One of the problems I’ve noted over the last few years is that some of my purple coneflower – Echinacea, is not forming its flowers properly. Instead of the nice purple flowers, the flowers are green, stunted and malformed. I’ve finally figured out the cause and it’s called Aster Yellows Disease.
It’s a disease which is passed from one plant to the next by the leaf-hopper and there is no practical cure. The only thing you can do is dig out the affected plants and get rid of them.
While this disease can also infect other plants, asters, marigold and coreopsis, it doesn’t seem to be infecting those plants in my garden.
One solution I’ve found is to save seed from healthy plants and plant the seed in November. Then you will have new healthy plants next year even if you have to discard diseased plants now.
I’m also testing out cutting off just the diseased flowers and see if that will stop the spread of the disease. Note – it doesn’t seem to work – I end up digginup and disposing of the plant. According to MoBot, do not put this in a compost pile or recycling bag. Put this in the trash so that the disease does not spread.
Here are some more resources about this problem: