Why would rain be bad for grapes?

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Dear Dr. Vinny,

I often read about how rain during harvest negatively affects grapes/wines. How come? And does it only affect them when it rains specifically during the hours of harvest or also when it rains more in general during the period/weeks of harvest? Thanks!

—Barry S., Netherlands

Dear Barry,

Ugh. Rain is the worst during harvest! First off, if it’s cloudy and overcast, the grapes are going to have a harder time getting ripe. They need sunlight. More than that, if it rains, the grapes will take on more water, which means the flavors become dilute and the sugar/acid balance that winemakers are looking for gets out of whack. With too much rain, the grape berries start to swell and even split, and then you have to worry about spoilage, mold and mildew.

Rain is also a concern in the spring. Grapes are formed from flowers, and a heavy rain can knock the blooms off the plant and reduce the size of the crop. Damp conditions can also lead to mildew and other diseases.

—Dr. Vinny

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