Aster yellows is a plant disease that can infect over 300 species of woody and herbaceous ornamentals, vegetables, and weeds in 38 plant families as well as a number of grain crops. Maryland gardeners notice this disease most often on purple coneflowers (Echinacea purpurea) and its cultivars.
Aster yellows causes:
- Malformed flowers with petals that are often abnormally green-colored.
- A mass of abnormal, brush-like shoots or small abnormal flowers arising at or close to the same point (witches brooms).
- Abnormal production of adventitious roots.
- Yellowing and stunting of plants.
- Wilt and dieback.
Aster yellows belongs to a group of plant pathogens called phytoplasmas. They are similar to bacteria, but lack cell walls. Like bacteria, they reproduce by fission. Phytoplasmas invade the plant phloem (the system that transmits the sugars and substances made by photosynthesis) and cause disease symptoms sometimes similar to those of viruses.
This disease is spread by the aster leafhopper, Macrosteles quadrilineatus. Leafhoppers acquire the pathogen, but there is an incubation period, sometimes referred to as a latent period, which may take 2-3 weeks. During this time the pathogen multiplies within the leafhopper, and then moves to the salivary glands. Only then is the leafhopper capable of transmitting the pathogen to another plant. After feeding, it can take 10 days to 3 weeks, depending on temperature and plant species for the appearance of plant symptoms.
The aster leafhopper can overwinter in the egg stage in Maryland, and that can result in mid to late-season infections. Frequently, however, these insects also begin developing down south early in the spring where they build up large populations. These southern adults will then migrate on the prevailing winds and jet streams that frequently move northward in the spring. Depending on weather and wind patterns these insects may arrive earlier in the season ready to transmit the disease.
The aster yellows pathogen survives in many weed hosts and leafhoppers feeding on infected weeds can spread the disease to healthy garden plants. Perennials infected with aster yellows generally die within the first season of infection.
Management
Early detection of infection is critical to prevent plant losses. Inspect new plant material and established plants for abnormal coloration and stunted growth. Destroy any infected plants.
Infections are systemic throughout the plant and therefore plants are not “cured” by removing symptomatic flowers. Promptly remove whole infected plants when symptoms are first noticed to prevent spread throughout the rest of the planting.
Weed control is very important since weeds can serve as sources of infections. Eliminate weeds such as wild carrot, field daisy, dandelion, thistles, ragweed, marestail, and pineapple weed. These can be symptomless reservoirs for the disease.
Revised with information provided by Dr. David Clement and Dr. Karen Rane, UMD IPM Alert, July 2023