Many table grape cultivars are susceptible but problems are most common with ‘Thompson Seedless,' ‘Flame Seedless,' ‘Calmeria,' and ‘Queen.' The disorder also affects wine and raisin grapes and is known in other areas as ‘bunchstem necrosis' (Australia), ‘palo negro' (Chile), ‘shanking' (New Zealand), ‘Stiellahme' (Germany), and ‘dessechement ed al rafle' (France). Severely affected vines can have nearly 100% of the clusters showing some symptoms and with a 50% crop loss in pack-out. More commonly, crop losses are in the range of 5-20% in affected vineyards. Labor to trim affected berries from cluster is an additional cost factor.
The affected berries become watery, soft, and flabby during ripening due to the interrupted flow of sugar and other constituents into the berries. The affected berries tend to have a more metallic, opaque, or dull green appearance; berry color will be lighter and more variable in red and black varieties. This results from the breakdown and necrosis of the cluster stem structure supporting cluster parts as well as individual berries. Ultimately, water movement may be affected, causing berry shriveling and drying. The earliest symptom is the development of small (1-2 mm) dark spots on the cap stems (pedicles) and/or other parts of the cluster framework. These spots become necrotic, slightly sunken, and expand to affect more areas. The cluster tips, shoulders, and upper laterals are most affected; the entire framework may be involved in severely affected clusters. The nectroic cluster stem tissue will have a dark brown to purplish-black or black color.
Waterberry is thought to be a physiological disorder which appears more in certain cultivars, years, and individual vineyards. It is associated with fruit ripening and most often begins to develop shortly after veraison (berry softening).
Studies in California have shown it to be associated with vineyard areas with a high nitrogen status; symptoms can be induced by foliar spraying with nitrogen compounds or fertilizing a low nitrogen status vineyard. Clusters with symptoms have higher total nitrogen and ammonium levels in the stem structure. Vine canopy shading and/or cool weather during veraison (berry softening) and fruit ripening can increase waterberry, further suggesting nitrogen metabolism involvement in this disorder. Other mineral nutrients such as calcium, magnesium, and potassium have not shown a direct relationship to waterberry; applications of these elements have not influenced the incidence.
There is no known control. However, it is recommended to avoid over fertilization with nitrogen due to waterberry's association with a high nitrogen status. Folier nutrient sprays of nitrogen should be avoided in waterberry-prone vineyards. Likewise, ammonium or urea fertilizers should not be applied via drip irrigation near the beginning of or during fruit ripening. Trellising and canopy management practices such as shoot thinning that minimize shading within the canopy may be beneficial. Supplementing other nutrients such as calcium, magnesium, and potassium has not been effective. Crop load differences or gibberellic acid treatments have not been shown to affect waterberry. Ethephon may possibly hasten its development but not increase the overall incidence by harvest. It has been well demonstrated that no pathogen is involved; fungicidal spray treatments are ineffective. Trimming off affected berries during harvest and packing is a common practice, although labor intensive. A delay between picking and packing may enhance the ability to see affected berries for trimming. This is because waterberries will tend to welt first on a cluster. However, the advantage of this practice must be weighed against the effects of water loss from the frtui to be packed.
Christensen, L. P. 1982. Waterberry -- what we know today. Table Grape Seminar Proceedings, pp 12-14, Feb. 10, 1982, Dinuba. Univ. Of Calif. Cooperative Extension and California Table Grape Commission.
Christensen, L. P. and J. D. Boggero. 1985. A study of mineral nutrition relationships of waterberry in Thompson Seedless. Am. J. Enol. Vitic. 36:57-64.