Recently I received this question from my friend Laura. She writes: Every year my coneflowers start out so beautiful, but a couple of the plants start looking weird around this time of summer. The leaves and stems start getting this weird white stuff on them. No matter what I do as far as pruning or watering, they do this. What is it and how can I prevent it?
The white fuzzy on leaves and stem are most likely powdery mildew. It can be avoided by good air circulation. Are your plants crowded? Divide them in the fall. They can also be sprayed with a fungicide that lists powdery mildew on the label. However, it will not cure the plant from the disease, but will keep it in check. If you decide to spray it is most effective when you spray before the powdery mildew appears. When is that? Powdery mildew is encouraged when it is dry but the air is humid (like our summers in SC)
The stipling on the leaves appears to be spider mite damage. Spider mites are very small and difficult to see. Use a magnifying glass and look on the underside of the leaf. They multiply quickly in hot dry weather. They can be controlled with Safer soap. Be sure to spray under the leaves not just the top.
Another note, if these are planted in full sun all day, a some light shade may help in avoiding these problems.