Aster yellows: the phytoplasma disease that distorts coneflowers and zinnias

I don't grow coneflowers at my parents' house and I don't have aster yellows in my own Long Island garden — I grow Echinacea but haven't seen it here.

—- title: "Aster yellows" slug: aster-yellows hub: problems category: Diagnostic guide description: "I don't grow coneflowers at my parents' house and I don't have aster yellows in my own Long Island garden — I grow Echinacea but haven't seen it here. What I do see in my neighborhood every summer." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 9 —-

I don't grow coneflowers at my parents' house and I don't have aster yellows in my own Long Island garden — I grow Echinacea but haven't seen it here. What I do see in my neighborhood every summer are coneflowers with deformed, lime-green flowers that look like something has gone badly wrong with them. This guide is sourced from extension publications because my personal experience with the disease is observational, not firsthand.

Aster yellows is the disease behind most of those deformed coneflowers. It is also behind deformed zinnias, stunted black-eyed Susans, and a range of problems in vegetable gardens. Once a plant has it, there is no treatment. The plant must come out. The good news is that understanding the biology makes prevention straightforward.

What aster yellows is

Aster yellows is caused by a phytoplasma — per Clemson HGIC, "a type of bacterium that does not have cell walls." Phytoplasmas are obligate parasites that live inside the phloem (the plant's sugar-transport tissue). Per Clemson HGIC, the aster yellows phytoplasma "systemically moves into the phloem and infects all the plant's tissues from the roots to the leaves." It cannot survive outside a living host — once the plant dies, the pathogen dies with it.

Phytoplasmas were formerly classified with viruses because they pass through bacterial filters. They are now understood as a distinct class of plant pathogen with an unusual biology: they require a living insect vector to move between plants.

Per Illinois Extension, aster yellows can infect over 300 species across 38 plant families. That host range includes ornamental annuals, perennials, vegetables, grasses, and weeds.

The vector: aster leafhopper

Aster yellows moves from plant to plant via the aster leafhopper (Macrosteles quadrilineatus), also called the six-spotted leafhopper.

Per Clemson HGIC, the aster leafhopper is "a small olive-green to tan insect with a wedge-shaped body." Adults are 1/8 inch in length and have three pairs of spots on their heads — the six spots that give the common name. Per Illinois Extension, aster leafhoppers overwinter in the southern Gulf states and annually migrate northward each growing season.

How infection works:

Per Illinois Extension, the transmission sequence:

1. A leafhopper feeds on a plant infected with aster yellows and ingests the phytoplasma.
2. Over several weeks, the phytoplasma multiplies within the insect's body, eventually infesting the salivary glands.
3. Once the salivary glands are infected, each new plant the leafhopper feeds on is inoculated via the insect's saliva.
4. The leafhopper remains infectious for the remainder of its 30—90 day lifetime.

Per SDSU Extension, there is approximately a two-week lag between when a leafhopper ingests the phytoplasma and when it becomes capable of transmitting it to new plants.

The leafhoppers are highly mobile — per SDSU Extension, they "are often blown in on wind," which makes insecticide control of the vector difficult and generally not cost-effective for home gardens.

Symptoms: what infected plants look like

Aster yellows is a systemic disease — it affects the entire plant, not individual stems or leaves. This systemic expression is one key to distinguishing it from certain look-alike conditions.

**On coneflowers (Echinacea):**

Per Clemson HGIC and Illinois Extension, symptoms include:

• Flowers that remain green or develop green-tinted petals instead of the normal pink, purple, or white coloration
• Deformed, malformed, or tufted flower structures
• Multiple flower heads that develop in an abnormal, clustered arrangement
• Distorted petals that may be narrow, leafy, or fail to develop normally
• Lopsided or disproportionately small flowers

Per University of Minnesota Extension, many thin, weak stems growing close together form a witches-broom effect on infected plants.

Foliage symptoms (all hosts):

Per University of Minnesota Extension:

• Discoloration from pale green to yellow or white
• In some plants, red to purple discoloration of leaves
• Small, stunted leaves
• Chlorosis (yellowing between leaf veins)

Other ornamental hosts:

On zinnias (Zinnia elegans): per Clemson HGIC, zinnias are commonly affected, developing green flowers and deformed petals. Black-eyed Susans (Rudbeckia sp.) are also commonly noted hosts.

In vegetable gardens:

Per University of Minnesota Extension, carrots infected with aster yellows develop taproots that are thin, small, covered in root hairs, and often taste bitter. The disease has caused significant damage to Minnesota garlic.

The look-alike problem: coneflower rosette mite vs. aster yellows

This distinction matters because one has a treatment; the other doesn't.

Per Illinois Extension, the coneflower rosette mite causes damage that closely resembles aster yellows on coneflower flowers. Both can produce deformed, green-tinted flower structures.

The diagnostic difference:

Per Illinois Extension, if foliage is normal and only the flower structures are distorted, suspect the coneflower rosette mite first. If both foliage and flowers are affected, aster yellows is more likely.

This distinction is worth making carefully. Removing and discarding a healthy plant with mite damage — when the correct treatment is a miticide before bud break — is an unnecessary loss.

