January 2013 Issue of
Wines & Vines
Not all shrivels are created equal; how to recognize and react
The ripening process for grape berries involves a coordinated series of changes in color, texture, volatile expression and (most importantly) the accumulation of sugars. Provided the berries follow these highly coordinated events of ripening, a soft and edible fruit with desirable organoleptic qualities will develop as the ultimate end product, one that is suitable for consumption and winemaking.
Conversely, perturbations in the cascade of events result in various ripening disorders, the most striking feature of which is the shriveling of the berries. Each shrivel form develops with distinct characteristics, resulting in distinguishable features in composition and appearance of the berry.
These ripening disorders are considered physiological in nature, resulting from altered metabolism and the disruption of normal ripening processes, eventually compromising fruit quality attributes and yield; what precisely causes them are not known. The images accompanying this story illustrate chief distinguishable features of each shrivel form that can help growers separate healthy clusters from afflicted ones at harvest to maintain fruit quality.
Vineyard conditions that favor sunburn include open canopies induced by deficit irrigation, leaf removal and reduced use of nitrogenous fertilizer. Sunburn is mostly observed in clusters that are exposed to direct solar radiation on the west aspect of the canopy.
Both red and white varieties succumb to sunburn, wherein one hemisphere remains constantly exposed to the sun (except on cloudy days), while the other hemisphere develops in shade. Most of the injury occurs on the exposed hemisphere and is manifested as loss of crystalline structure of the bloom, resulting in a shiny surface. As opposed to red varieties, sunburn in white grape varieties causes brown lesions in addition to the polished exterior surface.
The overexposed surfaces show reduced color development (pink color), which is very pronounced if sunburn occurs during early phases of ripening, when berries of red varieties transition into veraison. Sunburn during pre-veraison in both varieties causes dehydration followed by the collapse of the whole berry, which eventually turns brown in color.
Severe sunburn in post-veraison berries leads to complete desiccation and raisining in both red and white varieties. The feature that makes sunburn distinct from other ripening disorders is the polished appearance of the skin. Despite the overexposure to radiation and temperature, sunburned berries are suitable for making certain styles of wine.
Management: Since sunburn is primarily associated with poor canopy management practices, resolving it should be relatively straightforward. It can be avoided by reducing the fruit’s exposure to direct sunlight, especially in the afternoon by preserving shade through judicious removal of leaves around the cluster zone.
As this type of shrivel commonly occurs during advanced stages of ripening, it becomes more and more conspicuous prior to commercial harvest. The dehydrated clusters are observed on both east and west aspects of the canopy, but mostly on the west aspect.
The causes of late-season dehydration are unknown; it has, however, been attributed to prolonged dehydration coupled with cessation of sugar import and backflow into the vine late during the ripening period. Consequently, these lead to a systematic development of dimples in the form of small polygonal indentations throughout the once-stretched skin over the flesh. Visually, the dimpled berry resembles a golf ball, which is the main distinguishing feature of this type of shrivel.
Despite the dimpling of the skin, the bloom maintains its integrity, and the bunch stem remains healthy. Since dehydration causes weight loss, it leads to yield losses and eventually low payments to those growers whose contract identifies grape sale prices based on tonnage. On the contrary, in tandem with weight loss, concentration of sugars proceed without much affecting fruit quality, thus the clusters with dehydrated berries are suitable for making wine. Until recently, Syrah led as the poster child for this unique disorder. Lately, it has been known to occur more often in other varieties also, especially Cabernet Sauvignon.
Management: Generally, the weight loss from dehydration is about 15% but can reach a maximum of 30% before harvest. In hot American viticultural areas (AVAs) that experience several weeks of continuous sunlight, the berries may completely dehydrate, dry up and shrivel. Unlike sunburn, controlling late-season dehydration through management practices is a major challenge as the causes are unknown. In case of severe incidence, one strategy is to harvest the grapes and use them for making quality wines of a desired style. Minor incidences of dehydration may not compromise overall fruit quality.
