Botrytis Management: Distinguishing Noble Rot from Grey Rot

Problem Definition

Botrytis cinerea is a polyphagous fungus that can either enhance wine quality (noble rot, “pourriture noble,” “Edelfäule”) or destroy it (grey rot, “pourriture grise”). The difference depends on infection timing, weather conditions, grape variety, and enological response. Managing botrytis is among the most consequential decisions in sweet wine production.

Key challenges:

• Distinguishing noble rot from grey rot in the vineyard
• Harvesting at optimal botrytis development
• Managing oxidation and microbial risks from botrytis enzymes
• Achieving fermentation in high-sugar, enzyme-rich musts

Technical Context

Botrytis Biology

Botrytis cinerea infection proceeds through defined stages:

1. Spore germination: On grape surface in humid conditions (>90% RH)
2. Penetration: Through wounds, stomata, or direct cuticle penetration
3. Colonization: Mycelial growth through berry flesh
4. Sporulation: Grey conidiophores visible on berry surface

Noble rot development: Requires alternating conditions:

• Humid mornings (dew, mist): Promotes infection
• Dry, warm afternoons: Arrests fungal growth; desiccates berries
• Extended favorable period: Progressive concentration

Grey rot development: Continuous humidity promotes:

• Rapid fungal growth
• Berry splitting
• Secondary infection (acetic acid bacteria, other fungi)
• Irreversible damage

Chemical Modifications

Botrytis cinerea produces enzymes and modifies grape chemistry:

Enzymes:

• Laccase (polyphenol oxidase): Causes rapid browning; persists in wine
• Esterases: Hydrolyze grape esters
• Pectinases: Degrade cell walls

Concentration effects (noble rot):

• Water loss through infected skin concentrates sugars, acids, glycerol
• Sugar concentration can reach 35-50°Brix
• Glycerol increases to 10-25 g/L
• Malic acid consumed by fungus (reduced TA despite concentration)

Chemical products (both forms):

• Gluconic acid: Marker for botrytis; 0.5-5 g/L
• Glycerol: Elevated
• Botryticine: Antimicrobial peptide
• Volatile thiols: Distinctive “botrytis” aroma compounds

Distinguishing Noble vs. Grey Rot

Parameter	Noble Rot	Grey Rot
Appearance	Golden-brown, shriveled, intact	Grey, wet, splitting
Texture	Firm, concentrated	Soft, mushy
Odor	Honey, apricot	Sour, acetic
Sugar	Very high (>35°Brix)	Variable, may be low
VA	Low (<0.4 g/L)	High (>0.6 g/L)
Gluconic acid	Moderate	High

Options and Interventions

Vineyard Management

Promoting noble rot:

• Canopy management for air circulation
• Late leaf removal (reduce humidity)
• Site selection (fog-prone areas: Sauternes, Tokaj, Mosel)
• Variety selection: Thin-skinned varieties susceptible (Riesling, Chenin Blanc, Sémillon, Furmint)

Preventing grey rot:

• Early leaf removal
• Shoot positioning for airflow
• Reduced nitrogen fertilization (reduces canopy density)
• Fungicide applications (pre-véraison)
• Harvesting before rot becomes grey

Harvest decisions:

• Multiple passes (tries) to select optimally botrytized clusters
• Remove grey rot clusters and berries
• Harvest rapidly once conditions shift unfavorable
• Staff training for selection

Winery Processing

Must protection:

• Elevated SO₂ at crush: 75-150 mg/L
• Inhibits laccase activity (partially)
• Antioxidant protection

Laccase management:

• Laccase is not fully inhibited by SO₂ (unlike PPO)
• High SO₂ reduces but does not eliminate activity
• Avoid oxygen exposure (laccase substrate is O₂)
• Inert gas handling

Settling and clarification:

• Botrytized must has high solids, mucilage, glucans
• Pectinase addition essential (30-50 mL/hL)
• Beta-glucanase if glucan slime present (Botrytis cell wall component)
• Extended settling (24-72 hours)
• Flotation or centrifugation may be required

Fermentation:

• High sugar creates osmotic stress (see stuck fermentation article)
• Botryticine may inhibit yeast
• Inoculate with osmotolerant yeast strains
• YAN supplementation critical (botrytized must is often nitrogen-depleted)
• Extended fermentation acceptable (60-120 days)

Quality Assessment

Gluconic acid measurement:

• Indicates botrytis extent

• 0.5 g/L indicates botrytis presence

• 3 g/L suggests grey rot component

• Does not distinguish noble from grey (context required)

Volatile acidity check:

• Grey rot produces elevated VA (>0.6 g/L)
• Noble rot VA typically <0.4 g/L
• VA above 0.5 g/L before fermentation is warning sign

Sensory evaluation:

• Noble rot: Honey, apricot, marmalade
• Grey rot: Moldy, mushroom, acetic

Trade-offs and Risks

Waiting for noble rot:

• Risk of weather shift causing grey rot
• Crop loss to birds, insects
• Narrow optimal window

Harvesting early:

• Incomplete botrytis development
• Less concentration; less complexity
• May not achieve style goals

High SO₂:

• Necessary for laccase inhibition
• May inhibit fermentation
• Legal limits apply

Blending:

• Noble lot + grey lot = damaged wine
• Keep affected lots separate
• Declassify grey rot lots

Fermentation failure:

• High-sugar botrytized musts have elevated stuck fermentation risk
• Requires specialized protocols
• Extended cellar time

Practical Implications

Variety-specific considerations:

• Riesling: Thin skin facilitates noble rot. Mosel BA and TBA styles require perfect development. Acidity preserved better than in Sémillon (malic consumption less pronounced).

• Chenin Blanc: Vouvray AOC moelleux and demi-sec styles depend on botrytis. High natural acidity balances sweetness. Tries (multiple passes) essential.

• Gewürztraminer: Alsace AOC Sélection de Grains Nobles requires botrytis. Low acidity variety; botrytis concentration is necessary for balance.

Appellation-specific implications:

• Alsace AOC: Sélection de Grains Nobles (SGN) requires botrytis; Vendange Tardive does not mandate it but may include. Minimum must weights specified by grape variety.

• Vouvray AOC: Sweet styles (moelleux, doux) traditionally botrytized. Weather in Loire highly variable; noble rot not guaranteed annually.

• Mosel: Beerenauslese and Trockenbeerenauslese require botrytis-affected berries. Steep slopes and river humidity provide conditions in good years.

References

• Ribéreau-Gayon, P., Dubourdieu, D., Donèche, B., & Lonvaud, A. (2006). Handbook of Enology, Volume 1: The Microbiology of Wine and Vinifications (2nd ed.). Wiley. Publisher Link

• Thibon, C., Marullo, P., Claisse, O., Dagan, L., Dubordieu, D., & Tominaga, T. (2009). “Nitrogen Catabolic Repression Controls Volatile Thiol Production by Saccharomyces cerevisiae During Wine Alcoholic Fermentation.” Food Chemistry, 114, 1359-1373. DOI: 10.1016/j.foodchem.2008.11.008

• Magyar, I. (2011). “Botrytized Wines.” Advances in Food and Nutrition Research, 63, 147-206. DOI: 10.1016/B978-0-12-384927-4.00006-3

• Vannini, A., & Chilosi, G. (2013). “Botrytis cinerea.” In Encyclopedia of Food Microbiology (2nd ed.), pp. 285-293. Academic Press. DOI: 10.1016/B978-0-12-384730-0.00041-1