Last year I had an infestation of aster yellows in my front garden on the Echinacea, purple coneflowers. The recommendation is to remove the infected plants. I thought I had, though I found another plant this season infected with it.
Some think it is not a big problem but this disease is spread by leafhoppers. It can infect crops such as onions, celery, tomato, carrots as well as many ornamentals including coneflowers and marigolds. Listed here and here are some extension sites that go into more detail. Another good site is MOBOT's IPM site.
My first thought last year was that I had a mutation, nope- infection. Best advice-- removal!