Several different fungal diseases can cause the leaf spotting shown in the photograph. There are preventative sprays, but none that I know of that provide a cure once the problem is visible. While unsightly, there is no real danger that the infected plants are going to die or lose all their leaves, so most rhododendron growers ignore it. Since it's the lower leaves mostly, they were likely to be ready to fall anyway. Stress from heat or other causes does seem to play a role in encouraging such infections. Picking up and disposing of infected leaves can help in preventing any spread to uninfected plants.