Merci appius, j'ai trouvé un autre ... Mildew mais je ne le retrouve plus.
Sinon , pour comparer les 2 maladies , voici des explications en anglais :
Powdery mildew is one of the oldest plant diseases known to man -- the ancient Greeks described it, and several books in the Old Testament speak of its effects. Probably every gardener, unless they live at the Arctic or in high mountain regions, has witnessed its symptoms: a delicate webbing or thick felt that covers leaf surfaces and stems, giving the appearance that they've been dusted with flour. Leaves may turn yellow, die and fall off, reducing yields and weakening -- or killing -- the plant if infection is severe. On grapes and fruit trees, the disease is especially serious, attacking new growth -- buds, shoots, flowers and just-forming fruit -- as well as leaves and stems. Subsequent growth is dwarfed and distorted, often covered with the mildew; young fruit develop scars and corky skin.
The most favorable conditions for powdery mildew are nighttime temperatures of 60F and relative humidity of 90 to 99%, with 80F temps and 40 to 79% relative humidity during the days -- conditions common in late spring and early fall. Since the spores are sensitive to extreme heat and direct sunlight, plants in partial shade are especially vulnerable.
Downy mildew differs from its powdery cousin in a number of significant ways. Unlike powdery mildew, which appears on both sides of the leaf surface, downy mildew produces spores primarily on the undersides of leaves. The tops of leaves will have yellow blotches, while the undersides develop a faint frost of gray, white, blue or violet fuzz. Fruits of infected plants are often small and bitter, if they even make it to harvest.
On y apprends que le mildiou se repand par eclaboussure d'eau tandit que l'oidium se repand par le vent.
Le mildiou apparait sous les feuilles alors que l'oidium apparait des 2 cotés de la feuille.
Chose surprenante :
Since the spores cannot germinate in wet conditions, frequent water sprays or overhead sprinkling systems may help reduce the incidence of infection. Prune and train to promote good air circulation and remove any infected shoots, buds and branches that might harbor the disease from season to season. To avoid an outbreak of downy mildew, allow plants to dry between irrigations and keep leaves as dry as possible
En gros, en mouillant regulierement le feuillage ont reduit le risque d'infection.
C'est exactement l'inverse de ce qu'on pratique , c'est a dire eviter de mouiller le feuillage pour eviter les maladies.
Je suis perplexe maintenant....