Main symptoms

White, circular, powdery to fuzzy spots appear on the leaves * (Figure 1). Usually, they thrive on older, lower, more shaded leaves. These spots multiply, merge (figures 2 and 3), and finally gradually cover the upper and lower surfaces of the leaf blade, leading to premature senescence of the leaves. They are in fact made up of the mycelium and conidiophores of fungi at the end of which chain conidia are formed (Figure 4). Ultimately, the entirely oidized leaves give the impression of being covered with talc (figure 5); they turn yellow, more or less necrotize before drying out and shrinking. Plants age prematurely and when attacks are early and severe, they have more limited growth.

* It should be noted that on species or varieties of Cucurbitaceae not very sensitive to powdery mildew, the symptoms will be atypical and will materialize by the development of yellowish spots in places where the fungus will have some difficulty in establishing itself (figures 6 and 7 ).

The petioles and stems can also be affected by powdery mildew (figures 8 and 9), the fruits are much more rarely, or even never for some species, despite strong attacks on the foliage. Fruits may still show reduced growth. Let us add that when many leaves have been destroyed, they are more exposed to solar burns and therefore show superficial lesions on one of their face. On melon, we can sometimes observe attacks on young fruits (figure 10), subsequently causing corky scars and deformations.

Finally, let us note that the powdery spots of powdery mildew sometimes take on a dirty white tint as they develop, and that they can be covered by various secondary fungi, more rarely hyperparasites. Thus, several saprophytic fungi of the phylloplane (leaf surface), powdery mildew hyperparasites, settle on powdery mildew colonies or on the underlying altered tissues. Mention may be made of: Stephanoascus sp., Ampelomyces quisqualis , Tilletiopsis spp., Verticillium lecanii , various Penicillium spp., Cladosporium spp., Acremonium sp.

Let us add that the mycelial colonies of powdery mildew also sometimes harbor tiny globular structures, brown to black and often going unnoticed: cleistothecia , sexual reproductive organs (or teleomorphic form) of powdery mildew (figure 11).