James Hook answers one of the common winemaking questions using his experience in Viticulture (Adelaide Uni) and wine (WSET).
Botrytis is, in almost all cases 'bad' for wine grapes. There is on very specific wine style that is can help, called "Noble rot".
Firstly "Noble Rot" and "Botrytis Bunch Rot" in grapes are both caused by the same organism - Botrytis cinerea. Often in photographs both rots will look the same the difference being one is considered a problem the other a desirable trait!
Grapes are susceptible to this fungus as they ripen and produce sugar. Generally it causes a bunch rot that is bad for grape quality, it turns grapes mouldy, as mention above commonly known as "Bunch Rot", "Botrytis Rot" or "Grey Rot". It also creates conditions favorable for the growth of other nastiness like yeast, mould, and bacteria are we call secondary rots. These rots are even worse for grape quality with some being toxic!
The beginning of a Botrytis infection in Sauvignon Blanc.
Under certain ideal microclimatic conditions the fungus causes "Noble Rot", which is responsible for the production of some of the world's finest sweet white wines, and not as severe as what is pictured above. "Noble Rot" dries out fruit rather than turns it mouldy. The key is the weather conditions during infection and then what occurs as the fruit matures.
Development of Noble Rot
Temperature and humidity are the two critical factors influencing the development of "Noble Rot". During the botrytis infection phase, a temperature of 20-25°C and a relative humidity of 85-95% for a maximum of 24 hours are considered desirable. Once the infection has occurred the relative humidity should drop below 60%. This drop in humidity is a key factor in dehydration of the infected berries.
In Sauternes, France botrytis slowly develops into noble rot on ripe grapes and gives the wine unique aromas, colour, and flavour. Note the golden colour of grapes changing to grey as the infection increases - http://www.myquem.com/noble-rot/
During the course of development the botrytis penetrates the grape skin. The skin becomes permeable but does not split. This condition facilitates drying of the berries. The loss of water from the berries leads to the concentration of sugar and other constituents like flavour. The osmotic pressure inside the berry increases, consequently the metabolic activity of the fungus decreases. The limited activity of this mould causes certain changes in the fruit which enable vintners to produce unique and prestigious sweet white wines.
In simple terms the botrytis only infects the grape on the surface and sucks the water out of it leaving a concentrated berry behind that makes a unique wine.
Development of Bunch Rot
Following infection by Botrytis, if the relative humidity remains high, and drying of the berries does not occur, the fungus continues to grow and produce certain undesirable changes in the fruit. The berries swell and burst. This splitting of the berry makes it susceptible to attack by other spoilage organisms, especially molds and acetic acid bacteria.
Botrytis on Shiraz in McLaren Vale 2011. Note the severe rot. High levels of botrytis in red grape varieties cause unstable wine fermentation and other taints that are not desirable.
How do winemakers make Noble Rot wine?
The natural weather helps.
Sauternes is probably the best known region that makes wine botrytis. Botrytis takes advantage of autumn weather patterns specific that region. The Sauternes region is located 40 km (25 mi) southeast of the city of Bordeaux along the Garonne river and its tributary, the Ciron. The source of the Ciron is a spring which has cooler waters than the Garonne. The different temperatures from the two rivers meet to produce mist that descends upon the vineyards from evening to late morning. By mid day, the warm sun dissipates the mist and dry the grapes to keep them from developing less favorable rot. The humidity is high in the morning and low during the day.
The Ciron genteelly meeting the Garonne.
Other regions have weather that replicates this pattern of morning mists then dry weather. Internationally renowned botrytised wines include the aszú of Tokay-Hegyalja in Hungary and Beerenauslese or Trockenbeerenauslese wines from Germany and Austria.
Botrytis has also been imported for use by winemakers in California and Australia. In some cases inoculation occurs when spores are sprayed over the grapes, while some vineyards depend on natural inoculation from spores present in the environment. Growers are then relying on dry weather with low humidity to 'hold' the botrytis infection and keep it at a Noble Rot level.
References
Ribereau-Gayon, P. 1988. Botrytis: Advantages and Disadvantages for Producing Quality Wines. Proceedings of the Second International Cool Climate Viticulture and Oenology Symposium. Auckland, New Zealand, pp. 319-323.
Yquem website - http://www.myquem.com/noble-rot/