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grapegrowing In addition, parts of a vine treated with a spray application of gibberellin tend to accumulate sugars at higher than normal levels. Along with cytokinins, another group of plant hormones, gibberellin influences flowering in grapevines; external applications during flowering induce floral abscission and/or increased shot berries.13 Foliar applications near or during flowering may reduce bud fruitfulness and fruit yield the following year by encouraging formation of tendrils and inhibiting the formation of inflorescences. Short history: gibberellin for bunch rot control During the late 1950s and early 1960s, foliar applications of gibberellin to elongate the clusters of rot-prone grape varieties were studied in France, Germany, New York and California.29,21 University of California (UC) research indicated the best timing for cluster elongation, especially lengthening of the pedicels of seeded winegrape varieties, coincides with a cluster length of 3 to 4 inches. (At this time, the total range of cluster lengths may vary between 2 and 5 inches.) During the same period, shoots are between 15 and 20 inches long. These developmental stages occur about two to three weeks prior to bloom. UC research also showed that, as with all bunch rot control methods, application of gibberellin provides benefits only during years of high disease incidence. In this regard, it is a preventive treatment. Based on these research findings, many California wine grape vineyards were treated with gibberellin for bunch rot control. Widespread application, however, ceased around 1977 due to gibberellin misuse by a few growers, associated yield loss, litigation and the manufacturer rescinding the label. Nevertheless, some vineyard owners in the Sacramento River Delta acquired Special Local Need Registrations and have treated Chenin Blanc, Petite Sirah and Zinfandel with gibberellin since the 1960s without ill effect. During the 1980s and early 1990s, Special Local Need Registrations also were issued for vineyards in San Joaquin, Stanislaus and Merced counties. Around 2002 a gibberellin manufacturer reinstated a label for winegrapes, and during the past decade the number of treated acres in the Delta and Lodi has steadily increased. Scheduling foliar gibberellin applications Experience has shown the degree of elongation of the main cluster (rachis) stem—rather than cluster length or shoot length—is a more precise determinant for scheduling gibberellin application.2,3 Deciding when to apply gibberellin requires that growers focus on the lateral stems, which are attached along the length of the central or main stem of the cluster. The lateral stems have flowers attached to them and they appear as clumps of flowers at this time of year. Table I. Bunch rot susceptibility of selected wine grapes Negligibly Susceptible Alicante Bouschet, Muscat of Alexandra, Petit Verdot, Rubired, Ruby Cabernet, Tempranillo, Thompson Seedless Slightly Susceptible Cabernet Franc, Cabernet Sauvignon, Merlot, Mourvedre, Muscat Blanc, Muscat Hamburg, Roussanne, Sylvaner, Syrah, Viognier Moderately Susceptible Barbera, Burger, Carignane, Colombard, Grenache, Malbec, Malvasia Bianca, Meunier, Sangiovese, Semillon, Trousseau Gris (Grey Riesling) Highly Susceptible Chardonnay, Chenin Blanc, Gewurztraminer, Petite Sirah (Durif), Pinot Blanc, Pinot Grigio (Gris), Pinot Noir, Riesling, Sauvignon Blanc, Zinfandel Sources: Marois, et. al; Vail and Marois; Grant (personal experience) As the main stem begins to elongate, the lateral branches near the base of the cluster begin to separate from one another and, as they do, the main cluster stem becomes clearly visible between them (Figure 2). These events mark the onset of the period of rapid cluster elongation, during which the clusters are receiving an influx of gibberellin from inside the vine. During the next few days after the onset of cluster elongation, the cluster tip will continue to distance itself from the cluster base as the main stem of the cluster continues to elongate. During the same time, more of the main cluster stem becomes exposed (Figure 3). (Shoots are 72 p racti c al w i ne ry & v i ne yard MAY 20 13