Less obvious symptoms of Yellow Asters
We have discovered and removed some coneflower plants with very obvious yellow asters infection, but are wondering if some other symptoms are indicators of that or something else; what are the earliest observable symptoms of Yellow Asters? So far it's only been observed on the white variety of Echinacea purpurea planted intermittently with the purple. Thanks.
How likely is it that Yellow Asters will spread to nearby species (shrubs, ferns, and herbaceous plants) in bioretention ponds?
Baltimore County Maryland
Aster Yellows is caused by a microbe somewhat similar to bacteria which cannot live outside of its host plant or the leafhopper which transmits it. Therefore, the likelihood of its spread around a bioretention pond is low unless this particular species of leafhopper plus suitable alternate hose plants are prolific. Either way, there is no feasible way to stop its spread aside from removing and destroying infected plants as they appear.
References detailing symptoms do not list which come first, though in Echinacea, the malformed flowers are often the first clear sign. Mites can also cause flower malformations, but they have a different appearance and don't have the classic "flower-within-a-flower" growths that often manifest in Aster Yellows. This page from Ohio State has a few comparison photos: https://bygl.osu.edu/index.php/node/1634
The simplest test to see if Aster Yellows is affecting this plant is to cut off all the current flowers and buds and to see what flowers are produced next. Symptoms worsen over time with Aster Yellows, rather than improve, so if the next flush of blooms emerges normal then other causes were at work here. If they're deformed instead, you can presume the plant won't recover and it should be removed. While the foliage pictured here does appear to have sap-sucking insect feeding damage (like from leafhoppers) it isn't necessarily a guarantee that the insects at work were vectoring disease.
Thank you very much for your prompt reply.
Additional questions after removing five more purple and white coneflower plants since Friday. We haven’t seen leafhoppers though (would they be the green ones or the more brightly colored ones?).
There could be several species of leafhoppers that can transmit the pathogen, and they hop away at the slightest provocation, so may be challenging to find. They also cause have completed their life cycle and died off (the adults), and their prior feeding is what infected the plants. In some parts of the country, the Aster Leafhopper is the primary vector (we do not know if that is the case here, but the insects are present in MD); here are images: https://www.marylandbiodiversity.com/view/15482
If the plants are otherwise healthy, Echinacea re-blooms fairly quickly, so they should re-grow flowers within (a rough guess) a couple of weeks.
Seedlings should not contain the pathogen. That said, infected plants don't often set viable seed, at least once the flowers show symptoms. Since the pathogen needs living plant tissues to survive, it could persist nearby in weeds upon which the leafhoppers also fed. (Examples of weeds to look for are listed in the UM link above, though the potential host range is large and not all Aster-family members.) Symptoms in non-flower tissue mentioned by Ohio State include chlorotic and curled leaves and stunted stems. Symptoms will be more severe in hot weather; in cool weather, they will be much more subtle or possibly even asymptomatic.
Both plants and hoppers that are infected are infected permanently, so the pathogen will overwinter in either.