Botrytis Management — Avoiding Rot vs. Encouraging Noble Rot

Botrytis cinerea is a necrotrophic ascomycete fungus that colonises grape clusters under humid conditions, but its impact depends entirely on timing and the moisture regime. Gray rot (bunch rot or pourriture grise) develops when humidity remains elevated, causing wet rot, off-flavours, browning, volatile acidity spikes, and fermentation problems. Noble rot (pourriture noble in French, Edelfäule in German, Aszúsodás in Hungarian) occurs late in ripening when ripe grapes are first penetrated by the fungus and then exposed to drier conditions, allowing water to evaporate while sugars and aromatic compounds concentrate. The same species produces opposite outcomes; the difference lies entirely in the sequence of weather events and the ripeness of the fruit at the time of infection.

• Gray rot develops when humidity stays high throughout the day, causing berry splitting, mold blooms, and introduction of acetic acid bacteria and other spoilage organisms
• Noble rot requires ripe grapes infected under humid conditions, then exposed to warm, dry, sunny weather that promotes dehydration without secondary infection
• Botrytis penetrates grape skins through microscopic pores and fissures, breaking down cell walls and enabling controlled water evaporation in noble rot conditions
• Gray rot introduces laccase enzymes that cause problematic oxidation of phenolics and aroma compounds, making affected wines unstable and short-lived

Noble rot requires a precise microclimate: humid mornings with mist or fog to activate spore germination and skin penetration, followed by warm, dry afternoons that allow water loss without triggering secondary infections. This alternating wet-dry pattern naturally occurs in specific river-valley terroirs where cool tributaries meeting warmer main rivers produce morning mist that burns off by afternoon. Gray rot, by contrast, develops when humidity remains elevated all day, which is common in continental or Atlantic-facing vineyards during wet autumns or in dense canopies with poor air circulation.

• Sauternes (Garonne and Ciron confluence, France): the cool Ciron tributary meeting the warmer Garonne creates morning mists and foggy conditions that dissipate in sunny afternoons, the classic noble rot trigger
• Tokaj (Tisza and Bodrog river valleys, Hungary): south-facing volcanic slopes and river-generated fog create a microclimate conducive to Botrytis proliferation and subsequent grape desiccation
• Cool, damp regions: persistent autumn humidity without the drying afternoon sun shifts the balance toward gray rot; canopy management and leaf removal are critical preventive tools
• Austrian Neusiedlersee: morning fog from the lake followed by afternoon wind creates reliable noble rot conditions, as at Weinlaubenhof Kracher on the lake's eastern side

Noble rot fundamentally alters wine biochemistry through enzymatic action and physical concentration. The fungus consumes tartaric acid preferentially, shifting the acid balance toward malic and citric acid, while producing elevated glycerol, gluconic acid, and a range of sugar alcohols that contribute viscosity and complexity. Aromatic precursors for stone fruit (apricot, peach) are generated directly by the grape's metabolic stress response to infection, while honey notes appear to derive from the Botrytis fungus itself. Gray rot, by contrast, produces laccase enzymes that oxidise phenolics and aroma compounds, introduces volatile acidity, and fosters secondary infections that yield mouldy, vinegar-like off-flavours.

• Noble rot concentrates juice to 350–450 g/L sugar in the pressed must while also elevating glycerol, gluconic acid, and citric acid, creating remarkable textural richness and complexity
• Botrytis consumes tartaric acid, changing the tartaric-to-malic acid ratio from roughly 2:1 in healthy Furmint berries to 1:3 in botrytised ones, increasing perceived freshness
• Honey, apricot, peach, caramel, and citrus peel aromas arise through a combination of concentration, direct fungal metabolite production, and grape stress-response chemistry
• Gray rot causes oxidative browning, introduces mouldy and acetic off-flavours, disrupts fermentation, and produces wines lacking aromatic freshness and stability

