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Botrytis cinerea’s noble rot

Noble rot sounds like an oxymoron, right?

It is the name given to a singular infection produced by the fungus Botrytis cinerea on the grapes. Although it can cause disasters in agriculture, it is also very valuable.

First of all, we introduce the fungus

Botrytis cinerea is the name of the anamorphic* form of the fungus. While the teleomorphic* form is very strange in nature, the anamorphic one is extremely ubiquitous. The spores of this fungus are shaped like a bunch of grapes. In greek botrys means “group of grapes” and cinerea “ash”. When they accumulate, spores have a gray color that looks like ash. Its reproduction in this form is asexual through a kind of spores named conidia. These are borne on specialized stalks named conidiophores.

*In mycology, the science that studies fungi, the teleomorph is the sexual reproductive stage of a fungus and anamorph the asexual one. Fungi that have different forms may have different names. The teleomorphic form of Botrytis cinerea is Botryotinia fuckeliana.

B. cinerea is a phytopathogen, it can infect plants, nourish from their nutrients and use them for its own growth. In addition, it is not host-specific*, it can infect more than 200 species of plants. Therefore, infections produced by this fungus cause significant losses in different types of crops worldwide. B. cinerea is able to kill the cells of the plants through the growth of hyphae. These are branching filamentous that form the structure of many fungi.

*host-specific: parasite capable to infect only a certain specie.

Botrytis infections in viticulture

Normal Botrytis infection can cause grave damage in agriculture, as it is capable of infecting a broad range of plant structures (leaves, fruits, flowers, roots …). In viticulture this fungus can be a nightmare since the main target is usually the grape. Early infections originated in flowers or other tissues may lead to a latent infection. However, when acidity increases or the antifungal compounds in grapes decrease, the fungus can reactivate and develop. When this happens (usually under very humid conditions) a gray rot is formed on grapes, giving name to the infection.

Gray rot, but also noble rot!

It seems that this fungus only cause harm, but that is not entirely true. Under certain conditions, this infection can instead produce what is known as noble rot. Noble rot can increase the quality of the grape and the result are sweet wines with fruity aromas.


Grapes infected with botrytis

Image of infected grapes by Botrytis cinerea producing noble rot (Per davitydave (Flickr: P1010537.JPG) [CC BY 2.0], via Wikimedia Commons)

So, which conditions are necessary to produce a noble rot?

First steps of the infection are very similar in both cases. A penetration of the epidermis (skin) occurs. Some microfissures in the fruit allow the release of exudates, providing nutrients for the germination of conidia.

In general, fungi require moisture to grow, and Botrytis is not an exception. If moisture is constant throughout the day, the fungus can grow better and it will spread out and spoil the fruits. However, if moisture is high only during the first hours of the day (and after that the atmosphere becomes dry and hot), the fungus grows slower and only within the fruit grains, causing the noble rot. Consequently, these crops are usually located on shorelines.

The rest of the process is simple. The fungus consumes the water from the fruit so sugars are more concentrated. Therefore, wines produced with this grapes are sweeter since during the fermentation some sugars will remain in the wine without being fermented. 

The origin of these wines is full of legends. It remains a mystery why someone dared to make wine with rotten grapes for the first time as it appearance is still disgusting!


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