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56 p r a c t i c a l w i n e r y & v i n e ya r d a p r i l 2 0 1 4 g r a p e g r o w i n g T E C H N I C A L R E S O U R C E F O R G R O W E R S & W I N E R I E S 58 Why 'per acre' spray dose calibration no longer applies By Larry Whitted 59 Improving management of grape powdery mildew with new tools and knowledge By Walter Mahaffee, Seth Schwebs, Francesca Hand, Doug Gubler, Brian Baily and Rob Stoll 69 Helpful guide to train tasting room staff, sales representatives By Marnie Old practicalwinerylibrary.com Access Practical Winery & Vineyard article archives online. SPRING SAP FLOW S cant rainfall during winter or early spring can result in stunted shoots and poor fruit set in grapevines. Crop losses of up to 50% have been observed in some vineyards after a dry winter. However, when vine growth in a vine- yard that did not receive pre-bud break irrigation in 2005, after an unseasonably dry dormant period, was compared to a nearby vineyard that did, Markus Keller, professor of viticulture at Washington State University, noticed that the irrigated vineyard had more even bud break and superior shoot growth; clearly the effects of a dry dormant period could be miti- gated with irrigation. "Positive root pressure" in grapevines results from remobilized, stored nutrient reserves that are released into the water- conducting tissue of the vine (xylem), according to Keller. This raises the osmotic pressure in the xylem, causing the roots to draw more water from the soil, thus pushing the subsequent water and nutrient reserve solution (sap) to the canes. That pressure needs to be released somewhere, and it is typically released in the form of sap bleeding from pruning wounds (Figure 1). If the soil is too dry, root pressure may not develop; as a result, much needed water is not avail- able to growing tissues. Are there alternative, natural routes that this pressure can be released, such as through the breaking of buds and the growth of shoots? In other words, is sap flow related to bud break timing and speed? Starting in 2011, Keller, along with graduate student Colin Lee, began stud- ies using potted Vitis vinifera Merlot vines to see if there was a relationship between soil moisture content and subsequent plant development. Hemant Gohil, WSU-IAREC, Prosser, Wash. BY Role of soil moisture at bud break Photo BY michelle moYer Figure 1: Sap commonly bleeds through prun- ing wounds in the early spring and is a sign of adequate vineyard soil moisture content. This study continued in an expanded form in 2012, conducted by Giulio Car- massi, a visiting student from Italy, incor- porating different soil types that were representative of eastern Washington. These included a sandy loam from Prosser, Wash., and loamy sand from Pat- erson, Wash. Going into 2013, the trial was expanded to include a gradient of different soil mois- ture content in the two soil types. The gra- Photo BY hemAnt gohil Figure 2: Sap was collected from vines going through pre-bud break to bud break stages. Don Neel, Editor