Powdery mildew is by far the worst disease house grapevines. It is THE disease for the vine on facades and walls because of a lack of aeration of the leaves, and it is manifested by a musty fungus that attacks the green parts of the stock. An appearance often means the end of the stock, wich is brought to it's knees by the parasitic fungus and can only produce ugly leaves and no fruit.
(Uncinula necator / Oidium tucceri)
Unfortunately, the culture of a grapevine on a house wall increases the risk of powdery mildew. When standing in the garden with a wire frame supporting structure the other diseases are more common. But facade and wall greening is our main topic, that's why at FassadenGrüns powdery mildew is our main concern!
The most common cause for powdery mildew is the choice of an incorrect variety. Old European grapevines (vitis vinifera) varieties, i.e. virtually all known historical wine varieties from Chasselas to Pinot Blanc are highly sensitive to powdery mildew. The disease was first observed around 1810 in North America, later in 1845 in an English greenhouse and shortly afterwards on other grapevines on house walls. It was probably brought in from North America, arrived in Germany in 1850 and almost killed European viticulture until sulfur was discovered as a treatment...
Historical European grape varieties are usually unsuitable for cultivation on walls and in private gardens, athttps://www.fassadengruen.de/?id=3015 the very least when organic gardening is a requirement with no sulfur treatments. Selecting a resistant grapevine variety is the only way to prevent mildew and should be considered carefully first.
Powdery mildew is a fungal disease caused by airborne spores that infest vines and other related species. A vital, vigorous grapevine may cope with infection by a few spores just like a healthy immune system can cope with a cold wave. But in the neighborhood of a heavily infected and "terminally ill" stock that spreads spores and exerts intense infection pressure, even strong and vital stocks will be contaminated and fail to withstand the disease. A distance of 100 meters to an infected vine stock is critical already. It is crucial to quickly eliminate heavily contaminated grapevines, to protect all others in its vicinity!
Water is a natural but weak antagonist of the fungus forming on the top of the leaves. Rain droplets mechanically damage the sensitive fungus mycelium, and internal absorption of water may also damage the fungus. This could explain why vines are particularly hard hit when they are cultivated on a house wall under an overhanging roots shielding the leaves from dew and rain. At the same time, this would also explain the effect of strange grandmother remedies to mildew such as diluted milk spraying: the water in the spray causes some kind of damage to the mycelium...
The only real antidote available for private use however, is spraying the plant with sulfur net, several times a year and over the course of several years (maybe even forever), because the spores hibernate in the buds. Almost all insects in the vine stock are destroyed by sulfur, such as useful predatory mites etc.. Always read the manufacturer's instructions before treating with sulfur!