Erysiphe necator pig. 1834

Powdery mildew

Introduced less than two centuries ago in Europe (in England in 1845), this pathogen led to lightning attacks and crop losses reaching for example more than 50% in 1852. Already present in France in 1951, this fungus reveals a worldwide distribution. It should be noted that its attacks on berries are sometimes the cause of unpleasant tastes in the wine, in addition to the increase in the acidity of the juices generally linked to the delay in maturity of the grapes. Repeated and severe attacks on grapeviness reduce the vigor and productivity of the grapeviness, and can lead to their death. The observed yield reductions can be attributed to a reduction in the weight of berries and twigs.

Erysiphe necator is an obligate parasitic fungus which is strictly subservient to the genus Vitis . It was described in 1834 by Schweinitz in North America. Two groups of strains A and B (sometimes also called biotypes) were characterized a little over ten years ago in this species (1997), differentiating genetically and phenotypically:

• group A strains isolated mainly in the grapevinesyards of the Mediterranean rim (mainly on Carignan grape), with however recent detections in northern vineyards. They persist during the winter mainly in the form of mycelium sleeping in the buds and are therefore responsible for a large part of the powdery mildew symptoms of the "flag" type observed especially in the vineyards of the South.
• group B strains overwinter in the form of cleistothecia, but they can also form dormant mycelium in the buds, and thus induce "flag" symptoms at the start of the season. It should be noted that it is in this group of strains that the most numerous cases of resistance to fungicides are identified.

The existence of these two genetic groups of strains is not trivial although their exact role is not yet fully defined. Let us add that the genetic diversity of E. necator on its continent of origin, America, is much greater than in Europe: at least ten groups of strains have been identified.

Classification : Fungi, Ascomycota, Leotiomycetes, Leotiomycetidae, Erysiphales, Erysiphaceae,
Synonymy : Uncinula necator (pig.) Burrill, (1892)
English name : powdery mildew

Another powdery mildew on the grapevine

Another species of powdery mildew, Phyllactinia guttata (Wallr.) Lév.] (Hariot 1907), has been occasionally reported in French-speaking Switzerland on grape berries of the Gamay variety severely attacked by Uncinula necator. We also observed this species on grapeviness in Bordeaux during the 2000s. P. guttata has cleistothecia measuring between 216 and 245 µm in diameter. They develop 8 to 12 vitreous appendages with a length between 191 and 290 µm. These appendages are characterized by a bulge at their base which is typical of this species. The cleistothecia contain 4 to 5 asci which themselves contain 2 to 4 ovoid ascospores.

How to differentiate cleistothecia
of the two ascomycetes associated with grapevine powdery mildew

Erysiphe necator	Phyllactinia guttata