Noble Rot is the beneficial form of a grey fungus, Botrytis cinerea, affecting wine grapes.
It predominantly occurs on ripe vines when the weather remains wet, as infestation by Botrytis requires moist conditions. If grapes become infected but are then exposed to drier conditions and become partially raisined, it is known as noble rot. Grapes picked at a certain point during this ‘infestation’ can produce particularly fine and concentrated sweet wine.
Although not ideal, due to the blending process most houses implement, Noble Rot is less of an issue from flavour perspective for Brandy and Cognac, although they carefully monitor their vineyards as it can affect yields.