This is the most common problem I encounter, Partly due to the lack of recirculating air in the greenhouses.

This fungal disease attacks just my cucumbers and creates a powdery substance which covers the leaves and eventually kills them if left untreated. Fortunately after several years of the problem I seem to have found a good solution and a simple one at that.

When the problem starts to occur, I make a solution up of 1 part milk to 10 parts water, This is then liberally sprayed all over the leaves underneath and on top.

So far (fingers crossed) this has helped tremendously.
I simply repeat this every few weeks or so if the problem starts to occur again. Just make sure you empty the container you’ve used after spraying, Otherwise the container gets a little smelly after a few days. As I’ve found out already.