Oidium neolycopersici causes leaf spots quite characteristic of powdery mildew (Figures 1 and 2). These are powdery and white and rather cover the upper surface of the tomato leaflets (Figures 3 to 6). This white felting is in fact made up of a mycelial network superficially colonizing the lamina, surmounted by numerous conidiophores producing isolated hyaline conidia or sometimes in pseudo-chains of 4 to 6 spores when the relative humidity is high.

Locally, at the level of the spots, the affected tissues become chlorotic , turn brown locally and eventually necrosis (Figure 7). In severe attacks, the entire leaf blade may be covered by the mycelial network of the fungus and some leaflets turn yellow and completely necrotize.

Some spots comparable can be observed on the stem . The fruits do not appear to be affected .