Getting to the Bottom of Berry Scarring

Scarring can be a major quality defect of table grapes. Visibly scarred berries are generally clipped from clusters at harvest, increasing harvest costs and reducing efficiency; clusters with many scarred berries may be unmarketable.

Scarred berries may also develop tears in their skins as they grow because scars do not stretch in the same way as normal skin. Torn skin results in open wounds that serve as entry points for rot-causing fungi and bacteria. Such infections may then spread to adjacent healthy berries, and eventually taint the entire cluster.

Scarred berries may also develop tears in their skins as they grow because scars do not stretch in the same way as normal skin. Torn skin results in open wounds that serve as entry points for rot-causing fungi and bacteria. Such infections may then spread to adjacent healthy berries, and eventually taint the entire cluster.

The important consequences associated with scarring have led to the identification of several common causes of scarring and appropriate preventative measures to avoid them. The common causes are well known, but I still receive samples of scarred berries every year from growers who know how to prevent all the common causes of scarring.

Often the samples are from a variety that is new to the grower and it seems the most likely explanation is differences in varietal susceptibility to the same well-known causes of scarring, though it can be difficult to definitively identify the cause of scarring.

Watch Out For Fungi
Fungi, especially powdery mildew, are a common cause of berry scarring that can lead to rot problems, and table grape varieties, including new varieties, can vary substantially with respect to their sensitivity to powdery mildew. However, table grapes have a very low tolerance for powdery mildew regardless of scarring, and such infections are easily identified.

Avoid Abrasions
Mechanical damage is another common cause of berry scarring. The young berries of some varieties are particularly sensitive to abrasion from leaves, other vine parts, and trellising. Such injuries are usually referred to as “wind scars,” and are generally controlled by leafing. Observations in other countries suggest there may also be a role for protected cultivation in reducing wind scarring on particularly sensitive varieties grown on windy sites.

Common Cause of Scarring
Spray damage is another common cause of scarring in table grapes. Certain adjuvants and oils have long been linked to spray injury, which often results in ring-shaped scars, especially around the low point of berries, or around points where adjacent berries may touch. Even normally “safe” adjuvants may cause injury if too concentrated. Sulfur applied during flowering and fruit set has also been suggested as the cause of scarring in some varieties, though testing is needed to confirm this.

Scout for Thrips
Certain insects are another primary cause of berry scars. Thrips species, especially western flower thrips (Frankliniella occidentalis) can be extremely damaging in California. Western flower thrips may damage berries by feeding on them and by inserting their eggs into them. The feeding of western flower thrips larva can be particularly damaging on berries with “sticky” calyptras.

The petals of grape flowers normally remain fused at their tips, with their bases abscising from the flower. The fused petals form a cap-like structure called a “calyptra.” Sometimes the calyptra doesn’t shed as readily as it should, remaining on the tip of a berry for some time. A sticky calyptra provides shelter for feeding thrips larva, damage from which is ultimately manifest in a starfish-patterned scar, matching the cover provided by the calyptra.

Certain varieties seem to be particularly susceptible to this type of damage, possibly because they are more sensitive to thrips feeding, they are more likely than other varieties to have sticky calyptras, or for both of these reasons. Adult thrips insert their eggs into young berries, resulting in a small dark scar surrounded by a “halo spot.” Such scars are not always readily apparent but can lead to secondary infections in certain varieties. Careful monitoring and insecticide treatments are needed to minimize thrips damage in sensitive varieties.

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