The most well known wines from Sauternes and Barsac are produced from grapes affected by botrytis cinerea. This special type of fungus known as "noble rot" bores holes in the skin of the grapes, allowing water to evaporate and concentrating the sugars and all the other elements inside. The net effect is to greatly reduce the total yield while concentrating the sugar and imparting a particular and unmistakable botrytis aroma and flavor.
This noble rot is produced only when the weather cooperates. Cool, misty mornings are necessary to start the spread of the rot, while warm and sunny afternoons dry the grapes out and prevent the botrytis from spreading too quickly, which can cause the skin of the berries to be susceptible to another form of botrytis that produces only gray rot and makes the grapes unusable.
In Sauternes, fog is produced where the Cerons empties into the Garonne. The Cerons is a small, quick, cold stream, and the Garonne at this point is a large, warm, slow river. These morning fogs are trapped around the vineyards by the hills and a large forest, the Landes, which surrounds the area.
Botrytis begins as black spots on the healthy grapes. These black spots spread until the entire grape is covered, a stage known as complete rot or pourri complet. The enzymes produced by the rot degrade the skin of the grape, allowing the water to evaporate, concentrating the sugars and acidity. The final stage is known as roasted rot or pourri rôti
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