CropsMain symptoms
On vines, Botrytis cinerea is especially damaging on grape berries. This does not prevent it from attacking the buds and young shoots in early spring, leading to their necrosis and death. Infections are rather rare on the twigs during the spring growth phase, except when they show wounds or if they come in contact with infected tissue. Note that B. cinerea more easily invades young and tender tissues than hardened and lignified twigs for which the presence of wounds will be necessary. The twigs often become covered with mycelium and eventually crack or even break. It is not uncommon to see sclerotia developing on twigs that have fallen to the ground. Let us add that in the fall, badly augured shoots are susceptible to attack by this fungus. Then follows a bleaching of the wood, on which develop either a gray mold, or sclerotia having the appearance of black, oval crusts, 1 to 5 mm in diameter.
- Leaves
It produces leaf spots when the weather is wet and cool. The leaves then show spots often located at the edge of the blade, circular to irregular in shape, and initially moist (figure 1). They gradually spread and eventually become quite large, necrotic and reddish-brown in color (Figure 2). Sometimes the entire leaf blade can be affected by rot, sometimes reaching the petioles and young twigs later. Let us add that a more or less dense gray mold can be observed on certain areas of the damaged tissues.
- Inflorescences
B. cinerea can rot and dry out inflorescences which eventually fall off (Figure 3). For this, it colonizes the senescent petals which are particularly vulnerable. It uses the latter, but also other senescent parts of flowers or aborted berries as a nutritive base initially, in order to ensure subsequent infections on pedicels, rachis, and on fruits in particular. Raiding attacks are quite specific; they result in the development of a brown lesion, blackening and gradually spreading. Eventually, the peduncle (figure 4), the rachis, some pedicels end up being girdled, becoming non-functional. In this case, the downstream grape berries soon wilt, shrivel and fall off. These symptoms are called stem rot.
- Bunches and berries
The grape berries are receptive mainly after veraison. However, contaminations can occur on berries that are still green (figure 5). From veraison and at maturity it can take place directly through the cuticle or from various wounds. Several symptoms can be observed on berries:
- Tiny punctate lesions appear on the film; moist to brown, they are similar to insect bites, hence the name “bite” to characterize this symptom on table grapes (figure 6). Subsequently, these very localized lesions give rise to a circular beige to brown, even purplish alteration, evolving rapidly and radially mainly at the level of the skin of the bay. The latter ends up splitting in the center or on the periphery of the spots, or detaching as a result of friction. The juice flows then, this symptom is at the origin of the trivial name of "pissy berries", always on table grapes. Note that gray mold can form in the cracks and cover the tissues thus released (figure 7).
- Whole berries can rot quickly, taking on a brown to beige tint, even purplish on white grape varieties (figures 8 and 9), and dark to dark purplish on black grape varieties (figures 10 and 11). If the weather is dry, the rotten berries will eventually dry out and stay on the bunch. If the weather is wet, the berries may burst and the rot spread. In fact, gray rot can subsequently spread by contact from diseased berry to healthy berry until it reaches the whole bunch. This last symptom particularly affects very compact clusters on which the transmission of diseased berries to healthy berries is very important (figure 12).
- Signs
Whatever organs are affected, dying tissues become covered with a very characteristic dense gray mold (figure 13), made up of the numerous conidiophores and conidia of the fungus (figure 14). Note that on botrytis clusters, in the heart of the latter, other molds develop, forming colonies of various colors associated with or overcoming the gray mold (see other rots of the bunch) (figures 15 to 17).
We recommend that you also consult the Periods of onset of symptoms of B. cinerea on leaves and clusters .
- Organoleptic impact
Remember that this fungus is very damaging to the quality of grapes and wine. In addition to a drop in yields, it causes fermentation difficulties or even fungal and moldy odors.
When it invades wine grapes, the fungus converts simple sugars (glucose and fructose) into glycerol and gluconic acid and produces enzymes that catalyze the oxidation of phenolic compounds. In addition, the secretion of polysaccharides such as β-glucan hinders the wine clarification process. The wines produced from rotten grapes therefore have little flavor and are found to be sensitive to oxidation and bacterial contamination; they will therefore bear aging poorly.
Note that under certain specific climatic conditions, Botrytis cinerea can give rise to "noble rot". This is a slowly progressing rot that affects white grapes and that some winegrowers use to make sweet wines with particular aromas with a high sugar level, such as Sauternes, Tokay