Like table grape growers, wine grape growers can take advantage of the effects of gibberellic acid (GA3) – or ProGibb®. But with this crop, the fruit is not directly affected. Wine grape growers use ProGibb early in the season to make the rachis longer. In doing so, they elongate the clusters and improve air flow to help avoid diseases and problems associated with tightly bunched clusters such as bunch rot, powdery mildew, etc.
This approach is not new. Decades ago, University of California researchers sprayed Zinfandel vines with gibberellin at pre-bloom stage, and found that the clusters were loosened and the prevalence of rot sharply reduced by the sprays. They also found that optimum concentrations of gibberellin caused little or no reduction in yield of fruit. In addition, spraying two successive years produced no adverse effects.
In recent years, researchers have learned that if applications are put on late, close to bloom, the berries get stunted. The problem stems from the fact that wine grapes have seeds, and seeds naturally produce their own gibberellin. If the application is put on late, close to bloom, too much gibberellin is present and the berries do not develop normally; forming “shot berries.” Close to the size of peas, shot berries are very hard and don’t produce juice. Compounding the problem, because of the excessive gibberellins, there will be fewer clusters the following year (refer to the label for specific rate and timing instructions for each variety).
Application timing is critical, with the ProGibb application made at least 3 to 4 weeks before bloom. When the berries mature, the clusters are looser because they were stretched with ProGibb prior to bloom. If done properly, a grower can significantly reduce the amount of fungicides needed to produce a good crop.