I left my reisling vines unattended for three weeks and now they all look dead.
Pictures? More information? For example, maybe it is frost damage (if you had low temps recently).
Pretty sure it was powdery or downy mildew.
Even though the damage is often difficult to field differentiate, those are very different diseases.
Sulfur, for example, is the traditional go to solution for powdery mildew, but is of little control value for downy mildew. For downy mildew the historical control is Bordeaux mixture
, which will also control powdery mildew to some extent (but copper is more toxic to the vines and soil than sulfur which is why sulfur is preferable if you only have powdery mildew problems).
Or maybe it is neither of these diseases, but another.
If you know it is some fungus, but an unknown fungus, use Bordeaux mixture or a modern broad based fungicide.
My question is, are they salvageable? What should I do?
Begin and maintain a fungus control program, which is essential to have for any vinifera like Riesling.