It is always interesting to observe plant diseases and try to unravel the mystery of how a particular plant became infected and look toward solutions. So many of these ailments have an incredibly fascinating path to infection, often including multiple species when you consider the pathogen, host and potential vector species.
Aster yellows is a somewhat uncommon disease in Illinois with several species involved in transmission to a wide range of host plants, including a few vegetable crops, as well as a large swath of ornamental plants.
As its name implies, it infects a number of species in the large and diverse Aster family (Asteraceae), as well as some grasses and grains and even common weed species. All in all, this pathogen infects over 300 species across 38 families. In central Illinois, I’ve seen reports of it on black-eyed Susan (Rudbeckia hirta) and coneflowers (Echinacea spp.).
Tiny insects that suck the sap of plants spread this pathogen as they feed, with the aster leafhopper being one of the most common vectors. It is one of many insects that overwinter in Southern states and annually migrate up to the Midwest for the growing season.
The pathogen responsible for this disease is a specialized bacterium called a phytoplasma. As leafhoppers go from plant to plant piercing leaf tissue to access their food source, they become “infected” by the phytoplasma from plants that already have aster yellows.
In a few weeks, the phytoplasma multiplies to infest salivary glands of the insect. As infected insects go about their business of feeding, they infect each new plant encountered and continue to be infectious for the remainder of their 30-to-90-day lifetime.
Symptoms of this disease are most noticeable as flowers develop, causing flower heads to be deformed or tufted. Portions stay green, not developing their typical color, and many flowers become lopsided or remain small and disproportionately sized. In some cases, multiple deformed flowers develop or odd-shaped leaves spring out of the side of flower structures.
While symptoms in blooms of infected plants are most notable, they can sometimes mirror the impacts of feeding by other insect pests, requiring assessment of the whole plant for a true diagnosis.
Chloris, or yellowing of the tissue between leaf veins, is probably the most diagnostic symptom. Foliage or flower petals may also show mottling that looks like yellow specks or blotches that develop as tissue becomes infected from feeding. Growth is usually stunted or irregular, with smaller and narrower or discolored leaves when compared to healthy plants.
In coneflowers, the primary look-alike ailment is caused by feeding from a species of eriophyid mite. This mite has not yet been fully described taxonomically and therefore has no common or scientific name. It is typically referred to as the “coneflower rosette mite” due to the shape of the damage it inflects.
These insects feed on flowerheads, causing similar distortion as observed with aster yellows. Feeding typically results in rosette-like tufts of distorted growth on flower heads that often remain mostly green in color.
The difference in symptoms between coneflower rosette mite and aster yellows lies in the pattern across the plant. Aster yellows produces symptoms throughout the plant, on foliage as well as flowers, whereas mite damage is confined to flower structures. Both conditions require control to limit spread.
Coneflower rosette mites are best controlled with sanitation, by removing and destroying infected flowers.
Unfortunately, there is no treatment for aster yellows, and care must be taken to limit its spread. If you identify infected plants, they need to be completely removed, roots and all, since they serve as a constant reservoir for the pathogen.
To limit spread from leaf-feeding insects, consider floating row cover or other methods of exclusion. Selective spraying with insecticidal soap is another option, but try to limit your application to avoid killing other insects.
As mentioned earlier, rudbeckia and echinacea species are susceptible, along with quite a few other common ornamentals, such as zinnias, marigolds, petunias and chrysanthemums.
Ryan Pankau is a horticulture educator with UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his ‘Garden Scoop’ blog at go.illinois.edu/GardenScoopBlog.