and closed flowers were examined using scanning electron microscopy and fluorescence microscopy for evidence of anthesis and pollen tube growth, and pollen viability tests were carried out on pollen from both open and closed flowers. Pollen was shown to be present and viable on stigmas prior to cap fall, however pollen tubes did not begin to grow until after cap fall. Bagging of inflorescences throughout flowering in field vines, found that fruitset was not related to cross-pollination. In view of the effect of cap fall on pollen tube growth, the common commercial practice of fungicide application during flowering was investigated. Dilutions of commercial formulations of three fungicides were added to liquid germination media and pollen germination rates were measured. One of the fungicides was also used in a field study where vines were sprayed during flowering at 5% and 80% cap fall. All three fungicides prevented pollen germination at the recommended concentration for field sprays in the in vitro experiment, with pollen tube growth resuming at greater dilutions. In the field, percent fruitset was not affected by fungicide application at either stage of flowering, however average seed number at harvest was affected by fungicide application with the 5% cap fall treatment having a significantly lower seed number. As recent research suggests that bunch number is the yield component that contributes most to yield variation in cool climate wine producing areas, one option to manage yields from vines subject to variable spring weather, is to dormant prune to a yield target above the desired level, then summer prune or bunch thin to achieve the desired yield after fruit set. Different bud numbers were imposed on field grown vines over three seasons, varying from canopies of ten to forty buds. Yield components were measured at each harvest. Vines with smaller bud numbers had greater over-wintering starch levels, fruit had a lower pH and anthocyanin content, but there was no effect on sugar content. In a second field trial with the same pruning treatments, bunches were removed (thinned) at veraison, to create fruit loads equivalent to industry levels for small, medium and high yields. There was a reduction in berry number per bunch at the heaviest fruit load, but no treatment effects on bunch weight or berry weight. In both field trials bunch number per bud decreased with increasing number of buds retained in winter pruning, and berry number per bunch also decreased. Yield in the previous season also influenced bunches per bud in the current season. Results are discussed in terms of the influence on yield, of carbohydrate reserves and current photosynthesis in the period from bud burst to flower initiation, and on the potential to manage yield using bunch thinning or shoot thinning of vines dormant pruned to bud numbers locally regarded as overcropping. In particular the view that bunch number is the primary yield component is questioned. The results lead to a conclusion that bunch size is the main factor influencing yield variability, and contrary to most published literature bunch size, rather than yield, may be an important influence on wine quality.