cucumber plant problem

Almost certainly a type of fungus called powdery mildew, which a little counter-intuitively can be worse in dry conditions.

Not much hope for leaves as badly infected as in your pic, so take them off - a preventative measure (not a cure) is said to be a diluted milk spray (maybe 30% milk, 70% water), although good air circulation is also important, which you will have helped a little by removing some of the leaves.

Mildew seems almost inevitable on many cucurbits by the end of the season, although you are a bit unlucky to be seeing it now.

If it'still in a greenhouse try to increase the himidity.  I throw a can of water on the floor, or spray the leaves will a mist of water daily in hot weather.