In western Oregon the titratable acidity of grapes at harvest may in some seasons be higher than desirable for making quality wine, due to the retention of malic acid. The purposes of this study were 1) to investigate the effects of a vineyard cultural practice, cluster exposure at veraison by basal leaf removal, as a means of reducing the malate content and titratable acidity of grapes, and 2) to develop a rapid, simple, and inexpensive test procedure by which smaller wineries and vineyards could evaluate the effects of their own field experiments on the malate and tartrate content of their grapes. 1. At veraison, clusters of Chardonnay grapes were a) exposed to the sun by removal of all leaves opposite or below the clusters, b) treated as in a) but shaded with shadecloth, c) exposed to the sun by tying back leaves opposite or below the clusters, or d) left untreated as a control. Clusters of White Riesling were exposed to the sun by similar leaf removal a) 10 days before veraison, b) 10 days after veraison, or c) untreated. Clusters of Pinot Noir were exposed a) at veraison, b) 2 weeks after veraison, or c) untreated. Exposed clusters received 3 to 3.5 times more light than shaded clusters and up to 32% more heat, with temperature differences between exposed and shaded treatments being most pronounced during cool, sunny weather. None of the treatments had any effect on juice or berry malate, tartrate, or potassium content; however, exposed clusters of Pinot Noir had a lower pH (.03) and higher titratable acidity (.06%) than the control at harvest. Cluster exposure of Chardonnay increased sunburning of grapes, and cluster exposure of Pinot Noir at veraison caused a 1% reduction in juice soluble solids concentration at harvest. The detrimental effects of cluster exposure by basal leaf removal at veraison, as well as the lack of any major effect on the acid content of the berries, suggest that the practice has no value for acid reduction during a warm, dry maturation season in western Oregon. 2. A rapid, simple procedure for the estimation of the malate and tartrate content of grape juice is described. The procedure, which requires only a pH meter for instrumentation, does not directly measure malate and tartrate but instead measures their buffering effect. Samples are titrated between pH 2.70-3.00 and pH 4.50-4.80 and the titrant volumes required are compared to two sets of empirically derived standard curves. The malate and tartrate composition of the sample may be determined by a graphical or algebraic method. The use of the estimation method, its advantages, and its limitations are illustrated with different viticultural trials. The estimation error (estimated value - measured value) was influenced by many factors including maturity, season, vineyard location, and cultivar. Standard deviations of the estimation error for malate and tartrate in mature grapes were equal to 9% and 15%, respectively, of the mean malate and tartrate concentrations in pooled Pinot Noir and Chardonnay samples from different vineyards and years. The estimation error is probably due to interference from other buffers present in juice. Although not as accurate as existing analytical methods, the estimation method appears potentially useful for determining relative effects of treatments in vineyard trials where analytical equipment is unavailable or for monitoring malate decline during maturation of grapes. / Graduation date: 1987

Vineyard management strategies, including vineyard floor management and crop level management, can be used to influence vine vigor and fruit composition. Two studies were conducted to evaluate the impact of these practices on Pinot Noir in Oregon's Willamette Valley. Managing crop levels is common in cool climate vineyard production though it is a costly practice. With economic pressures, the premium winegrape industry is questioning whether they can reduce production costs and increase yields without compromising quality. A crop thinning trial was conducted in 2010 and 2011 to address these concerns and to better understand the role of vine balance on fruit composition. Crop levels were moderately (35% crop removed) or severely (65% crop removed) thinned at pre-bloom, fruit set, lag phase, or véraison and compared to full crop treatments. In both years, crop thinning reduced yields but had no effect on berry weight or cluster size. In 2010, poor fruit set reduced overall yields, and thinning treatments resulted in very few differences in vine growth, cluster architecture or fruit composition, including total soluble solids (TSS), pH, titratable acidity (TA), yeast assimilable nitrogen (YAN), anthocyanins, phenolics and tannins. In 2011, yields were much higher due to high fruit set and larger cluster size. No differences were found in vine growth (leaf areas or pruning weights) or fruit YAN, but thinned vines had higher TSS and pH and lower TA than full crop vines at harvest. Fruit thinned at lag phase and véraison had higher TSS and lower TA than fruit thinned pre-bloom. Intensity of thinning had a stronger influence on anthocyanin and tannin concentration than timing, while phenolics were not impacted by either factor. Ravaz index values (fruit yield/pruning weight) below 2.25 and leaf area to yield ratios of 2.25 to 3.25 m²/kg improved fruit composition in 2011 as did later season thinning, though data from the remaining years of this study will provide more insight into appropriate crop load metrics for cool climate Pinot Noir. A second study was implemented in 2011 to determine the impact of crop thinning in vines with different levels of vegetative vigor caused by three vineyard floor management techniques: permanent grass (Festuca rubra spp. rubra) cover (grass), alternating grass cover and tillage (grass & tilled), and tillage of every alleyway (tilled). Crop was thinned at the BB stage of berry development (EL stage 73) to one cluster per shoot (half crop) or not thinned (full crop); all cluster wings were removed at the time of thinning. Tillage treatments had been in place four years prior to the start of the study and competition for nitrogen in grass caused reduced early season vine growth, leaf chlorophyll and canopy size at both bloom and véraison while crop thinning increased canopy size at véraison. Yields were altered by tillage and crop thinning treatments, as grass had fewer clusters per shoot and berries per cluster, and crop thinning reduced yields to 64.7% of full crop across all tillage treatments. At harvest, grass had the lowest TA while TSS and pH were not affected by tillage. Crop thinning increased TSS but did not impact pH or TA. Anthocyanins were affected by both tillage and thinning and were found to be related to vine yield, YAN, leaf N, and leaf area index. Tannins were highest in grass but were not affected by crop thinning, and phenolics were not changed by either factor. Few interactions between tillage and crop thinning were found, but as variables such as yield per vine were impacted by both treatment factors, monitoring long term effects of crop / Graduation date: 2012

Graduation date: 2004