Botrytis Cinerea & Noble Rot

Botrytis cinerea is a necrotrophic ascomycete fungus classified within the family Sclerotiniaceae. It is one of the most widely studied fungal plant pathogens, capable of infecting over 200 crop species worldwide. In viticulture, it occupies a dual role that has fascinated winemakers for centuries: under the wrong conditions it causes devastating grey rot, but under a precise set of autumn weather conditions it produces noble rot, the biological engine behind some of the world's greatest sweet wines. The fungus penetrates ripe grape skins through natural openings and microscopic wounds, colonizing the berry and initiating a cascade of chemical and physical changes.

• B. cinerea belongs to the phylum Ascomycota, class Leotiomycetes, order Helotiales, and family Sclerotiniaceae
• The fungus is described as necrotrophic, meaning it kills host cells and then feeds on the resulting dead tissue
• Infection initiates through stomata, wounds, or microfissures on the grape berry surface
• The same fungal spores cause grey rot or noble rot depending entirely on weather conditions during and after infection

Noble rot emerges under a specific and delicate sequence of autumn weather: warm, humid mornings that allow botrytis to spread across ripe grapes, followed by dry, sunny afternoons that halt fungal progression and promote water evaporation from the infected berries. Botrytis infection requires temperatures around 20°C and humidity above 80% to initiate. If sustained wet weather prevails, the destructive grey rot form takes hold instead, causing bunch rot that disrupts fermentation and ruins harvests. The alternating humidity cycle is the critical factor that distinguishes noble rot from its destructive counterpart, and certain vineyard locations near rivers that generate morning mists are particularly prized for this reason.

• Warm, humid mornings around 20°C with humidity above 80% promote botrytis growth and skin penetration
• Dry, sunny afternoons halt fungal spread and accelerate water evaporation through the compromised skins
• Regions near rivers, such as the Ciron and Garonne in Sauternes or the Bodrog and Tisza in Tokaj, naturally generate the morning mists that favour noble rot
• If wet weather persists without afternoon drying, grey rot spreads rapidly, potentially destroying the entire harvest

Noble rot triggers a remarkable series of chemical changes inside the grape berry. The fungus breaks down skin cells, facilitating water evaporation that can reduce berry mass by up to 60%, leaving behind a shriveled, sugar-rich fruit. Crucially, botrytis primarily consumes tartaric acid rather than malic acid, a rare property among microorganisms, shifting the acid profile of affected berries significantly. The fungus also produces glycerol and gluconic acid, both of which are considered chemical markers of noble rot development and contribute to the distinctive richness and texture of botrytized wines. Elevated levels of sugar alcohols including glycerol also add to the characteristic mouthfeel and viscosity of these wines.

• Berry mass can decrease by up to 60%, concentrating the remaining sugars, acids, and flavor compounds
• Botrytis primarily consumes tartaric acid; in Furmint grapes, the tartaric to malic acid ratio shifts from 2:1 in healthy berries to 1:3 in botrytized ones
• Glycerol and gluconic acid, both produced by the fungus, are key chemical markers of noble rot and contribute to mouthfeel and complexity
• The fungus induces the production of aroma precursors for stone fruit notes such as apricot and peach, while honey aromas are attributed directly to the botrytis itself

The three most celebrated noble rot wine traditions are Sauternes in Bordeaux, Tokaj in Hungary, and the Pradikat wine regions of Germany and Austria. Sauternes, situated between the cool Ciron River and the warmer Garonne, benefits from a reliable autumn microclimate for noble rot development. The AOC requires a minimum must weight of 221 grams of sugar per liter, a minimum of 13% potential alcohol, and a maximum yield of 25 hectoliters per hectare, with no fixed minimum for residual sugar in the finished wine. In Tokaj, regulations revised in 2013 set the minimum residual sugar for Aszú at 120 grams per liter, eliminating the former 3 and 4 puttonyos categories and retaining only 5 and 6 puttonyos. Germany's Beerenauslese requires a minimum must weight of 110 to 128 degrees Oechsle depending on the region and grape variety, while Trockenbeerenauslese requires at least 150 degrees Oechsle in the Mosel and is produced exclusively from fully botrytized, raisin-like berries.

• Sauternes AOC mandates a minimum must weight of 221 g/L of sugar and a maximum yield of 25 hl/ha; harvesting requires multiple selective passes through the vineyard, sometimes up to 10 to 12 passes
• Tokaj Aszú, regulated since 2013, requires a minimum of 120 g/L residual sugar; puttonyos labeling is no longer mandatory, and wines formerly at 3 or 4 puttonyos are now labeled as late harvest or Szamorodni
• German Beerenauslese is defined by a minimum must weight of 110 to 128 degrees Oechsle depending on region and variety; botrytis is highly desirable but no specific percentage is legally mandated
• Trockenbeerenauslese is made exclusively from individually selected, botrytis-shriveled berries at a minimum of 150 degrees Oechsle and represents the highest and rarest Pradikat category

Harvesting botrytis-affected grapes is among the most labor-intensive and unpredictable processes in all of winemaking. Pickers must assess individual berries for optimal noble rot development and select only those at the right stage, leaving others for subsequent passes. In Sauternes, up to 10 to 12 passes through the vineyard may be required over several weeks, and yields are frequently far below the legal maximum of 25 hectoliters per hectare. At top estates such as Chateau d'Yquem, yields can amount to roughly one glass of wine per vine. Fermentation of botrytized must is also challenging: the extremely high sugar content stresses yeast, botrytis produces antifungal compounds that can halt fermentation prematurely, and the must is depleted of yeast-assimilable nitrogen and thiamine.

• Berry-by-berry selective harvesting, called triage in French, may require multiple passes over weeks as noble rot develops unevenly across a vineyard
• Extremely high sugar concentrations in botrytized must create challenging fermentation conditions, often resulting in naturally arrested fermentation at 13 to 15% alcohol with substantial residual sugar
• Botrytis depletes the must of yeast-assimilable nitrogen and thiamine, requiring careful fermentation management to avoid stuck fermentation
• In poor noble rot vintages, some top Sauternes producers declassify their entire crop to dry Bordeaux Blanc rather than compromise quality

Riesling and Semillon are the primary grape varieties used in botrytized wine production worldwide. In Sauternes, Semillon dominates plantings, supported by Sauvignon Blanc and Muscadelle; its thin skin makes it highly susceptible to botrytis penetration. In Tokaj, Furmint accounts for approximately 60% of plantings and is the backbone of Aszú wines, with Harslevelu and Yellow Muscat playing supporting roles. Riesling, with its naturally high acidity, is considered the finest variety for German and Austrian botrytized wines, as the acidity provides essential balance against extreme sweetness and supports decades of aging potential. Chenin Blanc in France's Loire Valley, Gewurztraminer, and Picolit in northeastern Italy are among other varieties used for noble rot wines in various traditions.

• Riesling and Semillon are the world's two primary varieties for noble rot wine production
• Furmint, the dominant Tokaj grape variety at approximately 60% of plantings, is prized for its susceptibility to botrytis and its naturally vivid acidity
• Chenin Blanc is the basis for botrytized wines in France's Coteaux du Layon and Quarts de Chaume appellations in the Loire Valley
• Botrytized wines are also produced in Austria, Australia, New Zealand, South Africa, and California, expanding the category well beyond its European heartlands