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Botrytis

botrite (I)
botrytis, pourriture noble (F)
grauwe schimmel (N)
botrytis bunch rot (GB)

Mostly used short name for Botrytis cinerea (synonym Botryotinia fuckeliana) for the mould fungus from the genus Botrytis, which is called grey mould rot (also grey rot, grey mould, sour rot) or positively occupied as noble fungus or noble rot. It belongs to the tubular fungi and spreads vegetatively via the so-called conidia (spore form). Depending on the stage of ripeness or the infested areas on the vine, it is also known as botrytis, stem botrytis or grape botrytis. It was already described in the 18th century and occurs in all temperate climate zones of the world. Wine-growing areas with geographical and climatic conditions that are particularly favourable for the infestation are Sauternes (France), Rheingau (Germany), Lake Neusiedl (Austria) and Tokaj (Hungary). Over 200 host plants are known. Particularly affected are flower bulbs, vegetables, ornamental plants and vines. In young grapes the infestation has a necotrophic (lethal) effect, in older biotrophic (host organism stays alive longer).

Botrytis - Anfangsstadium bei einer Rieslingtraube

In the vine, all parts except the trunk and perennial wood are affected, but flowers (if the fungus is present at this stage) and berries are preferred. Because on these there is a sufficient supply of sugar and nitrogenous compounds. The name grey mould is derived from the characteristic grey fungal turf that covers the infested plant parts. In the case of heavy grape infestation, it can be observed that a grey cloud consisting of the fungal spores rises when the harvest containers are emptied. In viticulture, botrytis is feared on the one hand, but on the other hand it is also very desirable "at the right time". Under appropriate environmental conditions, the noble rot, which is mainly desired in white wine varieties, develops. This is partly a prerequisite for noble sweet white wine specialities such as Ausbruch, Auslese, Beerenauslese and (a must under wine law) Trockenbeerenauslese. Due to their high sugar content, Botrytis wines belong to the category of sweet wines. With these, the caramel-like botrytiston (also brittle) is expressly desired.

The prerequisite is a late infestation of already ripe berries from about 80 °Oechsle (16 °KMW) in dry autumn weather. Above all, prolonged rainfall from the end of August offers optimal conditions. Ideal are dry and cool nights, in which the fungus growth slows down, and with 20 to 25 °Celsius warm and humid days, in which the fungus grows faster again. The fungus attacks individual berries and spreads very quickly from there to the whole grape. A second source of infection is infected, dead flower remains inside grapes. Infections often originate from feeding sites of the caterpillars of the grape berry moth, through which the fungus can easily reach the inside of the berries. All botrytis species live as parasites in the tissue of infected plants. This leads to progressive decay or cell death of the tissue (rot).

Botrytis - drei Weintrauben in verschiedenen Stadien

The skin of the berries is interspersed with tiny pores through which the water very slowly escapes. The ingredients are concentrated and the berries dry out like raisins. Tartaric acid and malic acid are reduced, but the formation of gluconic acid and sugars is promoted. Botrytis infestation always causes a chemical change and also destruction of the variety-specific aroma substances. Nitrogen compounds necessary for fermentation are reduced, so that part of the sugar remains unfermented. Metabolic products that change the taste of the sugar are produced. Octenol gives the wine a typical musty mushroom note of mushrooms and moist forest soil, while sotolon gives it a sweet, caramel-like taste. Anthocyanins and other phenols are oxidized by certain fungal enzymes and transformed into brownish polyphenols. Already only 10% of infested red wine grapes cause a visible change in the colour of the wine. This is why botrytis is usually undesirable in red wine varieties (although there are a few producers).

As a so-called grey rot (French: Pourriture grise), Botrytis causes great damage in the vineyards when the still unripe berries are attacked by it. This is, so to speak, a "noble rot at the wrong time" and then causes enormous damage. Mostly it starts in the middle of the grape and is transmitted from berry to berry. The mould spreads in the fruit flesh, breaks through the berry skin and the grapes rot due to wet rot. No more sugar can be stored in the destroyed berry and they remain small and sour. This is why it is also called sour rot. The berries turn pink and purple and finally turn brown. The berries can also be attacked by Acetobacter (acetic bacteria). Wine produced from this has an unpleasant, stuffy taste; red wines have a pale, grey-brown colour.

Botrytis - Endstadium für Trockenbeerenauslese

A preventive measure is the most comprehensive removal of the basis for infection and spread. Through appropriate training and pruning and the resulting good ventilation, the grapes can dry out quickly after rainfall. In this way, the fungus is deprived of the moisture base that is important for its growth. Grape varieties with dense foliage (drying after rain takes longer), as well as those with dense berries (the narrower the berries, the faster they spread) are more at risk. These are, for example, Gewürztraminer, Kerner, Müller-Thurgau, Pinot Noir and Scheurebe (seedling 88).

A new method is the spraying of grapes with antimicrobial substances to obtain loose berry-like grapes (see Phytoalexins). Spreading is also favoured by excessive nitrogen and magnesium deficiency in the soil. Therefore, attention is paid to this fact when fertilizing. Also the feeding damage of the grape berry moth favours the spreading, so that a reduction is necessary. In addition, control is carried out by means of appropriate fungicides; this may have to be carried out several times a year. When cultivating rootstocks or new varieties, special attention is paid to good resistance (hardiness) to botrytis or fungi in general. See also under vine enemies.

Riesling: From Tom Maack T.o.m. CC BY-SA 3.0, Link
Botrytis grape on the left: Valaiswine - own work, CC BY-SA 3.0, link
Botrytis grape middle: unknown
Botrytis grape right: From Uschi Dugulin on Pixabay

Hand with grapes: © DWI (German Wine Institute)

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