Why do my cucumbers get mildew in the greenhouse

Mildew is due to lack of sunshine and cold, damp conditions. If you get the temperature up, it should solve the problem.
The only ones that may be resistant to this are ridge cucumbers.

I don't think I fancy that Marguerite. I have sent you a PM re the ridge cues.

Thanks for answering the question DoctorB as this happens to me some years, well not me the cucumber plants, lol - may I ask, once they get mildew do they recover if they then get warmth and sunshine or is the plant ruined? Dawn

If you only have a minor attack you can remove it and they may well survive, but if you have a severe attack there is no chance.
Glad you explained it wasn't you getting it Dawn, that would not have been nice. LoL.

Well Doctor B, it is quite cold and damp at the moment with no sunshine so I might be susceptible to mildew, ha ha. Thanks for your info above.

Well all I can say Dawn is turn the thermostat up. Lol. then we can all get warm.