We've been getting a few calls lately from northern gardeners wondering if their coneflowers have aster yellows. That's one option—the other is an Eriophyid mite infestation. Here's what you need to know about these two plant problems.
This disease is fairly common and can occur on many plants, including tomatoes, marigolds, snapdragons, even lettuce—but it's most often noted here on coneflowers. The plant will show signs of decline, such as stunted growth or damage to the leaves. But the most dramatic symptom is the crazy, deformed flowers on the plant. The plant may have growths out of its cone or tufts of green where the purple petals should be. The growths can be very strange looking. I've even seen coneflowers with petals growing out of the center of their cones!
Aster yellows is caused by a phytoplasma that is spread to the plant by aster leaf hoppers, when they feed on the plants. The disease tends to be more common during hot years, which would explain why we're hearing more about it this year.
Aster yellows is not curable and it can spread, so the best thing to do is remove the diseased plant and throw it away. Do not add it to your compost. If you have several coneflowers and only one or two are showing symptoms, don't remove the others, but keep an eye on them next season for signs of trouble.
There are many types of eriophyid mites and they can cause a variety of galls and growths on plants. The most common one on coneflowers is the coneflower rosette mite. It's called that because it produces a rosette like growth on the cone of the coneflower. The growth may be green and may even send up shoots similar to aster yellows.
The mites are microscopic and live inside the flower bud as it develops, sucking nutrients from the plant. The cone of the coneflower often sprouts the rosette and petals around the rosette may fade. To treat for eriophyid mites, remove the flowers and do not compost. You can treat plants with a horticultural oil or a miticide before bud break. For now, the best treatment is to remove the infected flowers and clean up the garden well this fall.
For an excellent side-by-side comparison with photos of aster yellows and eriophyid mites on coneflowers, check out this page from Ohio State University.
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