BOTRYTIS 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, first appearing as tiny brown speckles which gradually grow and spread until the berries turn brownish-purple, begin to shrivel, and seem covered with a grey powder. Eventually the berries will become very dark, dessicate, and drop from their stems. Yields are greatly reduced and wine made from severely-infected fruit will taste moldy and oxidize easily. In some climates, grey rot is a severe problem with most all 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 the production of dessert wines like Sauternes in France, Tröckenbeerenauslese in Germany, or Tokaj in Hungary, that the mold is called "Noble Rot" in these locales (La Pourriture Noble in french, Edelfaule in German).
Dessert wines made with botrytised grapes are prized and somewhat rare, because weather conditions must be just right for "Noble Rot" 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.
Detailed and labor-intensive vineyard management is required to manage a Botrytis-infected crop. Harvesting is difficult and requires much care in handling the extremely tender and sticky berries and many passes to select them at their optimum.
This intense sweetness partially inhibits yeast and fermentation can be very slow, lasting for months. High concentrations of glycerol develop during these extended fermentations and the resulting wines can be fragrantly enticing, exceptionally smooth, and extremely long-lived, cellaring well for decades.
Chateau Suduiraut has a click-through photo gallery of a grape undergoing Botrytis infection (from their home page, click on English > Terroir > The Noble Rot > Photos)