The coneflower oddities pictured came from Sens Center Vegetable Demonstration Garden that the Bluebonnet Master Gardener Association manages in Bellville, Texas in June 2019.  Our research shows that these plants are infected with Aster Yellows disease caused by phytoplasma.  However interesting the effect, Aster Yellows is a serious garden disease impacting more than 300 plants species in 38 plant families.

The Texas Plant Disease Handbook lists the following as the most important impacted plants of Aster Yellows disease:

Crops: broccoli, buckwheat, cabbage, carrot, cauliflower, celery, endive, flax, lettuce, onion, parsley, potato, parsnip, pumpkin, red clover, salsify, spinach, strawberry and tomato.

Flowers: aster, anemone, calendula, Centaurea, China aster, chrysanthemum, Clarkia, cockscomb, Coreopsis, cosmos, delphinium, daisies, Gaillardia, hydrangea, marigold, Nemesia, Paris daisy, periwinkle, petunia, phylox, Scabiosa, snapdragon, statice, strawflower, veronica, and zinnia.

Weeds: cinquefoil, daisy fleabane, dandelion, horseweed, plantain, ragweed, thistle, wild carrot, and wild lettuce.

Yellows diseases are caused by phytoplasma.  All known forms of these small, specialized bacteria cause plant disease.  Phytoplasmas are naturally spread from plant to plant by sucking insects, particularly leafhoppers. The insects pick-up phytoplasma during their feeding on infected host plants, then spread the disease when they move to feed on other plants. The phytoplasma can overwinter in leafhoppers and on perennial host plants  and will re-emerge in the Spring.

Phytoplasmas commonly cause distorted, dwarfed, and yellowish leaves and shoots often referred to as “yellows”. Other symptoms include abnormal flower and leaf development, shortened internodes, and shoot proliferation (known as “witches’ broom”). The flowers of infected plants sometimes develop green, leaflike structures as seen the coneflower photo from the Sens Center Demonstration Garden.

Aster Yellows wreaks havoc on all parts of the plant. There are no chemical or organic treatments known to cure, suppress or kill the disease so once plants become infected, they remain infected and are a host plant for further infection throught the garden.  Failing to destroy the infected plant means it survives as a constant source of phytoplasma to be spread to other plants.  Garden sanitation is key to managing the disease.  Once the disease is discovered, all parts of the plant including the root system must be removed and destroyed.  Although heat may kill the pathogen, it is best not compost diseased plants. As with all phytoplasmas, the Aster Yellows pathogen cannot survive outside of the plant so the bacteria will not remain in the soil.

An integrated pest and disease management approach including destroying infected plants immediately upon discovery of the disease, maintaining proper garden sanitation practices and attempts to control the leafhoppers is recommended. If you believe you have plants of any kind infected with Aster Yellows or a similar disease,  contact the Texas Plant Disease Diagnostic Lab for information about diagnostic testing and recommendations for minimizing the spread of disease in your garden.

Web sources used for this post include:  Texas Plant Disease Handbook, Texas Plant Disease Diagnosic Lab, Missouri Botanical Gardens, Ohio State Cornflower Clean-up, Wisconsin Horticulture Division of the University of Wisconsin – Madison.