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My first thought is that this is downy mildew:

Irregular yellowish to brownish spots on upper leaf surfaces; grayish powder or mold on undersides. This disease is caused by the fungus Peronospora parasitica and can attack both seedlings and mature vegetable plants. Infected plants develop a gray mold on the lower leaf surface. The upper leaf surface of infected plants first turns yellow and then may turn brown or necrotic. Leaves wither and die. Symptoms differ from powdery mildew in that the downy mildew fungus grows only on the lower surface of the leaf. Development of the disease is favored by moist conditions.

Preventative measures:

• Improve air circulation.

• Plant resistant varieties.

• Rotate crops.

• Keep garden free of plant debris.

If disease becomes severe enough to warrant application of controls, Neem oil, is commonly used to treat downy mildew disease.  The chemicals chlorothalonil will give good control and copper fungicides will give fair control.

Another possibility is Harlequin bugs or stink bugs:

 Leaves have whitish or yellowish spots; leaves are deformed; plant wilts.. Harlequin bugs are black with bright red yellow or orange markings. They suck fluids from plant tissue causing white and yellow blotches. Handpick and destroy bugs and egg masses. Keep garden free of crop residue and weeds where bugs breed. Stink bugs are gray or green shield-shaped bugs about ¼-inch long; they feed on fruits. Remove garden debris and weeds where bugs can overwinter. Hand-pick egg masses and bugs and destroy.