greenhouse cucumber problem

Hi all,can anyone help,my neighbour has a problem with her greenhouse cucumber plants,they have turned yellow,the stem the leaves and now the new growth of cucumbers have started to go yellow.
The plants are not wilting but yellow,the greenhouse is well ventilated

Any help appreciated,thankyou

I was just about to post about feeding cucumbers .... what do you feed them, fed mine with tomato feed today, hope it's ok