Seedless varieties, particularly Thompson Seedless and Flame Seedless
Shatter can reach levels of 20% or above and represents serious loss. The percentage varies among vineyards and years.
Shatter refers to the loose berries, those that have detached from the stem.
For some reason, seedless varieties are usually less well attached to the capstem than seeded varieties. This is defined with less of a "brush" than in seeded varieties. When a seeded variety is pulled from the capstem, a substantial amount of flesh remains attached to the stem. Conversely, a seedless variety may be removed with little or no flesh. The force required to remove a seedless berry from its capstem is much below that required for a seeded berry.
In general, shatter increases in severity with increasing maturity and/or the longer the fruit remains on the vine. It is also believed to be associated with any stress to which the fruit is subjected such as heat, water insufficiency, weakly growing vines, etc. Shatter varies considerably from season to season, and among vineyards.
Girdling at fruit set, as done for berry weight increase, also increases the strength of the berry attachment to the capstem and so reduces the shatter potential. However gibberellin, also applied at fruit set, weakens this attachment and is dosage dependent. As the rate of gibberellin is increased, the berry weight benefit decreases with each additional increment, but the shatter potential continues to increase.
Shed packing, with some delay between harvesting and packing, decreases the shatter somewhat as compared to field packing where the fruit is packed soon after harvest. Much of the shatter occurs during the packing operation itself with additional shatter all the way to the final retail sale. Bagging the clusters reduces the shatter during packing. Also the loose berries are inside the bag and are sold along with the cluster. With bagging less fruit is packed per box. This in itself reduces shatter because the fruit is not so crowded. Over 60 percent of the grapes were bagged for the 1995 season.