Botrytis (gray mold) of the vine is a disease much feared by winegrowers because of the damage it causes and its impact on wine quality.
Symptoms:
Before August, brown spots appear on the twigs, which turn white after hardening, and black blisters appear. Twigs are generally parasitized following wounds. On leaves, symptoms often appear on the edge of the leaf blade (the broad, flattened part of the leaf), with the appearance of spreading necroses giving the leaf a burnt appearance. In wet weather, a gray felting appears around the edges of the spots. Attacks on leaves generally have no economic impact.
On bunches, during flowering and tying, symptoms are visible on the inflorescences and cluster stalks (the woody, peduncular support of the bunch on which the grapes hang), which show brown spots. During sprouting and as harvest approaches, the berries take on a characteristic rotten appearance, with the development of a typical gray mold. Attacks result in a loss of quality for future wines, due to the degradation of coloring matter, destruction of the film containing aromatic substances, increased S02 fixation in wines and higher volatile acidity.
Botrytis is mainly visible from veraison onwards (the time of year when the grapes swell), and its development takes place through the creation of new foci and then the extension of existing ones (contamination of berries from one to the next). At this stage, and right up to harvest, the disease's development is strongly conditioned by meteorology, particularly rainfall, especially as the berries approach ripeness.