Coneflowers (Echinacea spp.) have long been one my favorites in landscapes and naturalized areas because of their attractiveness to pollinators of all sorts. Of course, this is the time of the season when we may walk away from them as they start looking pretty rough. However, we should continue to focus our attention on detecting and managing two serious problems that will only get worse next season: Coneflower Rosette Mite and the phytoplasma disease known as Aster Yellows.
Coneflower Rosette Mite: Tufted Seed Heads
Tufted flower parts that rise rosette-like from coneflower cones are produced by the Coneflower Rosette Mite. The mite is an eriophyid (family Eriophyidae) that has yet to be taxonomically categorized, so it has no scientific name or approved common name. However, the mite is generally referred to as the Coneflower Rosette Mite based on the damage that it causes to coneflowers.
You will not detect these mites with an unaided eye; you won't even see them with most hand-lens'. Eriophyid mites are unique among other mites both in their size and anatomy. While most mites can be clearly seen with a 10x hand-lens, you need to use 40x magnification to clearly see eriophyid mites. Most mites are round to oblong in their body shape and they have four pairs of legs that extend laterally from the sides of their body. Eriophyid mites are cigar-shaped and they only have two pairs of legs that extend from the front of their body. No other mite has only two pairs of legs at any stage in their development.
The coneflower rosette mites live inside the developing flower buds and suck nutrients from the base of the flowers. As a result, green to reddish-green elongated rosette-like tufts of stunted and distorted flower parts will sprout from the tops or sides of the cones of coneflowers.
The damage caused by the rosette mite is not only unsightly; it can also seriously reduce seed production and thus natural re-seeding. Sanitation is key to managing the mite. Cutting and destroying flower heads deformed by mite activity will reduce mite populations.
Aster Yellows: Stunted, Deformed Flowers and Plants
This serious, chronic disease occurs throughout North America and may affect over 300 species of plants in 38 families including a number of vegetables such as carrots and potatoes. However, as its common name implies, aster yellows occurs most often on members of the aster family and coneflowers are particularly susceptible.
The disease is caused by a phytoplasma. All known forms of these small, specialized bacteria are plant pathogenic and they infect plant phloem tissue. Phytoplasmas are naturally spread from plant to plant by sucking insects, particularly leafhoppers. Symptoms of aster yellows include chlorotic, curled foliage; stunted stems; and bizarrely distorted flower parts. Flower petals may appear as a ring of tiny greenish-yellow spoons arrayed around the base of highly deformed cones. Cones may appear as tightly clustered rosettes. This symptom is sometimes mistaken for damage caused by the eriophyid mite commonly known as the Coneflower Rosette Mite, and vice versa.
Aster Yellows wreaks havoc on all parts of the plant. There are no sprays that will suppress the disease and once plants become infected, they remain both infected and infectious which means they serve as a constant reservoir of the phytoplasma to be spread to other plants. Thus, sanitation is key to managing the disease. All parts of the plant including the root system must be removed and destroyed. As with all phytoplasmas, the Aster Yellows pathogen cannot survive outside of the plant so the bacterium will not remain in the soil.