What is botrytis in wine grapes?

Quick answer

Botrytis in wine grapes usually means infection by the fungus Botrytis cinerea. When berries stay wet or humid for too long, the fungus can cause grey mould—also called botrytis bunch rot—which softens, splits and damages the fruit.

In particular late-season conditions, however, infected berries may dry out rather than collapse into widespread decay. This can produce “noble rot”, a desirable production outcome in which berries lose water and become more concentrated. Noble-rot grapes are commonly used for certain sweet wines.

The same fungus can therefore have two very different reputations. Ordinary botrytis bunch rot is generally a vineyard and winemaking problem. Noble rot is a specialised outcome that depends on the right balance of infection, grape ripeness, moisture and drying weather.

What is Botrytis cinerea?

Botrytis cinerea is a mould that can affect grapevine flowers, shoots, leaves and berries. In wine discussions, “botrytis” is often used as shorthand for both the fungus and the effects of its infection.

You may also see these terms:

• Grey mould: a common name for the disease caused by B. cinerea. “Grey” is the usual Australian spelling.
• Botrytis bunch rot: the technical term for damaging infection and decay in grape bunches.
• Noble rot: a favourable form or outcome of botrytis infection in which berries dehydrate and become concentrated.
• Botrytised wine: a broad term for wine made from grapes affected by botrytis, especially noble-rot grapes.

These terms are related, but they are not interchangeable. Noble rot is not a different fungus, and “rot” on grapes does not always mean botrytis.

When botrytis damages grapes

Botrytis is more likely to become destructive when moisture remains on berries or inside dense bunches. Rain, high humidity, mild conditions and slow drying can all support infection and spread. Ripening berries are more vulnerable, particularly if their skins have been damaged by insects, birds, weather or physical contact.

Damaged berries may:

• turn brown or reddish;
• soften and break down;
• split and leak juice;
• shrivel;
• develop fuzzy grey fungal growth;
• spread decay to neighbouring berries.

This is botrytis bunch rot, or grey mould. It can reduce yield and downgrade fruit quality. For a winemaker, unwanted botrytis can also bring practical problems: compounds from the fungus may contribute to oxidation and browning, mouldy characters and difficulty clarifying or filtering the wine.

So if someone says a vineyard has “botrytis”, that is not automatically good news. In many situations it means fruit needs careful assessment and selection before it reaches the winery.

When botrytis becomes noble rot

Noble rot requires a much narrower combination of circumstances. The fungus must infect the berries, but conditions afterwards need to allow them to dry and lose water rather than remain persistently wet and collapse into destructive decay.

As the berries dehydrate, the remaining juice can become more concentrated. Sugars and other dissolved components may be present at higher concentrations, while the fungus also changes some of the berry’s chemistry and aroma compounds.

This is why noble rot is commonly associated with sweet wine. Concentrated grapes can provide juice with high sugar levels, although the final sweetness still depends on fermentation and winemaking choices.

Noble rot is not a perfectly predictable or uniform process. Berries in the same bunch may be affected differently, and the result depends on grape variety, ripeness, weather, picking decisions and cellar practice.

How does botrytis affect wine?

The outcome can range from distinctive and desirable to flawed and degraded.

Noble-rot wines may show aromas or flavours described as honeyed, dried-fruit, marmalade-like, spicy, floral, earthy or savoury. These are possibilities, not a checklist. Botrytis does not guarantee that every wine will taste honeyed, nor does it determine a single style.

Dehydration and concentration can contribute to a fuller or more viscous impression, but texture is also shaped by residual sugar, acidity, alcohol, grape variety and winemaking.

Unwanted botrytis can have a very different effect. It may contribute mouldy character, oxidative browning and processing difficulties. The fungus can produce laccase, an enzyme associated with oxidation, and fungal polysaccharides can make clarification or filtration more challenging.

This is why “botrytised” is not a quality rating. It tells you something about the grapes, not whether the finished wine is sweet, excellent or flawed.

What does botrytis mean in Australia?

Botrytis is a relevant Australian viticultural and winemaking issue, but its impact varies from place to place and vintage to vintage.

Australian Wine Research Institute guidance identifies prolonged rain or high humidity, combined with cool or mild conditions, as circumstances that can keep berries wet and encourage disease. Vineyard structure, grape condition and local weather also matter.

That variability is important when thinking about noble rot. The drying conditions needed for a favourable outcome may not arrive consistently after infection. In Australia, noble-rot results should therefore be understood as site- and vintage-dependent rather than as a uniform national characteristic.

Common misconceptions

“Botrytis is always desirable.”

No. Ordinary botrytis bunch rot is generally damaging. Noble rot is a particular favourable outcome under more specific conditions.

“Noble rot is a different organism.”

No. Noble rot is associated with the same fungus, Botrytis cinerea, but under different environmental and grape conditions.

“All botrytised wine is sweet.”

Not necessarily. Noble-rot grapes are commonly linked with sweet wines, but the final style depends on grape composition, fermentation and winemaking.

“Any mould or rot on grapes is botrytis.”

No. “Rot” is a broad consumer term. Other fungi and bacteria can also cause grape decay, so not every damaged bunch has botrytis.

The simple takeaway

Botrytis is best understood as a context-dependent grape condition. In wet, slow-drying conditions, Botrytis cinerea can cause grey mould and reduce grape and wine quality. If infection is followed by the right drying conditions, it can instead produce noble rot, concentrating berries for certain sweet wines.

When you see “noble rot” or “botrytised” in a wine description, think of a specific production outcome—not a guarantee that the wine will be sweet, honeyed or better.

Suggested FAQs

Is botrytis a yeast?

No. In this context, botrytis is a mould, usually Botrytis cinerea, rather than a wine-fermenting yeast.

Is noble rot the same as grey mould?

They involve the same fungus, but describe different outcomes. Grey mould generally refers to damaging decay, while noble rot describes a favourable dehydrating form of infection.

Does botrytis make grapes sweeter?

Noble rot can remove water from berries, concentrating sugars in the remaining juice. It does not simply create sugar, and not every botrytis infection produces noble rot.