JULY 23, 2018. BY STAN GRANT.
Wine grapes with certain defects are undesirable for winemaking. These defects fall into two principal categories – compositional and physical. Only a few compositional flaws are discernable in wine grapes while still on the vine, such as a preponderance of vegetative flavors, excessive acidity, and sweetness outside of the range desired for a particular wine style. Physical imperfections, on the other hand, are easy to see in the vineyard. They are the focus of this article.
Most wineries include provisions in their grape contracts that allow them to downgrade or reject fruit with certain physical defects. Probably the most frequently listed defects are bleaching, sunburn, and fungal diseases. These flaws are associated with diminished wine quality through affects on color, flavor, and aroma. Other physical defects, such as shriveling and raisining, are less often included in winery contracts. Still, they are of concern because they impact vineyard revenues through reduced fruit yields. Fortunately, careful vineyard management can minimize most physical defects.
Powdery mildew and bunch rots are the most common fungal diseases affecting grape clusters in California (Figure 1). Even small percentages of berries infected with these diseases can negatively impact the sensory characteristics of wines. Accordingly, the tolerance for diseased fruit in grape loads is low.
Fortunately, there are highly effective integrated management methods for controlling powdery mildew and bunch rot. These include the combined use of cultural and chemical techniques. Vine and vineyard sanitation, shoot thinning, fruit zone leaf removal, altered irrigation schedules, gibberellin, and fungicides are among them.
Exercise care when using sulfur, the most commonly applied fungicide for powdery mildew control. It can cause dark scarring on the sun exposed portions of berries if applied immediately before or during periods of hot temperatures (Figure 2). Also, when applied too close to harvest, sulfur residues on crushed grapes may impart rotten egg (hydrogen sulfide) aromas to wines.
Sun Affected Fruit
Halos, bleaching, sunburn, and raisining represent degrees of berry damage due to oxidative stresses associated with high temperatures and high light intensity (Figures 2 and 3). Such damage can be minimized by ensuring fruit is not exposed to prolonged, direct sunlight. At the same time, some sunlight exposure is essential for optimal wine grape quality, especially for color in red varieties. The objective, then, is to expose clusters to intermittent or dappled sunlight through a discontinuous cover of a single leaf layer with gaps. In addition to controlling sunlight exposure, such a canopy allows cooling air movement into the fruit zone. Use canopy manipulations to achieve a single leaf layer fruit zone canopy and carefully manage soil moisture to maintain it.
Here are two canopy management strategies to achieve desirable fruit exposure for different production goals. For optimum quality, prune vines to one-bud spurs spaced about 4 inches apart, thin shoots to one shoot per spur, and, if necessary, remove lateral shoots from the fruit zone. For larger fruit yields, prune to two-bud spurs and remove both leaves and lateral shoots, leaving only a thin veneer of leaves on the afternoon sun (south or west) side of the fruit zone. While pursuing large yields, be sure to maintain balanced growth (14 to 20 exposed leaves per shoot) because overcropping increases fruit sensitivity to temperature and susceptibility to shrivel and sunburn. For both production strategies, remove fruit zone foliage as soon as possible after fruit set while berries are most tolerant of sunlight.
Soil moisture management practices that maintain grapevines on the wetter side of moderate water stress (-10 to -12 bars), such as regulated deficit irrigation (RDI), will avoid leaf damage and loss in fruit zones, thereby conserving the fruit exposure set by canopy manipulations. In addition to basal leaf loss, severe water stress can directly lead to fruit defects, such as berry shrivel and raisining.
Basal leaves may also be lost due to intense insect damage (e.g. leafhoppers and Willamette mites), severe deficiencies of certain mineral nutrients (e.g. potassium, magnesium, and phosphorus), and toxicities (e.g. sodium and chloride). Therefore, sound pest, mineral nutrient, and salinity management are also important in the conservation of desirable fruit exposure and minimizing fruit defects.
It is essential to temporarily abandon deficit irrigation immediately before and during periods of high (i.e. triple digit) temperatures, and liberally apply irrigations to hydrate and cool foliar tissues to avoid or at least, minimize, heat and light stresses. These stresses will damage leaves, but may also lead directly to fruit bleaching (Figure 3), sunburn, and shrivel (Figure 4). Liberal irrigation can also lessen berry shrivel that comes with very high sugar levels.
A tissue protectant, such as Purshade and Surround, can provide an added measure of protection against fruit defects. In addition, Surround minimizes leafhoppers and their damage to leaves. Low biuret urea applied shortly before periods of high temperatures can prime foliar tissues for antioxidant production and thereby, lessen damage.
Soft berries are susceptible to cracking, especially late in the ripening period. Researchers have yet to identify the causes of cracking, which appear to involve localized forces or some other localized phenomenon within berries. Some varieties, such as Zinfandel, appear more prone to cracking than others, perhaps due to the compactness of their clusters.
Cracking has been associated with high humidity, rain, and liberal irrigation, often following a period of severe water stress. Accordingly, it is prudent to carefully manage vineyard water, avoiding both severe water stress and over irrigation. In addition, ensure well-aerated fruit zones for low humidity and prompt drying following rains.
Contaminants in Grape Loads
Although technically not defects, contaminants such as leaves, off-type fruit, dirt, oil, and other foreign matter in grape loads degrade quality. Obviously, contaminants must be minimized to maintain the integrity of the product delivered to a winery customer.
A version of this article was originally published in the Mid Valley Agricultural Services June, 2005 newsletter, and was updated for this blog post.
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