Aster Yellows — What is it and what do I do about it?
As a long-time gardener, I know that sooner or later I will probably have to deal with some sort of disaster in the garden. For the most part, insect infestations don’t faze me. Occasional outbreaks of minor fungal diseases are a nuisance, but they don’t alarm me either. Not even the occasional snake startles me. But the sight of some distorted blossoms on my purple coneflowers (Echinacea purpurea) a couple of summers ago sent me into a panic. Although I had never seen this problem in my garden before, I knew from my training as a master gardener that this must be the dreaded aster yellows disease!
What is aster yellows disease?
Aster yellows is a chronic, systemic plant disease caused by microscopic organisms called phytoplasmas. Phytoplasmas are similar to bacteria but lack cell walls. The disease invades plant cells and can cause devastating damage to more than 300 species of herbaceous plants and food crops. It occurs throughout much of the world’s temperate zones, including North America and Europe. It gets its name of aster yellows because it commonly affects members of the aster family (Asteraceae). There is no known organic or chemical cure for this disease!
How do plants become infected with aster yellows?
Aster yellows is spread from plant to plant by an insect called a leafhopper, specifically, the aster leafhopper (Macrosteles fascifrons also known as M. quadrilineatus) or six-spotted leafhopper. To be clear, only a small percentage of leafhoppers are carriers of the aster yellows pathogen. But those that are carriers can infect a variety of plants including flowering annuals, flowering perennials, vegetables, and weeds. Without the leafhopper’s intervention, the organisms cannot be transmitted so readily.
The infection cycle starts when an otherwise disease-free leafhopper dines on an infected plant. As the leafhopper uses its piercing/sucking mouthparts to feed on the plant, it sucks up the phytoplasma organisms. The now-infected insect then hops to other plants where it spreads the pathogen to all parts of those plants from their roots to their flowers. While the organism won’t kill the host plant, it can severely disfigure it.
What are the symptoms?
Aster yellows symptoms don’t present themselves until 10 to 40 days after infection. The symptoms can vary from plant species to plant species. In general, symptoms presented on annuals and perennials include the following:
- Small, distorted flowers that remain green and don’t develop the proper color for the species.
- Green tufts of deformed leaves embedded in the flowerhead itself. This is a condition known as phyllody, which is a reversion of flowers to a leaflike form.
- 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.
- Excess growth may appear at the tops or sides of flowers.
- Yellowed (chlorotic) leaves with veins that remain green.
- Twisted or curled foliage.
- Stunted plant growth, particularly on younger plants.
- Abnormally bushy growth.
- Flowers may not produce seeds.
Symptoms may vary depending on the age or size of the plant at the time it was infected. For example, plants that are small when infected tend to be stunted and have leaves that are narrower than the leaves on healthy, uninfected plants.
What are some examples of plants affected by aster yellows?
Examples of annuals and perennials affected by aster yellows include asters, begonias, purple coneflowers, coreopsis, daisies, marigolds, zinnias, chrysanthemums, gladiolas, petunias and snapdragons.
In addition to a wide variety of ornamental plants, Aster yellows can infect many vegetables and field crops, most notably carrots, celery, lettuce, spinach, tomatoes, potatoes, grains, and Jerusalem artichokes. Symptoms on vegetable crops vary from species to species. For example:
- Carrots – Stunted growth, distorted and discolored leaves, and thin, bitter-tasting roots that have an abnormal number of feeder roots.
- Lettuce – Twisted inner leaves plus the inner part of the head is pale. Tan or pink spots on lettuce leaves.
- Potatoes – Distorted stems and rolled leaves that are a purplish color.
Some weed species that host this disease include clover, dandelion, horseweed, plantain, Queen Anne’s lace, ragweed, and thistle.
What are some treatment options?
There is no known cure for this insect-borne bacterial infection. It is incurable, but you do have options for preventing it in the first place and for keeping it from spreading to other plants:
- Once you spot the telltale symptoms of yellow asters, promptly remove the plant, roots and all, and destroy it. Although the pathogen can affect all parts of a plant, it cannot survive outside of plant cells. Simply put, it cannot live in the soil.
- Clean up any dead or dying leaves or other debris from the area where the infected plant was growing to prevent other leafhopper insects from consuming the pathogen.
- Keep your garden and surrounding area weed-free. This is particularly important with perennial weeds such as dandelions, plantain and other broad leaf weed species. If infected with the aster yellows pathogen, these weeds can harbor it in their cells from one year to the next.
