From Distillers Wiki
There are hundreds of fungi that attack grapes, and the majority of them have effects that can be disastrous to a vineyard. A fungus called Botrytis, however, is a beneficial fungus that becomes magical and noble.
What is Botrytis?
Botrytis is a mold that develops on grapes. When carefully cultivated, Botrytis causes the grape to shrivel, concentrating, and intensifying both the sugar and flavor. In addition, the acid levels remain high, which prevents the resulting wines from being too sweet. It also changes the makeup of the grape, so the wines made from grapes that have the botrytis fungus are not just sweet but have an intense honey flavor. Most winemakers are exhilarated when noble rot descents on their grapes because it gives those fruit from which to make very elegant, intensely flavored dessert wines.
Few winemakers will attempt to make botrytis wines. The fungus requires specific weather conditions and the winemaker has to leave the grapes on the vine until late fall, risking loss of the entire crop.