Healthy cucumber plants going dry.

It's 70 days old plant. Was fine a few days ago and has already given a few good cucumbers. (Although most of them were bitter). Since 2-3 days, leaves have started getting dry. Drying up starts at edges. Then whole leaf, then whole vine becomes dry. Earlier there was leafminer problem, which got treated with Neem spray. (It's already done twice at 20 days interval) I can see very small (smaller than a pencil dot) hardly visible, brown colored insects on back of each leaf.