Twenty winegrape varieties are planted for viticultural evaluation at the Kearney Research and Extension Center (KREC). Degree days, calculated from March 15 thru October 15 using a lower threshold of 50° F, regularly reach 4500 placing it in region V on the Winkler scale. Many the varieties are being evaluated in California for the first time. We believe that the world holds many winegrape varieties that are capable of making valuable contributions to the California wine industry but are untested in California. Identifying these varieties is dependent on continued research.
Data was first collected from this trial in 2006. During that season several deficiencies in the trial design were identified. It was apparent that modifications to the trellis and irrigation schedule were required. These modifications were made prior to the 2007 season. As a consequence of these changes it was determined that data collected in 2006 would not be included in mean calculations made over years. The 2008 season therefore, provides the first opportunity to present both the current year’s data and average data from multiple years.
In the 2008 season, as in previous years, all varieties were pruned to 15 shoots per meter and excess shoots were removed. No attempt was made to adjust cluster size or clusters per shoot. Measurements of leaf water potential made across varieties were employed to schedule irrigation frequency and volume. The protocol called for all varieties to be harvested at 24º Brix.
In 2008, harvest occurred from September 9 to October 15, a 36 day period. Mean harvest Brix across all varieties was 23.9. Ten varieties were harvested within 0.5 Brix of 24 Brix. Three of five varieties harvested more than 0.5 below 24 Brix were harvested on the last day and may be considered over-cropped. Yield ranged from 6.5 to 19.0 kg per vine, Carmenere and Cinsaut respectively. Cluster weight was the yield parameter most correlated to both yield and cluster rot. Cluster rot was greatest for Cinsaut where almost 15%of the crop was lost to rot.
From a winemaker’s perspective, winegrapes grown in warm regions often have the undesirable characteristics of high pH and low acid at harvest. In 2008 half of the varieties had pH values above 3.75 and another half had titratable acidity below 6.0 g per L. Among the varieties in this trial Tannat stood out as a variety with high acid and low pH. Juice of Tannat at 25.3 Brix had titratable acidity of 6.8 g per L and 3.46 pH in 2008.
Careful attention to plant water status by Larry Williams provided the data necessary for him to properly manage vineyard irrigation. In addition to managing irrigation plant water status measurements are revealing varieties that may be more efficient in their water use. In particular Tinta Francisca has higher leaf water potential than the other varieties following the same level of irrigation.
Interest in this project resulted in a collaboration with Constellation Brands, Inc. Constellation agreed to make and analyze wine from nine of the 20 varieties in the plot. The results will help us more thoroughly describe these varieties.