Noble rot is geographically concentrated in river-valley microclimates with specific humidity and thermal patterns. Sauternes (Bordeaux, France), where the cool Ciron river meets the warmer Garonne, remains the benchmark. The Sauternes AOC, established in 1936, mandates the use of Sémillon, Sauvignon Blanc, and Muscadelle, with a legal maximum yield of just 25 hL/ha. Tokaj, Hungary, situated at the confluence of the Tisza and Bodrog rivers and declared a UNESCO World Heritage Site in 2002, produces botrytised Aszú wines with more than a thousand years of documented history. Alsace produces Vendange Tardive and Sélection de Grains Nobles from botrytised grapes, while German and Austrian producers occasionally achieve Beerenauslese and Trockenbeerenauslese levels through Botrytis.

• Sauternes AOC: Château d'Yquem (sole Premier Cru Supérieur since 1855), Château Suduiraut, Château Climens, and Château Rieussec are benchmark producers; yield capped at 25 hL/ha
• Tokaji, Hungary (UNESCO World Heritage Site, 2002): Tokaji Aszú legally requires botrytised berries; Disznókő, Royal Tokaji, and Oremus are leading estates; Furmint is the dominant variety
• Alsace (France): Sélection de Grains Nobles (SGN) is produced solely from noble rot-affected grapes; Riesling and Gewurztraminer are favoured varieties; Zind-Humbrecht is a noted specialist
• Germany and Austria: Beerenauslese (BA) and Trockenbeerenauslese (TBA) categories require individually selected botrytised berries; Schloss Johannisberg (Rheingau) is among the historic pioneers

Preventing gray rot requires aggressive canopy management to reduce humidity near fruit through leaf removal and shoot positioning, as well as harvesting before prolonged wet periods. Fungicide applications at key phenological stages (bloom, bunch closure, and veraison) are widely used, with copper-based products approved in organic and biodynamic viticulture. Conversely, encouraging noble rot demands restraint: late-harvest timing, minimal canopy intervention to preserve humidity around the fruit zone, and strategic selection of affected bunches through multiple harvest passes. At top estates in Sauternes and Tokaj, individual berries are hand-selected during successive tris, making noble rot viticulture among the most labour-intensive practices in all of wine production. Understanding your vineyard's specific fog patterns, drainage, and varietal response is paramount.

• Gray rot prevention: canopy thinning, leaf removal in the fruit zone at or after veraison, fungicide programmes (copper sulfate, sulfur, approved synthetic options), and harvest before major rain events
• Noble rot encouragement: delayed harvest into autumn, minimal post-veraison canopy intervention, avoidance of fungicide applications once noble rot is desired, and multiple selective harvest passes
• Selective harvesting: the finest Sauternes and Tokaji estates pick berry by berry in successive passes (tris) to gather only fully botrytised fruit at optimum dehydration
• Varietal choice matters: Sémillon's thin skin makes it highly susceptible to noble rot; Riesling and Furmint also have strong affinity; thick-skinned red varieties are largely unsuitable

Warming trends are reshaping botrytis geography and timing. Earlier ripening, driven by rising temperatures, is compressing the autumn window in which noble rot naturally develops in many traditional regions. Drier autumns in some areas reduce the morning humidity pulses that initiate the infection cycle, while wetter conditions elsewhere increase gray rot risk. At the same time, some producers are adapting by adjusting harvest timing, modifying canopy architecture, and reassessing varietal composition. The unpredictability of noble rot has always been part of its appeal, but climate change is intensifying that unpredictability, demanding greater vigilance and flexibility from growers in every botrytis-dependent appellation.

• Earlier phenological stages mean ripening increasingly occurs in warmer, drier September conditions in some regions, shortening or altering the classic noble rot autumn window
• Warmer, wetter autumns in other areas raise gray rot pressure, requiring more intensive canopy management and faster decision-making around harvest timing
• Some producers in traditionally marginal areas are experimenting with artificial botrytisation techniques, inoculating harvested grapes under controlled conditions to produce botrytised wines
• Adaptation strategies include adjusting harvest date, revisiting canopy management, and exploring varieties with different susceptibility profiles to maintain noble rot production