Grapevine - Bunch rots * and unfavorable compounds

**Bunch rots * and unfavorable compounds**

Fungus-earthy wine

Several microorganisms attached to grape berries, in particular responsible for rots, can produce various compounds that will be found in wines: compounds at the origin of aromatic defects with musty, earthy and mushroom characteristics, toxic compounds called mycotoxins.

Main known fungus-earth compounds

The geosmine (trans-1,10-dimethyl-trans-9-decalol) was predominantly involved in the main deflection earthy aromatic encountered several French vineyards (Bordeaux Beaujolais, Val de Loire, Burgundy, etc.). This compound is already present in the field in the rotten clusters, mainly concentrated in the internal berries of the cluster. It is rarely found in the outer berries and in the stalk. At the level of the berry, it is the skin that contains it mainly, the contents noted in certain clusters (8000 to 36000 ng / kg) suggest that a small quantity is enough in the harvest for the wine obtained afterwards. winemaking has an earthy character. It is also found in musts and wines (alcoholic fermentation having only a slight impact on its content in the solution). The concentrations measured in the wines are above its perception threshold which is 60 ng / L.
Geosmine is not the only compound at the origin of all the emerging faults of a musty earthy or even fungus nature detected in the various French production areas. Other potential compounds are found:

2-methylisoborneol (MIB) (musty-earthy), 2,4,6-trichloranisole (TCA) (corky taste), 2,3,4,6-tetrachloroanisole (TeCA), 2-3.5 -dimethylpyrazine (musty taste of certain corks),.
oct-1-en-3-ol (fungal odor of Basidiomycetes), and other C8 alcohols and ketones that cause fungal odors, including oct-1-en-3-one.

Mycotoxins

Among the 185 species of Aspergillus recorded, Aspergillus niger and Aspergillus carbonarius (widely distributed in the world) are recorded on the vine, but also A. alliaceus Thom & Church (1945), and A. fumigatus Fresen (1863). Note that A. carbonarius produces ochratoxin A, a mycotoxin found in several food products, and in particular in wine. This fungus seems to find climatic conditions that are probably more favorable to its development in Mediterranean vineyards.

In the species Penicillium reported on grapes, several are known to produce mycotoxins which pose problems in the food industry in products such as, for example, fruit juices and wines or even dried fruits. For example, two toxins are synthesized by P. expansum : patulin, a mycotoxin quite common in animal feed and citrinin. Only the first seems to be detected in apple and grape juice.

Alternaria alternata , which is the most reported on the vine, synthesizes at least 3 compounds: alternariol, alternariol monoethyl ether and altertoxin. Several of these compounds are sufficiently stable to be found in fruit juices in particular.
The mycotoxin of Trichothecium roseum , a trichothecene, can be found in wine.

Aureobasidium pullulans has been reported from Canada, Ontario, where it causes problems during the fermentation of icewines.

* The mycoflora of healthy and rotten berries

A fairly diverse mycoflora is found on healthy and rotten berries made up of fungi fairly classically associated with plant organs ; they belong to the genera Alternaria , Stemphylium , Ulocladium , Epicoccum , Aureobasidium , Pestalotia, etc.
The genus Penicillium is particularly well represented on healthy berries as well as on rotten berries. The different species present are responsible for blue rot ( P. expansum ) (figure 2) and / or colored molds developing on and in the heart of bunches affected by gray rot, this in complex with B. cinerea (figures 2 and 5 ).
We insist on the fact that these fungi are frequent in certain production regions, but that they often go unnoticed in the heart of botrytised clusters. The opening of several clusters makes it easier to appreciate the phenomenon. In addition, let us also underline the strong association demonstrated in the vineyard between the presence of geosmin in the heart of the clusters, the more or less significant development of B. cinerea on them, and the secondary invasion of the latter by Penicillium. spp. *, and in particular by P. expansum .
We also find all the other fungi responsible for the rots described above, such as Aspergillus from the Nigri section, Rhizopus stolonifer , Trichothecium roseum, etc.
Several of them have the potential to produce compounds with fungus, moldy and earthy characteristics.

Exemple d'species of Penicillium signalees on Vigne sont: P. aurantiogriseum , P. brevicompactum , grams of flesh , P. chrysogenum , P. citreonigrum , P. citrinum , P. season 's, P ** spread , P. glabrum , p herqueii **, p miczinskii , P. minioluteum **, p pinophilum , P. purpurescens , P. purpurogenum , P. roqueforti , P. rubrum , p * simple , p spinolusum *, p usual ssp. crustosum , P thomii , grams variable , P fox , etc
** species most frequently isolated in French vineyards.