Description of the fungus

Erysiphe necator is an obligate parasitic fungus that can only be preserved on its living host. As for Plasmopara viticola , another biotrophic pest, it is transplanted in order to perpetuate it on young vine leaves or leaf discs (figure 1). In the case of powdery mildew, it should be noted that the challenge is much more difficult because the plant material must be sterile.

• The mycelial hyphae 4-5 µm wide of Erysiphe necator are hyaline and septate. Mononuclear cells develop appressoria multilobed which terminate the germ tube or develop laterally on hyphae. Each multilobed appressorium allows E. necator to attach to the surface of the host and emit a haustorium which penetrates the cells and thus ensures its nutrition. These two structures form a sucker; the haustorium is usually located in the center of the appressorium and enters the host's epidermal cells through a fine perforation. The creeping and branching mycelium leads to dense felting (Figures 2 and 3).
• The conidiophores resulting from asexual reproduction are partitioned, flexible at the base, often erect or even oblique, measuring 100 to 400 µm in length. They form at regular intervals, have a granular content and produce 3 to 12 conidia (Figures 4 and 5). The conidia are hyaline, cylindro-ovoid in shape, and distributed in chains. They measure 22-48 x 12-22 µm and do not form fibrous bodies ( fibrosin bodies ) present in other powdery mildews (figure 6).
• The cleistothecia globular and closed form on the surface of the host plant organs (Figure 7). At first colorless to yellowish, their color evolves towards intense yellow (following the accumulation of lipids), orange then black brown. The same is true for the diameter which is 10 to 20 μm in the juvenile state to reach 70-130 μm once well developed. or Appendages fulchers are produced by the outer cells of the ascocarp wall (Figure 8). These fine filaments, cylindrical and long (measuring one to six times the diameter of the cleistothecium), flexible and partitioned, are more or less curved into a butt at their end. Each cleistothecium (Figure 9) contains four to six asci ovoid measuring 40-75 x 25-40 µm, in which two to eight ascospores hyaline are observed (Figure 10). These have yellow colored oil droplets and have the following measurements: 15-25 x 9-14 µm (Figure 11).