Erysiphe necator is an ectoparasite of grapevine that grows largely on the surface of green aerial organs. Its presence on the latter is materialized by a more or less powdery conidial felting at the origin of the Anglo-Saxon name of the disease powdery mildew and characterizing these symptoms.

 

Several symptoms can be observed at the beginning of the season on the vegetation of the vine:
- small scattered, slightly chlorotic, downy spots developing on the underside (figure 1), then the upper side of the leaves (figure 2). These spots, often consecutive to primary contaminations by ascospores, spread and become more and more powdery (figure 3). There are islands of necrotic brown cells. The strongly attacked foliage shows more or less deformed leaves (figures 4 and 5) and takes on a grayish tint, giving the impression of being covered by soil dust;
- young twigs showing a few spots at first which evolve rapidly. They eventually become covered with powdery mildew and then take on a dirty gray tint, are often stunted and bear twisted leaves (Figure 6). In this case, we are in the presence of the "flag" symptom which is frequent in the vineyards of the south and on grape varieties such as Cinsaut and Carignan. These spectacular spring attacks are initiated by the dormant mycelium present in the buds.

Whatever the powdery mildew facies, the affected leaves are often more or less deformed and sometimes chlorotic; in severe attack, they can dry out and drop.

 

Young twigs are also colonized by E. necator . Covered by its mycelium, they present spots, even irregular and ill-defined areas more or less powdery. The hardening of oidized twigs is often partial and in the fall, the areas affected by E. necator take on a brownish to blackish tint that is always visible during the winter (Figures 7 and 8).

 

The inflorescences and berries of grapes can be attacked from flowering to closure of the bunch (Figures 9 to 10). If the former are heavily infected, then there is significant sagging. The berries become covered with powdery mildew which gives them a gray-green tint, their growth is greatly slowed down and their skin becomes thicker and corky. Bursts (figure 11) may appear, revealing the pips. Let us add that these wounds favor contaminations of Botrytis cinerea and microorganisms responsible for acid rot .

The severity of powdery mildew attacks on berries is very dependent on the intensity and precocity of the infections occurring after flowering. It should be added that the development of E. necator is gradually reduced as the sugar and anthocyanin content of the berries increases.