Several diseases and pests affect coneflowers. While your coneflowers may never get any of them, it can be troubling if they do.
This disease typically occurs in overly moist conditions or from a lack of airflow. Plants that are already weakened by pests or disease may be especially susceptible. Avoid powdery mildew by spacing plants properly and planting in well-drained soil.
Floral damage caused by eriophyid mites mimics some of the symptoms of aster yellows (see below). In this case, the damage is only cosmetic and won’t affect the health of the plant, although it will reduce seed production. Mites feeding on the flowers cause tufted growth or rosettes to form on the cones. The deformed flowers are unsightly and similar in appearance to aster yellows. Removing the affected flowers will improve the ornamental display and reduce the mite population.
The beetles feed on foliage and flowers, leaving ragged holes behind. Fortunately, their damage is mainly cosmetic. Hand picking adult beetles and disposing of them works best.
Normally resulting from overwatering (or from a plant being sited in an overly moist site), crown/stem rot results in the entire plant turning brown and essentially rotting away. Again, well-drained soil is key.
This is perhaps the most grievous problem because there is no prevention or cure—all parts of the plant are infectious and potentially harmful to other plants. A specialized bacteria called a phytoplasma spreads from plant to plant by leafhoppers, which are sucking insects that feed on coneflowers. Disfigured and chlorotic leaves, stunted stems, and distorted flowers are symptoms of aster yellows. Affected flowers may also show green spoon-shaped rays and/or a rosette of leafy growth on the cones (photo). The phytoplasmas multiply in the roots over winter, so symptoms worsen every year. Sanitation is best as soon as symptoms appear—all parts of the plant, including the roots, must be destroyed.
See more about coneflowers in What’s New With Coneflowers?
Richard Hawke is plant evaluation manager at the Chicago Botanic Garden in Glencoe, Illinois.
Photos, except where noted: Danielle Sherry