I have seen mildew (Blumeria graminis - Мучнистая роса) run away under these conditions -15oc, high humidity - and there is a bit lurking around in the bottom of susceptible varieties. Will need to keep an eye on this to see if it moves up the plants although free water actually inhibits spore germination so wet crops are at less risk.
The only realistic chemical option here is metrafenone which is a protectant and will needs applying before the disease spreads to the upper part of the canopy.
