Main symptoms

Pseudoperonospora cubensis grows mainly on the leaves of cucurbits. It causes leaf spots more or less wide which may present some common points but also some differences, this depending on the parasitized species and the climatic conditions in particular:

- whatever the species attacked, the spots are first wet, oily (figure 1), then they turn yellow (figures 2 and 3) and gradually necrosis (figure 4 and 5). A discreet greyish to blackish down develops on the underside of the limbus at the level of the injured tissues, showing the sporulation of P. cubensis (figure 7). If conditions are right, the spots can spread quickly;
- on some cucurbits, such as cucumber and zucchini , the spots are mainly angular because they are delimited by the veins, which gives the blade a mosaic appearance (figure 6). Thus, the leaves eventually show a mosaic of yellow and / or brown spots if the lesions are necrotic. These pseudo-mosaics should not be assimilated to those caused by viruses;
- for other cucurbits, the spots are more irregular in shape, rather rounded, as on melon for example. On this plant, the spots are moist at first, then quickly turn yellow and brown. Ultimately, a chlorotic border surrounds them. 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. 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 (figure 7). These gradually merge and entire sectors of the leaf blade dry out (Figure 8).

On watermelon , the leaf spots may or may not be angular, and brown to black in color. On the leaves of squash and pumpkin , they are said to have a rather orange hue than yellow.

It should be noted that when the climatic conditions are very favorable to its development, the mildew progresses very quickly on the plants and in the plots.