Zinfandel is California’s winegrape variety and for over a century, Zinfandel has been at the heart of the Lodi, Sierra Foothill, and eastern Contra Costa County wine industries. Over the years, this versatile variety has lent itself to various grape and wine products to accommodate changing market conditions. A the same time, Zinfandel has made highly appreciated, darkly colored and full-bodied red wines with intense fruity flavors and aromas that are often reminiscent of raspberry and other bramble fruits (fig. 1). Yet, in spite of extensive experience with this variety, we are still learning how to best manage Zinfandel for red wine production. In recent years, with the conversion of many vineyards from grape production for white wines to production for red wines, such knowledge has grown in importance.
One distinctive characteristic of Zinfandel is its capacity to produce very large fruit yields, sometimes in excess of what vines can fully ripen. This trait, which has served the growers of white Zinfandel well, is a challenge to red wine grape production. Very large crops create internal competition for vine resources, which slows ripening and diminishes the intensity of some compounds that impart color, flavor, and aroma. For Zinfandel, this situation is serious due its propensity for variability in berry development within clusters and associated uneven ripening (fig. 2). This variability complicates basic crop management activities, including yield estimation and fruit sampling for harvest scheduling.
Here is a crop management strategy to enhance berry uniformity for red Zinfandel production. First, set the stage for uniform fruit development by promoting fruit bud differentiation and storage of nutrient reserves in woody tissues during the previous growing season. Specific management activities to achieve these goals vary with vineyard location, so they are not listed here. However, in all instances, they ought to result in the following effects. Moderate shoot growth vigor, balanced growth of crop and leaf area, canopies with most leaves positioned on the exterior and exposed to sunlight, basal sections of canes exposed to diffuse sunlight, and root zones with abundant organic matter, a balanced supply of mineral nutrients, and available moisture that prevents severe stress in vines.
Second, engage in management activities that directly enhance berry uniformity within Zinfandel clusters, which include the following. Prior to bloom, thin clumps of clusters. Thinning in this way will decrease early season competition for internal vine resources. Then, immediately before bloom, apply fertilizer nutrients involved in pollen viability and fruit set to vine foliage. These include phosphorus as phosphate, boron, zinc, and molybdenum. As soon as possible after fruit set, initiate regulated deficit irrigation (RDI) and continue it through the ripening period to advance fruit development. Also immediately after fruit set, thin any remaining clumps of clusters and any external clusters shading internal clusters to improve fruit uniformity. Depending on location and vineyard design, these targeted crop load adjustments usually result in yields between 4 to 8 tons per acre for bilateral cordon trained, single canopy vines.
Zinfandel is prone to two mineral nutrient deficiencies that can compromise efforts to enhance fruit uniformity and quality. These are deficiencies of magnesium and boron, and they are common in Northern Interior Zinfandel vineyards. Magnesium deficiencies usually become evident after fruit set as a creamy yellowing (chlorosis) on the edges of the leaves where chlorophyll is lacking (fig. 3). Under these conditions, photosynthesis is reduced and fruit ripening inhibited.
As mentioned above, boron is critical for normal fruit set, but it is also required for the synthesis of red pigments (anthocyanins) and other phenolic compounds that affect red wine aroma, flavor, and mouthfeel. Lignin, the substance of wood, is also a phenolic compound and boron deficiency in Zinfandel can compromise cane ripening and winter hardiness. In addition, boron is a cofactor for the mechanisms involved in nitrate uptake by roots and boron deficiencies can induce nitrogen deficiencies. A mottled creamy chlorosis, which in Zinfandel is often accompanied with reddening mid season, is the leaf symptom of boron deficiency (fig. 4).
Harvest scheduling is our final consideration regarding red Zinfandel production. Due to the inherit variability of berries within Zinfandel clusters, harvest ought to be scheduled when soluble solids are little on the high side (> 25 degrees Brix) to ensure the harvested fruit meets winery specifications (fig. 5). At the same time, avoid extending ripening much beyond these soluble solids levels to minimize yield loss from fruit desiccation and bunch rot.
Actually, season-long efforts to manage bunch rot are critical components of any management strategy to maximize the quality of red Zinfandel grapes. These may include dormant season sanitation, cluster elongation with gibb, fungicides, shoot thinning, leaf removal, and careful irrigation scheduling to limit cluster compactness and fruit zone humidity. For new Zinfandel vineyards, clonal selection is a key component of bunch rot management. In this regard, the Primitivo-03 selection is much less prone to bunch rot than most other Zinfandel selections.
This article was originally published in the Mid Valley Agricultural Services December 2010 newsletter.
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