Main symptoms

Plasmopara viticola develops exclusively on the aerial herbaceous organs of the vine, and more particularly on the youngest in the growth phase and saturated with water. It does not attack the hardened tissue in any way.

• Leaves

Two types of lesions can be observed depending on the time of year and / or the quality of the leaf tissue:
- more or less circular and extensive spots, at first translucent to oily, yellowing quite quickly, then necrotizing (mostly on young leaves) (figures 1 to 3). A more or less dense white down forms on the underside of these spots (figures 4 and 5); it is made up of the sporangiophores and the sporangia of P. viticola . Measuring up to 1 cm and sometimes confluent, these spots can cover almost the entire blade and cause the leaves to dry out and drop, the vines becoming heavily stripped (figure 10);
- spots of limited extension, polygonal in shape because limited by the veins, more or less chlorotic to necrotic (rather on old leaves). This rather particular facies, which appears later, is at the origin of the name: " mosaic mildew " or "tapestry stitches" (figures 6 to 8).

Under favorable climatic conditions, the rapid development of mildew on the plant leads to the destruction of many leaves (figures 9 and 10), and many vines are affected, first divided into foci before the plot is fully affected ( figure 11).

• Twigs, petioles and tendrils

The portions of young twigs affected show superficial longitudinal lesions, brown in color, which can lead to deformation of the latter. Cracks can appear on these lesions, and P. viticola can also fruit on the surface. Similar changes can form on tendrils (Figure 12) and petioles.

• Inflorescences

The inflorescences can be more or less attacked, brownish spots appearing on the peduncle, the pedicels of the flowers, and sometimes directly on the flower buds. As before, P. viticola can fruit on infected tissue. If this occurs on the corollas of flowers, the efflorescences formed with a grayish appearance are at the origin of the “ ” facies gray burp (figures 13 and 14).

• Bunches and berries

Grape berries are susceptible to mildew until veraison. They can sometimes be reached very early, after fruit set, on fruits a few mm in diameter from inflorescences already attacked. Again, many sporangiophores and sporangia can form. Berries affected less early have purplish to blackish, depressed ("push"), non-fruiting spots. In this case, we are in the presence of " brown rot " (Figures 15 to 20).

Some grape varieties showing partial resistance to downy mildew can express symptoms with different facies. To see the particular forms that mildew can take on resistant grape varieties, consult the website of the national observatory of resistant grape varieties OSCAR .