Mouldy fungus (Botrytis cinerea). It makes the berry skin of ripe grapes permeable, it spreads and it consumes relatively little sugar, but acid. This shifts the ratio in favour of the sugar. Due to evaporation of water through the rotten skin, a significant concentration of the juice occurs. The noble rot, that is caused by botrytis, is needed in order to produce wines like Auslese, Beerenauslese and Trockenbeerenauslese.