Noble rot

Noble rot is the beneficial form of a grey fungus, Botrytis cinerea, affecting wine grapes. Infestation by Botrytis requires warm and humid conditions, typically around 20 °C (68 °F) and above 80% humidity. If the weather stays wet, the undesirable form, "bunch rot" or "grey rot", adversely affects winemaking by disrupting fermentation and changing the taste, aroma, and appearance of the final wine.

Grapes typically become infected with cinerea when they are ripe.
If they are then exposed to drier conditions and become partially raisined, this form of infection is known as noble rot.
Grapes picked at a certain point during infestation can produce particularly fine and concentrated sweet wine.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 3.0