noble rot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A parasitic fungus (Botrytis cinerea) that attacks ripe grapes and causes an increase in their sugar content. Certain wines, such as the French Sauternes and the Hungarian Tokay, are produced from such grapes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A benevolent form of the fungus Botrytis cinerea which, in the right conditions, leads to a concentrated sweet wine.