Powdery mildew is by far the worst disease for house grapevines. It is THE disease par excellence for the vine on facades and walls because the leaves can't breathe enough; it manifests as a musty fungus 'fabric' that attacks all green parts of the stock. Its arrival often means the end of the stock, which is brought to its knees by the parasitic fungus and can then only produce ugly, stunted leaves and fruit.
(Uncinula necator / Oidium tucceri)
Unfortunately, the cultivation of a grapevine on a house wall increases the risk of powdery mildew. When standing in the garden with a wire frame supporting structure, other diseases are more common. But facade and wall greening is our domain-- so at FassadenGrün, powdery mildew is our main concern.
The most common cause for powdery mildew is the choice of an incorrect variety. Old European grapevine (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 therefore usually unsuitable for cultivation on walls and in private gardens, at the very least when organic gardening (thus avoiding any 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 from a few spores just like a healthy immune system can cope with a common cold. But in the neighborhood of a heavily infected and "terminally ill" stock that spreads spores quickly, even strong and vital stocks will be contaminated and fail to withstand the disease. A distance of 100 metres from an infected vine stock is critical. 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 which develops on the upper surface of the leaves. Rain droplets structurally damage the sensitive fungus mycelium, and internal absorption of water may also weaken the fungus. This could explain why vines are particularly hard hit when they are cultivated on a house wall under an eave/overhang where they are sheilded from dew and rain. At the same time, this would also explain the effect of strange 'grandmother' remedies like 'diluted milk spraying': it is the water in the spray that likely causes some kind of damage to the mycelium...
The only real antidote available for private use, however, is spraying the plant with sulfur several times a year and over the course of several years (maybe even for the entire lifespan of the vine), because the spores hibernate in the buds. Almost all insects in the vine stock are destroyed by sulfur, such as useful mites, etc.. Always read the manufacturer's instructions before treating with sulfur!