Two-Faced FungusBotrytis cinerea is a fungal disease that can blight many species of plants, including flowers, fruits, and vegetables. Depending upon weather conditions, Botrytis can take one of two forms in grapes, one as destroyer, the other as enhancer.
As "grey rot" it appears and grows during lengthy periods of humidity early in the season. Settling in on immature grapes, it multiplies rapidly. The bunches appear to be covered with a grey powder and eventually darken and drop. Yields are greatly reduced and wine made from this fruit taste moldy and oxidizes easily. In some climates, grey rot is a severe problem with most grape varieties.
In certain white grape varieties, such as Semillon, Sauvignon Blanc, Riesling, and Furmint, an infection of Botrytis can be so beneficial, even critical to dessert wines like French Sauternes, German Tröckenbeerenauslese, or Hungarian Tokaj, that the mold becomes known as "Noble Rot." Weather conditions must be right for this to occur. Ideally, a short period of humidity or rain in mid to late season, when the grapes are more ripe than green, will be followed by a sustained period of cool, dry weather, where daytime temperature hovers near 60° F.
Under these somewhat rare conditions, the Botrytis fungi penetrate the grape skins with mycelia to feed and take water from the grapes, which shrivel. Overall acidity decreases. Gums form, along with glutinic and citric acids, and the grape sugars become very concentrated.
This intense sweetness partially inhibits yeast and fermentation can be very slow, lasting for months. High concentrations of glycerol developed during these extended fermentations and the resulting wines can be exceptionally smooth and extremely long-lived, cellaring well for decades.