I think it may be downy mildew disease caused by fungus Pseudoperonospora cubensis. To confirm further we need to see underside of infected leaves. If it is downy mildew you will notice brown to grey fungal growth on the lower surface of the spots. The best time to observe this is early morning hours ie., when the leaves are covered with dews.
The main symptoms of downy mildew are appearance of yellow to brownish irregular spots on upper surface of leaves initially. As the disease advances the brown spots appears on both surface of leaves. Later this spots will enlarge quickly and turn black resulting in withering and dying of entire leaf. Typically the petiole of infected leaves stand straight even though the whole leaf is dead (see your first image). The disease is favored by high humidity and cool temperature.
Brown spots on both surface of leaves
Appearance of yellow to brownish irregular spots on upper surface of leaves.
Posted by: Dr. Ravishankar Narayana
Posted: July 31, 2016