Host range: what plants are susceptible

Per Illinois Extension, aster yellows infects over 300 species across 38 families. Common ornamental hosts include:

• Coneflower (Echinacea spp.)
• Black-eyed Susan (Rudbeckia hirta)
• Zinnia (Zinnia elegans)
• Marigold (Tagetes)
• Petunia (Petunia)
• Chrysanthemum
• Aster (Symphyotrichum and Aster spp.)

Per Clemson HGIC, the disease infects over 300 species of herbaceous and woody plants, though woody perennials are generally less commonly reported as hosts.

Vegetable hosts: carrot, celery, potato, lettuce, onion, leek, and garlic are reported per extension sources. Weeds that harbor the pathogen include dandelion, plantain, and other common broad-leaved lawn weeds.

Per SDSU Extension, plants claimed to be resistant or tolerant and not yet reported as hosts include verbena, salvia, celosia, and nicotiana.

No treatment exists

This is the most important fact about aster yellows:

Per Clemson HGIC: "There is no cure once a plant is infected with aster yellows."

Per University of Minnesota Extension: "Once infected with aster yellows, a plant will never recover."

Per Illinois Extension: "There is no treatment for aster yellows and care must be taken to limit its spread in the landscape."

No fungicide, no bactericide, no systemic treatment will clear a phytoplasma infection from plant tissue. Infected plants are a permanent reservoir for the pathogen and a source of infection for any leafhopper that feeds on them.

What to do: removal protocol

Per Clemson HGIC, the correct response to an infected plant:

1. Remove the entire plant, including roots. Bag it and put it in the garbage.
2. Do not compost infected plants unless the compost pile is active (high-temperature decomposition) and the infected material is completely buried. Per Clemson HGIC, "the infected plants can be buried in a compost pile, but make sure they are completely covered with compost or soil so insects can't feed on the infected tissue."
3. Remove weeds from around susceptible plants. Per Clemson HGIC, weeds harbor aster leafhoppers and the pathogen, serving as a source of infection for the insect vector.

Per SDSU Extension, removal of infected perennials is particularly important because aster yellows can survive in the crown and roots of perennial plants for several years.

Prevention

Weed control. Per Clemson HGIC, weed management is the primary prevention strategy. Dandelions, plantain, and other broad-leaved weeds serve as phytoplasma reservoirs and leafhopper hosts. Keeping the area around susceptible ornamentals weed-free reduces the local pathogen load.

Row covers/exclusion. Per Illinois Extension, floating row cover can exclude leafhoppers from susceptible plants. This is practical for vegetable crops; less so for large ornamental plantings.

Insect control. Per SDSU Extension, "control of the aster leafhopper can be done up to roughly 2 weeks prior to harvest" for vegetable crops. Because of the two-week lag between feeding and transmission, treating leafhoppers after infection has already occurred is not effective. Per SDSU Extension, leafhoppers are often blown in on wind, making "full control very difficult and expensive." Insecticide management is most justified for high-value vegetable crops.

Plant resistant species. Per SDSU Extension, plants that are not reported hosts include verbena, salvia, celosia, and nicotiana. If aster yellows is a persistent problem in your garden, substituting susceptible species (zinnia, echinacea, rudbeckia) with non-host species in the most affected areas is a rational response.

Heat as natural control. Per SDSU Extension, the phytoplasma "struggles to be transmitted when temperatures are frequently above 90°F." Aster yellows tends to be more severe in cooler summers and more limited in hot summers.

Common problems table

Recommended gear: Best Floating Row Covers for Pest Exclusion (2026) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Frequently asked

Is aster yellows in my soil?

Per Clemson HGIC, the aster yellows phytoplasma "will not survive" once the host plant is dead. It is not a soil-borne pathogen. The disease persists in living plant tissue — infected perennial crowns and roots, infected weeds — not in the soil itself. Removing infected plants and weeds is effective because it eliminates the living reservoir, not because it cleans the soil.

Can I save an infected coneflower?

No. Per University of Minnesota Extension and Clemson HGIC, there is no cure. No treatment will clear the phytoplasma from the plant's tissues. An infected plant that appears to produce a few normal flowers may temporarily seem to recover, but the phytoplasma is still present throughout its tissues and it is still a source of infection for leafhoppers.

Should I spray for leafhoppers?

Per SDSU Extension, leafhopper control is generally not cost-effective for home ornamental gardeners because leafhoppers migrate in on wind from a wide area and full control is "very difficult and expensive." Per Illinois Extension, if insect control is attempted, use insecticidal soap selectively to avoid killing non-target beneficial insects. The better investment of effort is weed removal (eliminating the local phytoplasma reservoir) and prompt removal of infected plants.

What plants are safe to grow where aster yellows has been a problem?

Per SDSU Extension, plants not reported as aster yellows hosts include verbena, salvia, celosia, and nicotiana. Illinois Extension suggests substituting susceptible species with hardy geranium, salvia, and verbena as perennials, or cockscomb, impatiens, and nicotiana as annuals, if aster yellows is persistent.