Main symptoms

Pseudoperonospora cubensis only grows on the foliage of cucurbits causing mainly leaf spots. In France, these are observed on cucumber, melon and more recently on zucchini. Downy mildew is a disease that should be diagnosed early because its epidemics, like those of many other mildews on other plant species, are rapid and very damaging.

On melon, the spots leaf are first wet (Figures 1 to 3), then they turn yellow (Figures 2 and 4), turn brown, and rapidly necrosis (Figures 3 and 5). Ultimately, a chlorotic border surrounds them (Figure 4). These spots are sometimes initiated near the veins which turn brown over a more or less limited portion. During periods of high humidity, affected tissue can break down and fall off.

The distribution of spots on the leaf blade may resemble that of a "checkerboard" and this symptom can be misinterpreted as a patchwork mosaic (Figures 6 and 7).

When conditions are favorable, felting varying in color from light gray to dark mauve covers more or less the surface of the spots on the underside of the leaf blade (Figures 8 and 9).

In addition, late blight progresses very rapidly on leaves, plants and in plots. The melon leaves dry out completely and remain attached to the plant, erect and rolled up (Figures 10 to 13).