Botrytis

Noble rot and grey rot

Noble rot, caused by Botrytis cinerea under specific conditions of humidity and warmth, is a blessing for winemakers seeking concentrated sweet wines. The fungus penetrates the grape skin, allowing water to evaporate and concentrating the sugars, acids and flavours. The resulting wines, made famous in Sauternes and Tokaji, are among the most prized in the world.

Grey rot, the same fungus in the wrong conditions, is a disaster. The grapes become soft, mouldy and unworkable, and the resulting wine is thin, brown and off. Grey rot is the most damaging disease in UK vineyards, particularly in damp, cool autumns, and growers spend a significant part of the year trying to prevent it.

How the fungus works

Botrytis cinerea releases enzymes that break down the grape cell walls, allowing water to evaporate and sugars to concentrate. The fungus also produces glycerol and other compounds that add body and complexity. In noble rot conditions, the dehydration is gradual and the chemistry of the grape concentrates cleanly. In grey rot conditions, the breakdown is uncontrolled and the grape collapses into a wet, mouldy mass.

Botrytis in the UK

Grey rot is a constant threat in UK vineyards, particularly in wet autumns. Botrytis-susceptible varieties, including Pinot Noir, Chardonnay, Sauvignon Blanc and the German crosses, require careful canopy management and timely spraying to keep the disease at bay. The UK industry uses a combination of cultural controls, biological controls and conventional fungicides, with a growing band of organic and biodynamic producers using the first two.

Noble rot, on the other hand, is a rare and unpredictable event in the UK. A few warm, dry autumns have produced small quantities of botrytised wines from Bacchus, Ortega and Chardonnay, but the volumes are tiny and the conditions are hard to repeat. The most ambitious UK producers, including Chapel Down, Nyetimber and Three Choirs, have all produced small batches of botrytised wines as limited releases.

Managing botrytis risk

UK growers manage botrytis risk through a combination of canopy management, varietal selection, spraying and harvest timing. An open, sun-exposed canopy dries off quickly after rain, slowing the fungus. Varieties with loose bunches and thick skins, such as Seyval Blanc and Regent, are less vulnerable than tight-bunch varieties such as Pinot Noir and Chardonnay.

Spraying, particularly with specific botryticide chemistry, is a routine part of UK viticulture. The timing is critical: the spray must be applied before the fungus establishes, not after. Once the fungus is in the bunch, no spray will help. A wet September often means a difficult harvest, with growers working overtime to pick the cleanest fruit before the rot sets in.

Reading botrytis on the label

English wines made from botrytised grapes usually say so on the back label, often with a phrase like "made from hand-picked, botrytis-affected fruit" or "from a late-harvested, noble-rotted parcel". The wines are usually classified separately, with a higher price and a smaller volume than the standard range. They are a niche within the niche, but a celebrated one.