Garden Pests and Diseases: Please help identify disease: purple spots with white centres, 3 by Andrew_Hayes

Andrew_Hayes wrote:
Please help me identify this disease. Purple spots white centres began appearing on my plants and are now spreading. They start of as small purple spots and then grow in size and eventually form white centres. I will most likely destroy the badly affected plants but it has spread to other plants that I really want to keep. What is this disease? How do I stop it? Is there any hope for the infected plants?