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.

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.

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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.