Noble rot

This variety of ascomycete fungi naturally present in grapevines belongs to the Sclerotiniaceae family. This mould is therefore a necrotrophic parasite: it destroys the living cells of its host, the grape. It develops in autumn on the surface of grains, its mycelium then forming a whitish, ash-like fuzz... Giving rise to cinerea, meaning "gray" in Latin!

Phytopathogenic, this fungus evolves in two ways on the grapes: as grey rot, which is deleterious, or as noble rot, which is beneficial to the production of sweet white wines.

Divergence in Botrytis cinerea development is mainly dependent on grape berry condition and vineyard autumn climate.

Appearance of noble rot

Noble rot occurs only on healthy white grapes(intact skin) and :

• reached complete maturity on foot,
• exposed for 2 to 4 weeks to alternating wet, dry and sunny periods.

Cool nights, abundant dew and morning fog favor the development of the fungus. Its microscopic filaments pierce the grape skin. Then porous, it no longer fulfills its protective barrier function: under the heat of the afternoon sun, the water in the grain begins to evaporate, concentrating sugars, acids and aromas. Botrytized grapes are then said to be "full rotten": they keep their shape without bursting, and their color turns chocolate brown.

As skin decomposition progresses and the grain dries out, the fruit shrivels and shrivels. This wilting produces a "roasted", blue-purple grape ready for harvesting, whose sweetness kills off Botrytis cinerea.

Appearance of grey rot

Grey rot appears on more or less healthy grapes, white or red, subjected to excessive humidity.

A grape is considered damaged when its skin is not intact. This can be caused by heavy rain, hail, birds, insects or certain vine diseases.

The late harvest of botrytized grapes begins in September-October and can last until December. Botrytis cinerea grows heterogeneously on the berries of a single bunch. Harvesting therefore requires several passes, or "selections", in order to harvest only those grapes that have reached the desired maturity for the wine produced, which is different for each selection.

Yields are lower than for dry white wines, averaging 25 hectoliters per hectare (around 3,000 bottles).

While Botrytis cinerea concentrates sugars, it also forms glycerol, mannitol, ethanol, citric acid, acetic acid and lactic acid. After a vinification similar to that of dry white wines, but with a necessarily longer alcoholic fermentation, , the result is a wine with a residual sugar concentration in excess of 50 g/l : a "sweet wine", with a golden yellow or even amber color and characteristic aromas of honey, roasted dried fruit, spices and candied fruit.

Sweet white wines are mainly produced from the following grape varieties:

• Semillon Blanc and Sauvignon Blanc in the Bordeaux region (Barsac, Monbazillac, Sauternes, etc.),
• Gewurztraminer, riesling and pinot blanc in Alsace,
• Chenin in the Loire Valley (Quarts de chaume, Vouvray, Coteaux du layon, etc.).