How To Spot Winegrape Shrivel
December 29, 2010 
Shriveled winegrapes in vineyards have several causes, some poorly understood. There are four main types of shrivel that are often confused with one another. However, the types are quite different, which means growers should know what they are in case they can do something to improve the quality of their crop.
Growers should also be able to spot the differences so that they can alert their Extension agent or consultant, says Mark Krasnow, a researcher who has worked on the issue in California for many years. Krasnow is now a senior lecturer at the Eastern Institute of Technology in Napier, New Zealand.
Sunburn — The best known of the four causes, only the fruit that is exposed to direct sunlight is a problem. But, there is no proof that sunburn is caused by simply high ultraviolet (UV) light. Krasnow says in New Zealand they give the fruit complete exposure, and they have much more UV, being closer to the ozone hole at the South Pole, yet winegrape sunburn rarely happens. “I’ve seen fruit here that’s actually in the canopy and it’s been fried because it’s on the hot side of canopy,” he says. “Temperature may be the key.”
Late-Season Dehydration — Dehydration, also known as raisining, is the most common cause of berry shrivel, mainly because growers cause it themselves. Brought on by the grapes losing weight to water loss and the sugars getting concentrated, it’s especially prevalent in Syrah.
Bunchstem Necrosis — Diagnosing “BSN,” as it’s often termed, is done through examining not the fruit, but the rachis. While the berries will be shriveled, as with the other types, the rachis may be weak or, in advanced cases, will be dead. The first visible symptoms of BSN are often small black spots on the bunch’s pedicels, the branches of the rachis that attach to the berries, and progress to the lateral stem structure. It can occur in many varieties, but is especially prevalent in Cabernet Sauvignon. It’s important to identify BSN, says Krasnow, because while it generally doesn’t affect a lot of fruit, it can hit early in fruit development, leaving the grapes with precious little sugar.
Sugar Accumulation Disorder — With the apt short-hand name “SAD,” this disorder can be found in many varieties and has been known to hit entire vineyard blocks, says Krasnow. Some believe SAD and BSN are actually one and the same, though Krasnow doesn’t think so. In any case, the grapes are smaller and have a lower pH, in addition to lower sugar.
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