What are the differences between “noble rot” and botrytis?

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Dear Dr. Vinny,

What are the differences between “noble rot” and botrytis? Are there instances where either one is bad for the grape?

—Janet H., Palo Alto, Calif.

Dear Janet,

“Noble rot” is just a nickname for Botrytis cinerea, a humidity-loving fruit fungus that causes grapes to shrivel and dehydrate, concentrating flavors. It sounds ugly (and grapes with botrytis actually are quite ugly) but the resulting dessert wines can be lovely—sweet, viscous and spicy, with honeyed notes. Botrytis-affected non-dessert white wines can also offer spicy, honeyed nuances.

Botrytis is often encouraged in the vineyard during the production of sweet wines, but it becomes scary when it attacks grapes not meant for dessert wine, or when the timing is off. Bad timing on botrytis can be disastrous, as the fungus often thrives at the end of the growing season. In ideal dessert wine production, the weather stays relatively dry after botrytis infects the grapes, allowing the berries to shrivel … if it's too humid, the berry skins can split, exposing them to bacteria. Worst case scenario: Botrytis progresses into gray rot, which bears nothing but rotten fruit.

So, as magical as botrytis can be to a certain style of wine, it’s not always a desired affect, and not all rot is noble.

—Dr. Vinny

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