Bunch stem necrosis
Bunch stem necrosis (BSN) is another type of shrivel that occurs after veraison, either early or late in the ripening process. It is commonly observed in parts of the canopy where clusters are shaded by the shoot system. Visible symptoms of BSN include darkened lesions that may develop anywhere on the bunch stem (collective term for peduncle, wing, rachis and pedicel.)
These lesions expand and girdle the affected region, causing necrosis and loss of green color, the prominent feature of BSN. The girdling of the bunch stem blocks the supply of water, nutrients and sugar to the berries distal to (farther away from) the necrotic region. Consequently, these berries dry up, shrivel and eventually develop raisin-like wrinkles.
The bunch stem in the region distal to the necrosis also dehydrates and turns brown. On the contrary, all structures proximal to the necrotic region remain healthy. Of all the ripening anomalies, BSN is the most-researched disorder with no distinguishable causal factors. However, it has been associated with vine vigor, over cropping, severe hedging, heavy or frequent rainfall (high humidity) and calcium or magnesium deficiency.
Compared to late-season dehydration, BSN results in a very high concentration of sugars (e.g., 43ºBrix), rendering such clusters unsuitable for making table wine. Nonetheless, these clusters can be harvested and used for making wines of a different style depending on the timing of inception.
Management: Canopy shading and/or cold weather conditions during ripening can increase the severity of BSN. In Europe, foliar sprays of nutrients (e.g. magnesium and calcium) to clusters beginning at veraison have been shown to reduce incidence of BSN; elsewhere, however, such sprays have not been very effective. These contradictory results suggest that additional abiotic forces work in tandem with nutrients to influence BSN.
SOUR (suppression of uniform ripening) shrivel, also known as SAD (sugar accumulation disorder) and berry shrivel, causes great yield losses and deterioration of fruit quality characteristics in both white and red cultivars. It is a recurring phenomenon, especially in Washington, where the disorder is observed every year to some degree in both young and established vineyards.
Since SOUR shrivel occurs in a wide variety of soils, cultural and environmental conditions, no particular pattern of occurrence could be deduced from its distribution in the vineyard. Thus, SOUR shrivel may well distress any vine in a given vineyard. Following the affliction, SOUR shrivel is either confined to just a single cluster, irrespective of its position in the shoot, or all clusters in that shoot become symptomatic.
Within a cluster, either only a few berries are afflicted or the whole cluster is transformed into SOUR-shriveled berries. Any berry, regardless of its position within the cluster framework, may develop symptoms, although berries at the tip of the cluster frequently develop symptoms. Furthermore, the shoots on the same cordon adjoining the shoot with SOUR shrivel may either develop healthy or afflicted clusters. Also, the vines adjacent to the afflicted vine are either free of SOUR shrivel clusters or get affected to a varying degree depending upon the severity of incidence.
Irrespective of spatial variation in occurrence, all SOUR shrivel berries are flaccid and soft, their skin wrinkled and pulled away from the receptacle forming deep grooves in the flesh reminiscent of a deflated soccer ball. This is the most characteristic feature of SOUR shrivel, which can be used to distinguish it from other disorders such as bunch stem necrosis.
Another interesting feature is that the whole bunch stem appears to be healthy. The collapsed flesh is responsible for the flaccidity of the berries; despite this breakdown the seeds remain healthy, indicating that SOUR shrivel initiates after veraison. These modifications do not alter the wax (bloom) structure. Even though SOUR shrivel berries have viable seeds, they cannot be used for making wine or any other product due to low sugars.
Management: Sour shrivel afflicts grape clusters to varying degrees of severity during every growing season. In a given block, yield loss can be up to 50%. A lack of knowledge about causal factors makes it very difficult to manage SOUR shrivel through cultural practices. Since the clusters afflicted with SOUR shrivel are not ever salvageable, the rational course of action to optimally achieve desired fruit and wine quality is to drop the clusters prior to harvest.
Bhaskar Bondada is a grapevine physiologist on the faculty of Washington State University, Tri-Cities, with interests in both basic and applied research geared to address industry goals. His particular area of research is integrative plant physiology and plant anatomy, utilizing the knowledge of structure-function relationships to improve vine health and fruit quality.
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