Leaf spots first appear on the older leaves in the middle and upper part of the plant. The spots start as small, necrotic flecks. They enlarge and contain concentric rings and are surrounded by a yellow halo. Other parts of the plant are not directly affected. An indirect effect is seen the fruit production. Alternaria leaf spot may cause reduced yield and smaller fruit.

Source- https://wiki.bugwood.org/Alternaria_cucumerina_(Alternaria_Leaf_Blight_of_cucurbits)