- The aster yellows pathogen does have an “Achilles heel.” It can’t tolerate prolonged periods of hot weather. Temperatures of at least 88°F for 10 to 12 days inactivate the pathogen in both the leafhopper and infected plants.
- If aster yellows is a persistent problem for you, grow other flowering plant species that aren’t susceptible to this disease — flowering perennials such as hardy geranium, salvia, and verbena or flowering annuals such as cockscomb, impatiens, and nicotiana to name a few.
- For vulnerable edible crops, use floating row covers to prevent leafhopper access.
But wait — If the problem isn’t aster yellows, then what is it?
Before leaping to the conclusion that your plant has aster yellows, rule out two other possibilities. The first is herbicide exposure, which can occur from misapplied weed control products. If the herbicide is applied on a windy day, it can drift from the application site onto nearby plants where the damage may not show up for several days. Depending on the plant and product used, symptoms of herbicide damage include discolored and twisted leaves, cupped foliage, or plant dieback. If multiple plant species are affected, that’s another telltale sign of herbicide damage.
The second possibility is mite damage. Having encountered aster yellows on some purple coneflowers (Echinacea) in my own ornamental garden, I followed the generally accepted protocols for controlling the pathogen. I dug up and destroyed all parts of the affected plants, cleaned up and removed all plant debris from the soil, and removed all weeds from the immediate vicinity. Satisfied that I had done due diligence in ridding my garden of the disease, I congratulated myself on stopping this dreaded pathogen in its tracks. Only later did I learn that there’s another disease that looks similar to aster yellows and is caused by the Eriophyid mite. Suddenly, I wasn’t so sure my diagnosis of aster yellows was correct.
How does Eriophyid mite damage compare with aster yellows on coneflowers?
Eriophyid mites are microscopic in size and often go undetected because they live unseen inside flower buds. A number of Eriophyid mites exist but the specific species of mite that affects coneflowers is the coneflower rosette mite. It gets that name because it causes abnormal rosette-like (circular) growths on the cone of the flower. As the buds develop, the mite sucks nutrients from the base of the flowers causing stunted and distorted flower parts. Characteristics of the coneflower rosette and aster yellow diseases include the following:
Coneflower Rosette Disease:
- Affects only the seed head (cone portion) of the blossom and presents as a tufted portion of the cone.
- Green to reddish-green elongated rosette-like tufts of stunted and distorted flower parts that sprout from the tops or sides of the cones.
- Foliage and stems are not affected.
Aster Yellows Disease:
- Affects all parts of the plant.
- Bizarrely distorts the entire flower. 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.
- Affects the foliage with yellowed, curled foliage
- Stunted stems.
A link to an Ohio State University Extension article on Coneflower Cleanup is included under Sources at the end of this article. It provides several very good comparison photos of aster yellows and coneflower rosette mite symptoms.
What are the treatment options for Eriophyid mites?
While there’s no cure for aster yellows, fortunately, Eriophyid mite damage can be treated. Remove damaged flowers and destroy them. Clean up all plant debris in the garden this fall. Next year, treat susceptible plants with a horticultural oil or a miticide before bud break.
Aster yellows is a devastating disease of many herbaceous ornamental plants and edible food crops. The disease is incurable and is easily spread from plant to plant by leafhopper insects. Promptly removing the affected plant roots and all, keeping the area free of weeds, and cleaning up any infected plant debris will help control the disease. But do make sure you accurately diagnose the problem as aster yellows, which has no cure, and not Eriophyid mite damage, which does have a cure.
As for those diseased purple coneflowers in my garden two summers ago, I will never know for certain which disease they had – aster yellows or coneflower rosette disease caused by mites. But now that I know there’s more than one explanation for the problem, I’ll do my research first before I panic.
Featured Photo: Purple coneflowers displaying symptoms of aster yellows. Photo: Courtesy of Missouri Botanical Garden
Aster Leafhopper, University of Wisconsin-Madison Extension and Research
Aster Yellows, Missouri Botanical Garden Fact Sheet
Aster Yellows, University of Illinois Extension
Aster Yellows Disease on Flowers, University of Maryland Extension
Aster Yellows vs. Eriophyid Mites on Coneflower, Minnesota State Horticultural Society Website
Coneflower Cleanup, The Ohio State University Extension website