Sauternes? Tokaji Aszú? Trockenbeerenauslese Riesling? These sweet wines count among the world's finest, and their existence would not be possible without the aid of one particular fungus: botrytis. Botrytis (pronounced "bow-try-tiss"), or "Botrytis cinerea" by its full scientific name, is a fungus that sweeps humid vineyards in mild climates. In welcome conditions, botrytis affects vineyards masked with a morning fog that later retreats to reward warmer, sunnier afternoons. In these circumstance, botrytis earns the more romantic epithet "noble rot," whereby it penetrates ripe, thin-skinned berries and subsequently shrivels and coats them in what looks like a grey ash. Noble rot effectively concentrates a grape's sugars, making it the ideal material for dessert wine production. Several of the world's most pedigreed sweet wines are derived from botrytized grapes, such as Bordeaux's Sauternes and Barsac , Coteaux du Layon and Quarts de Chaume from the Loire Valley, Sélection de Grains Nobles bottlings from Alsace, Hungary's Tokaji Aszú , and the Trockenbeerenauslese wines from Germany and Austria. Alas, not all botrytis is beneficial. Under inopportune conditions, botrytis can serve as one of the most vicious fungal diseases a vineyard can face, and as such it adopts the name "grey rot" or "grey mould." Grey rot often affects vineyards that are overly humid without relief, but is equally undesirable when grapes are either dark-skinned or underripe. When grey rot attacks, it often splits the grapes' skins, leaving room for other fungal and bacterial infections. The resulting wines typically taste and smell off.