Botrytis cinerea: also known as Botryotinia fuckeliana. This fungal infection, is usually very damaging but may be welcomed depending on circumstances. Botrytis cinerea belongs to the Sclerotiniaceae family of the Ascomycetes class (which has over 1,600 genera and 64,000 species, including yeasts, truffles and Penicillium).
In the form of grey rot Botrytis cinerea is most undesirable. It thrives in wet, humid conditions on vigorously growing vines. It can affect buds and young shoots, turning them brown. Later in the season, it can form grey mould patches on the leaves and infect flowers. It may then become dormant until the grapes are developed. White grapes will turn brown and black grapes may take on red tones. The grapes are covered in a grey or grey/brown mould and berries can split. Grey rot will result in off-flavours in wine, partially owing to the modified chemical composition of the grapes. However, its effect on black grapes is greater than white because it causes loss of colour, tannins and flavour. Outbreaks at harvest time (often induced by rain) can wreck a vintage - there will be a substantial loss in yield and a serious impact upon quality. The image on left below shows Botryris affected grapes. The one on the right is of grapefruit spray being used as a method of prevention
In its benevolent form, Botrytis cinerea occurs as 'noble rot' on some varieties of white grapes when certain climatic conditions occur. The ideal conditions are damp, misty early autumn mornings giving way to very warm, sunny afternoons. Some of the great sweet white wines of the world are produced from affected grapes.