Highlights
- Botrytis cinerea is a fungus that can affect wine grapes.
- The fungus can cause destructive grey rot—or, under suitable conditions, the desirable result called noble rot.
- Noble rot usually needs moisture or humidity followed by drying conditions.
- As affected grapes lose water, their remaining sugars, acids and flavour compounds become more concentrated.
- Botrytis does not automatically make a wine sweet, high-quality or suitable for winemaking.
- Noble rot is different from drying grapes through appassimento, leaving grapes to ripen late, or freezing grapes for Icewine production.
- In Australia, the Riverina is a recognised context for Botrytis Semillon.
Quick answer: what are Botrytis and noble rot?
Botrytis cinerea is a fungus that infects grape berries. Depending on the weather, timing and condition of the fruit, it can cause destructive grey rot or contribute to the winemaking outcome known as noble rot.
Noble rot is not simply “good mould”. It is a carefully managed form of Botrytis infection in which berries lose water, become concentrated and develop distinctive flavour chemistry while remaining suitable for selective harvesting and pressing. Moist or humid conditions can help the fungus develop, but drying conditions afterwards are important. If wetness continues, the fruit may collapse into destructive bunch rot instead.
Noble-rot grapes are often used for sweet wines because their juice can become highly concentrated and fermentation may stop with residual sugar remaining. However, Botrytis itself does not guarantee sweetness. The final style depends on the fruit, harvest decisions and winemaking.
What does Botrytis do to wine grapes?
Botrytis can enter grape berries through natural openings, tiny skin injuries or damaged tissue. An infection may remain quiet before becoming active as grapes soften and their natural protection declines.
When conditions favour noble rot, the fungus damages the grape skin and contributes to dehydration. Water leaves the berry, concentrating the juice’s remaining sugars, acids and other components. The fungus also changes the berry’s aroma and flavour chemistry.
This is why botrytised wines can be concentrated and distinctive. Possible descriptions include honeyed, dried-fruit, apricot-like, rich or viscous. These are possibilities, not guaranteed tasting notes. The grape variety, infection stage, vintage and winemaking all affect the result.
Botrytis can also create serious risks. It may contribute to mouldy characters, browning, oxidation and problems with clarification or filtration. A vineyard can contain a mixture of suitable noble rot, overripe fruit, destructive rot and other contamination. Appearance alone does not establish whether grapes are appropriate for a wine.
Noble rot versus grey rot
The same fungus can lead to very different outcomes.
Noble rot generally involves infection followed by drying conditions. Berries dehydrate and concentrate, and suitable fruit may be picked selectively for a planned wine.
Grey rot or Botrytis bunch rot is destructive. Persistent wetness can encourage tissue breakdown, spreading decay and secondary infections. Fruit may leak, collapse or contribute unwanted characters to wine.
The boundary is practical rather than a promise that every berry fits neatly into one category. Weather, berry condition, bunch structure and harvest timing all matter. That is why “Botrytis” and “noble rot” should not be used as interchangeable terms: Botrytis is the fungus; noble rot describes a desirable, managed outcome.
How is botrytised wine made?
Making wine from Botrytis-affected grapes can be demanding.
First, vineyard and winery teams assess the spread and condition of the fruit, along with the weather and likely harvest timing. Suitable berries may be selectively harvested, while severely degraded or mould-affected fruit is excluded.
Dehydrated berries can be difficult to press and may produce relatively little concentrated juice. During winemaking, Botrytis-associated compounds can make clarification and filtration more difficult. The fungus can also increase the risk of browning and oxidative spoilage, so oxygen exposure becomes an important consideration.
Highly concentrated juice may be challenging to ferment. Some wines retain residual sugar, but this is an outcome of the fruit and fermentation—not a definition of Botrytis. A wine can be botrytised without being sweet, and a sweet wine is not automatically botrytised.
For the wider winemaking sequence, see How Wine Is Made. For the finished style, see Botrytised Wine: that page covers the wine, while this one explains the fungus and noble-rot process.
How does noble rot compare with other grape-concentration methods?
These methods can all concentrate grape juice, but they do so in different ways.
| Method | What creates concentration? |
|---|---|
| Noble rot | A suitable Botrytis infection and the dehydration that follows |
| Late harvest | Leaving grapes on the vine longer to reach greater ripeness or concentration |
| Appassimento | Drying or partially drying grapes |
| Icewine | Pressing grapes that have naturally frozen on the vine |
Late harvest describes when grapes are picked; it does not mean they have noble rot. Appassimento is a drying process and does not require Botrytis. Icewine relies on natural freezing and does not require either Botrytis or drying.
For more detail, explore Appassimento, Icewine production and Late-harvest winemaking.
The Australian example: Riverina Botrytis Semillon
For Australian wine learners, the Riverina provides a well-established example of Botrytis in wine production, particularly through Botrytis Semillon.
In suitable years, still or humid conditions during ripening can support Botrytis development, while the right drying pattern can allow grapes to become concentrated rather than collapse into destructive rot. The result can be a concentrated, sweet Semillon dessert wine.
That does not mean every Riverina Semillon is botrytised, or that every vintage develops suitable noble rot. Botrytis is weather-driven, so the outcome varies with conditions and harvest decisions.
You may encounter “botrytised”, “Botrytis” or “noble rot” in Australian wine education and descriptions of sweet wines. Treat these as terms to understand, not as proof that every sweet wine was made using noble rot.
FAQs
Is noble rot safe to assume from a mouldy-looking grape?
No. Noble rot is a managed winemaking outcome, not a general description for mouldy fruit. Fruit condition, weather, microbial context and winery assessment all matter.
Does noble rot always make wine sweet?
No. Botrytised grapes are often used for sweet wines because water loss concentrates their juice, but sweetness depends on fermentation and winemaking decisions.
Is Botrytis the same as oxidation?
No. Botrytis is a fungus. It can increase oxidation risks during processing, but it is not the same thing as oxidation in wine.
Is all sweet wine botrytised?
No. Sweet wines can be made in many ways, including late-harvest, appassimento, Icewine and other methods. Botrytis is only one possible route.