When and why does grape "shatter" occur in the vineyard?

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Dear Dr. Vinny,

When and why does grape "shatter" occur in the vineyard?

—Nick F., Chesterfield, Mo.

Dear Nick,

Grape shatter (known as coulure in French) occurs when a grape cluster fails to develop completely during its infancy. It's either because the grapevine's flowers weren't pollinated and therefore never developed into berries, or the tiny berries fell off soon after they formed. This is what vineyard managers mean when they say there is "poor fruit set."

Shatter happens during the spring blooming period, and is triggered by rain, wind, rough handling, or extremes in temperature—either too high or too low. Shatter can happen to any grape, but some varieties of grapes are more at risk, such as Merlot and Grenache.

—Dr. Vinny

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