Gray rot

Aside from being UV-protectants, in a number of species certain stilbenes act as phytoalexins. Resveratrol (6.24 trans-3,5,4 -trihydroxy-stilbene), its cA-isomer, as well as their glucosides and dehydrodimer mm-e- iniferin (6.25) are present in grape leaves and berries and play a role in the defense against gray rot caused by the fungal pathogen Botrytis cinerea. 

Several other varietal properties influence susceptibility to noble rot. Very thin cuticles and the compact grape clusters favor gray rot, whereas thick cuticles resist Botrytis attack (Ribereau-Gayon et ah, 2000). High stomatal number, which is variety- or even clone-dependent, favors infection by Botrytis (Pucheu-Plante and Leclair, 1990 Pulcheu-Plante and Mercier, 1983). 

The acids present in a given wine are determined by the grape variety, climate, presence of gray rot [Botrytis cinerea), yeasts, bacteria, and various treatments to which the wine may be subjected (sulfur dioxide, ascorbic acid, acidification, desacidifica-tion). There are at least 50 different acids in wine ranging in concentration from 1 or 2gl (tartaric, malic, succinic acids) to hundreds of mgl (citric, lactic, acetic acids) to tens of mgl (pyruvic, shikimic acids). However, tartaric, malic (depending on the MLF), acetic, and succinic acids constitute 80-90% of total complement of wine acids. 

Gray rot rarely occurs between frnit set and viraison. In 1983 and 1987, northern Enropean vineyards suffered early attacks, sometimes affecting up to 30% of the berries but the reasons for the loss of resistance in green grapes are still not known. 

In the first place, the green grape skin, covered by a thick cuticle, constitutes an effective barrier against parasites. Since Bonnet s (1903) initial research, a resistance scale of the principal Vitis species has been established based on the cuticle thickness of their respective berries. American varieties whose cuticle thickness varies from 4 p,m (Vitis rupestris) to 10 tim (Vitis coriacea) have better protected berries than European species (Vitis vinifera), whose cuticle thickness is from 1.5 to 3.8 p,m. This observation led to the production of V. vinifera and American species hybrids that are effectively more resistant to gray rot, but these hybrids do not usually produce quality grapes on the best terroirs. 

Under the cuticle, the exocarp also participates in the resistance to B. cinerea. According to Karadimtcheva (1982), the external layer of the hypodermis in certain varieties resistant to gray rot comprises more than seven rows of thin, elongated cells, with a total thickness exceeding 100 p,m. In sensitive varieties, it contains only four to six cell rows, with a total thickness of 50-60 p,m. 

The gray rot infection process by B. cinerea is identical to the noble rot process previously described, but early fungal development is difficult to detect on red grapes. The external development of the fungus is certainly the most characteristic trait of gray rot. The conditions leading to the death of the fungus in the case of noble rot do not occur. A mycelial felt forms on the surface of... 

The chemical composition of grapes is greatly modified in the course of a gray rot attack. All of the intermediary products between noble rot and gray rot can be encountered. 

In contrast to noble rot, which gives sweet white wines their specific qualitative aromas, gray rot often causes aromatic flaws. The grapes and wines obtained often are marked by characteristic mold or undergrowth odors. The responsible compounds are cuticular fatty acid (l-octen-3-one, l-octen-3-ol) or terpenic compound (unidentified) derivatives formed during pellicular maceration by the mycelial biomass (Bock et al., 1988). 

Exceptionally, B. cinerea can be observed to develop exclusively, forming a crescent-shaped mycelial mass to obstruct the fissure (Doneche, 1992). Changes in the chemical constitution of the grape are thus also characteristic of a gray rot attack. 

Various vine diseases alter grape crops. As a result, disagreeable tastes often appear in wine. Farly draining may help to lessen the severity of these alterations. Gray rot Botrytis cinerea) is a typical example. Certain vineyards are susceptible to gray rot, since the maturation period coincides with the rainy season. Fortunately, the pesticides currently available have greatly reduced the frequency of this disease. Botrytis has multiple effects on grape constitution and wine character (Chapter 10). Their impact influences maceration decisions. 

Even a relatively small percentage of botrytized grapes in the crop always seriously compromises the aromatic quality of dry white wines. Gray rot on white grapes results in a decrease in varietal aroma, a greater instability of fermentation... 

Botrytis enzymatic extract. The occurrence of gray rot is more detrimental to neutral variety wines, containing essentially fermentation aromas. 

Finally, gray rot seriously affects Ihe aromatic distinctness of dry white wines. Varietal aromas are masked while dusty, dirty and moldy aromas appear. It also promotes the development of rancid, camphorated and waxy odors, appearing later during maturation and especially in the bottle. This type of olfactory flaw is comparable to a premature oxidative aging of white wines (Volume 2, Section 8.2.3). Gray rot is not solely responsible and white wines made from healthy grapes can also contain the flaw. The responsible compounds and their formation mechanisms remain to be discovered. 

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