Botrytis cinerea, or noble rot, is a gray mold that infects a variety of plants, including wine grapes here in Switzerland. When it forms on grapes it's called Botrytis Bunch Rot. In contrast to its unpleasant appearance, its effect on wine grapes can be quite pleasant. In short, the Botrytis cinerea fungus pierces grape skins causing dehydration, which concentrates the sugar in the remaining juice. The resulting sweet wine can be exquisite.
The three regions best known for Botrytis cinerea affected wine are Tokaji in Hungary, the Rheingau in Germany, and the Graves area of Bordeaux (home of Sauterne). Exceptional examples are also coming from Coteaux du Layon and Vouvray in the Loire, Alsace, Austria, Australia, Burgundy, Chile, South Africa and here in Switzerland. Successful vineyards for botrytised wine are often near rivers and lakes. For example, Tokaji has the river Bodrog, the Rheingau is on the Rhein, and Graves is near the Ciron and Garonne.
Getting noble results for Botrytis cinerea depends on two things; climate -- particularly humidity -- and the grape variety. A species of the plant pathogen Botryotinia fuckeliana, the spore-producing Botrytis cinerea thrives in cool moist conditions and the "noble" effect occurs when drier conditions follow the wet. Continuously damp conditions can lead to a loss of whole crops.
Close-bunched white grapes tend to be most successful for producing the noble rot. In Tokaji the main grape variety is Furmint, for the famous Sauternes from the Graves region it is Semillon and Sauvignon Blanc, and in Loire valley, Chenin Blanc.
Making "botrytised" wine requires a certain optimistic dedication, as making these Noble wines is labor intensive, the yields are low, and the whole process is not without a fair amount of risk. Because Botrytis cinerea does not effect bunches, or even grapes within a bunch, at the same rate, only individual grapes are picked and the crop needs to be harvested in a number of passes called "tris". Further, because grapes need to be picked in the late stage of Botrytis cinerea development (early stages actually consume sugar without concentrating juice), experienced pickers are needed. In its late stage the effect of dehydration has concentrated the juice, and the fungal action produces glycerol, dextrin and other compounds that add to the texture plus give it the unique honey and apricot characteristics.
Production amounts for these wines are low, and the low yields are a function of three factors. First, many vineyards limit the amount of grapes produced by the vines, thereby increasing the health and quality of the remaining grapes. Second, only the Botrytis cinerea affected grapes are picked, and the rest are left on the vine. Third, dehydrated grapes produce less juice.
Leaving grapes on the vine after normal harvesting times involves a leap of faith on the part of the wine maker, but it is in the late season that if Botrytis cinerea spores land on a bunch of grapes, and if there is enough evening humidity, and if the evening humidity is followed by dry days, and if it does not rain heavily, and if there is no frost, then there is a good chance of a good harvest. But the risk is not over at that point.
Dehydrated grapes are difficult to press and may need several pressings to get a decent yield. However by about the third pressings there is the risk of some oxidation. Then there can be problems with starting fermentation: high sugar content can slow down the action of the yeasts, and high concentrations of Botrytis cinerea can hamper fermentation because it produces botryticine, which is an antimicrobial. Once started, the fermentations may "stick". Lastly, just when you think it's safe, because of the high residual sugar there is the problem of fermentation stopping before the yeast consumes all the sugar, or the possibility of re-fermentation in the bottle.
So why bother with all the risk and complications? Because the luscious results can be very seductive. While it's true that low yields, skilled labor and exacting cellaring leads to a fairly high priced wine, some good values can be found from the less well known producers. Well worth a search, as these wines are rich and complex, and when matched to the right food: sublime.