The Vintage thus Far
Michelle Moyer, Viticulture Extension Specialist
Sometimes average is good, especially when it comes to weather. Coming out of two vintages noted by below average temperatures, it is a welcome reprieve that 2012 is heading down the middle of the road, on par with long-term average temperature accumulation (Fig. 1). As a result, much of the Valley is at mid-véraison, tracking towards a “normal”, or slightly advanced, harvest date.
Precipitation, however, is another beast (Fig. 2). While seasonal accumulation is above normal, most of this occurred in early June, and the lack of significant rainfall the last couple of weeks, coupled with routine temperatures above 90ºF, may be placing some vineyards under drought stress, particularly those practicing deficit irrigation. Reference evapotranspiration, a calculation of the amount of water that is lost through a plant, has been above normal for August (Fig. 2), highlighting this potential for water stress.
When vines are under drought stress, they will produce abscisic acid (ABA), which tells stomata (plant pores involved in transpiration) to close, which helps reduce plant water loss. Unfortunately, this water loss also helps cool plant tissue, and when stomata close in hot, sunny days, tissue can super heat. Photosynthesis and many metabolic processes involved with berry ripening are temperature sensitive, and will shut down when temperatures reach extremes (both hot and cold).
While we associate higher temperatures with advanced ripening, when these high temperatures are coupled with severe water stress, ripening will actually slow down or stop. Just be aware that watering decisions should be coupled with both visual and measured signs of plant water stress.
Water Stress and Atypical Aging Flavor Defect
Thomas Henick-Kling, WSU V&E Program Director
Water stress has been linked to Atypical Aging (ATA) in white wine. Wines with the ATA defect lose their varietal flavors quickly, sometimes within the first year, and atypical flavors such as candle wax, Linden tree blossoms, dusty roadside, furniture varnish, and dirty dish rag will appear. The wines will also develop a metallic, bitter taste. Research done in Europe and work I did at Cornell University has linked drought stress to the onset of ATA. Particularly, the time period immediately surround véraison (about 2 weeks before and after) appears to be the most critical; however, the exact period is not known. Also, we do not know exactly, how severe the water stress needs to be, nor how long it needs to last.
We know that water stress close to the point where photosynthesis shuts down (stem water potential below -1 MPa), for a period of about 1 week, can cause ATA. Of course, when photosynthesis shuts down or very nearly shuts down, flavor development also stops. With the recent hot temperatures, and our rapid advancement through véraison, keep an eye on vine water status.
A more in-depth look at ATA and how to manage it in the winery will be presented in the Fall issue of the Viticulture and Enology Extension Newsletter (VEEN).
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