It looks like the plant is loosing moisture faster than it can replace it. Hot weather causes plants to transpire a lot of moisture and it the leaves loose it faster than they can bring it up from the roots they get some "scorching" on the leaf edges.

I don't think it is moisture. These problems definitely look like disease. I couldn't attach the other photos, but three of the four plants have yellow-brown areas that look like a blight. I'm having a bad garden year with lots of blight and viruses on tomatoes and beans too. Ugh! Depressing.