For red-wine makers, Botrytis cinerea is simply rot, a nasty fungal infestation with potentially disastrous consequences. But for estates making dessert wines in regions such as Germany and France's Sauternes and Loire Valley, Botrytis cinerea goes by the sobriquet "noble rot." Given the proper conditions, noble rot supercharges flavor, sweetness and concentration, yielding some of the world's richest and most ageworthy dessert wines.
Botrytis cinerea is "noble," rather than nasty, when it occurs in the right place at the right time. The right place is a vineyard relatively close to water, typically a river, in a region with foggy fall mornings. These conditions spark the development of botrytis, and afternoon winds and sun keep moisture levels in check. A balance of these variables ensures steady development of noble rot and also inhibits the growth of unwanted fungi.
Timing is equally crucial: To be beneficial, botrytis must occur after grapes have achieved physiological ripeness. Otherwise the fruit can develop "sour rot" (a condition that makes grapes smell of vinegar), as was the case for some German producers in the 2000 vintage.
Additionally, noble rot benefits only certain grape varieties. The classic examples are Sémillon, the mainstay of Sauternes; Chenin Blanc, the source of Vouvray and other late-harvest Loire whites; and Riesling, the expressive grape of the finest German vineyards. These varieties tend to have tight clusters that limit air flow and retain moisture, conditions conducive to botrytis growth. Other white varieties sometimes used for late-harvest wines include Pinot Gris, Gewürztraminer, Muscat and Chardonnay.
However, excessive moisture may lead to the development of "gray rot," a potentially devastating form of botrytis bunch rot that taints grapes with rotten flavors.
It should be noted that red grapes are not suitable candidates for making botrytized wines because red wines rely on maceration for color and flavor extraction; botrytis-affected red-grape skins become drained of color and tend to impart unwanted flavors to wine.
Winemaker Olivier Humbrecht of Domaine Zind-Humbrecht in Alsace makes botrytized wines almost every vintage. Humbrecht focuses on Riesling, Pinot Gris and Gewürztraminer. Depending on the weather, botrytis takes from five days to a month to spread through the Zind-Humbrecht vineyards. Since Humbrecht assumed production responsibilities at his family estate in 1989, only twice has he been unable to make late-harvest bottlings (1992 was too cold and rainy for noble rot; 2003 was too hot and dry).
As botrytis develops, grapes become sweeter and exhibit different spectrums of flavors. In Germany's Mosel region, for example, ripe, unbotrytized Riesling often features lime, peach and slate nuances, sometimes with an herbaceous or a scallion undertone. Botrytized Riesling, on the other hand, offers a richer palette that includes honey, dried and roasted fruit character and spice notes such as clove, nutmeg and pepper. The wines also have higher viscosity, primarily because of higher levels of sugar and glycerol. Botrytis also increases the proportion of natural acidity, thereby providing necessary balance to the extra sugar.
Regardless of the grape variety or the region, the successful development of noble rot follows a fairly predictable pattern. First, the grapes achieve ripeness. Then, in the presence of adequate moisture, fungus spores grow, producing enzymes that break down the grape skins and turn them a mauve color. Botrytis mycelia breach the skins, allowing the fungus to consume water, natural sugars and acids. The skins do not rupture but become permeable, leading to desiccation. When conditions permit advanced noble rot, berries resemble deflated, ash-caked balloons.
Botrytis develops unevenly in grape clusters. At Château d'Yquem in Sauternes, noble rot (pourriture noble in French) usually takes about six weeks to spread through the 250-acre vineyard. Like other producers striving for extremely concentrated wines, Yquem requires its harvesters to make multiple passes, or tries, through the vineyards, picking only the most botrytized berries.
In difficult harvests, pickers may make as many as 10 passes. But Xavier Planty, owner and winemaker of Château Guiraud in Sauternes, achieves the best results when noble rot advances more quickly through the clusters, as was the case in 1989 and 2001. "If you allow the botrytis to stay on the grapes too long, you get unpleasant flavors," Planty says.
Much as a virus makes its host more susceptible to other infections, Botrytis cinerea weakens the defenses of grapes. If conditions are less than ideal for noble rot to achieve dominance, unwanted microbes might sneak in and introduce undesirable flavors. That's especially likely if it rains on botrytized grapes. Rich fruit and spice flavors can quickly turn medicinal, and if grapes remain wet, they will soon taste rotten.
"There are so many kinds of mold [in vineyards] that it can explode. It's really a fungal zoo out there," says Greg Allen, winemaker at Sauternes-style Napa producer Dolce, which lost its entire 1996 crop to fall storms.
Most winemakers try to prevent the growth of fungi in their vineyards. Late-harvest estates need to encourage it. Planty emphasizes the importance of eschewing chemicals or soil treatments that kill or inhibit botrytis. Depending on the region and vineyard, producers sometimes need to break with standard viticultural practices. At Dolce, for example, which lies in a relatively dry region, Allen wants the canopy to be thick to trap humidity and reduce the desiccating effects of air flow: "You have to grow the vineyard in a manner that promotes the development of the mold," he says.
Botrytized grapes also present challenges in the cellar. The first is that desiccated grapes are difficult to press, and they yield minute quantities of juice (yields at the finest Sauternes estates, for example, rarely exceed 700 bottles per acre, perhaps one-third the standard crop of a top-tier red Bordeaux producer). The fungus consumes nutrients necessary for fermentation, so vintners often need to add nutrients to the must, such as the vitamin thiamine or some source of nitrogen. Botrytis also produces a natural antibiotic called botryticine, which kills and inhibits other microbes, including yeast. This can result in slower, more labored fermentations. "The effect is that you have fewer active yeasts in conditions that make it more difficult for yeasts to multiply," says Humbrecht.
Therefore, wineries that make late-harvest styles shouldn't be over-sterilized, Humbrecht says; if the cellar lacks a healthy yeast population, fermentations can be difficult. But even under the best circumstances, the fermentation of botrytized grapes presents difficulties. Grapes intended for dry wines typically contain enough sugar at harvest to yield 12 percent to 16 percent alcohol. Desiccation from noble rot frequently ups that figure to more than 20 degrees potential alcohol. Yeasts can struggle in the presence of so much sugar, so the fermentation of botrytized grapes can sometimes take months.
Once vintners decide that the alcohol and residual sugar level is appropriate, they stop the fermentation, typically by cooling the vat and adding sulfites, which inhibit yeasts. German vintners might bottle a trockenbeerenauslese with 6 percent alcohol, whereas a Sauternes usually contains about 14 percent. To ensure that fermentation does not recommence in the bottle, however, practically all late-harvest wines are filtered to remove yeasts and other microbes that might feed upon the residual sugar.
That residual-sugar sweetness is, of course, what most distinguishes late-harvest wines. But the sugar has another benefit: It helps make this type of wine remarkably ageworthy. Sugar protects wine from the gradual exposure to air that is often part of extended cellaring (that's why late-harvest bottlings lose sweetness with age). So in addition to concentrating flavor and boosting richness, noble rot allows vintners to make wines than can last for decades, perhaps even for generations.
Nik Weis of St.-Urbans-Hof, who has tasted late-harvest German Riesling dating back to 1911, has been amazed by the longevity of these wines. "It all works together," he says. "It's mostly the high sugar concentration, but there's also the antibiotic compounds produced by the botrytis, which protect the wine and the concentration of acids, minerals and flavor. The wines are incredible."