Aster yellows was confirmed in Iowa in 2010 for the first time. Not to be confused with the damage associated with Aster Rosette Eriophyid mites, aster yellows is caused by a phytoplasma that is transmitted from plant to plant by aster leafhopper feeding. And in 2024, based on symptoms shared in social media posts in Iowa gardening groups (Fig. 1), aster yellows in running rampant in Iowa. Before we dig into why this might be such a problem this year, let’s review a bit about the disease.
Distorted growth in the form of phyllody, which appears as vegetative tissue growing in the place of floral tissues, is the most characteristic symptom of aster yellows in perennial flower gardens. Symptoms on other hosts can vary, including dwarfed, yellowish growth. The symptoms of phyllody in purple coneflowers tend to be especially distinct (Fig. 2). The phytoplasma that causes this aster yellows can cause disease in over 300 plant species.
However, we’ve only been able to find a publication with a partial host list.
Once a plant is infected with aster yellows, there is no cure. The impacted plants must be removed completely (including roots) and destroyed. Because of the wide host range (many which can be found in perennial gardens), finding plants with aster yellows symptoms in your garden can be especially troubling.
So what gives this year? As mentioned, this phytoplasma (bacteria-like) pathogen is transmitted from plant to plant by leafhopper feeding, primarily the aster leafhopper.
Aster leafhopper populations, like the populations of all insects, vary naturally from year to year based on several factors, like weather and rainfall. Aster leafhopper is a bit more unique in that the individuals we experience in Iowa primarily migrate for the winter to more southern locations and must make the trip back the following spring. Thus, factors such as wind events, primarily winds from the south, at times during migration could result in higher populations.
Winter temperatures were warmer across much of the U.S. this past winter (Fig. 3 and Fig. 4). This could have resulted in additional survivorship of aster leafhoppers and other insects (however, some insects could experience less survivorship during warmer winters). In several locations across Iowa, however, there have been significant gusty wind events during the migration period of the aster leafhopper. These wind events could have propelled additional leafhoppers into more northern locations this summer, thus providing additional populations in fields and an increase in transmission of the phytoplasma that causes aster yellows. This is purely speculation, of course, as the exact causes of increased insect populations year-to-year is challenging to prove. We also have limited data on the number of leafhoppers in fields this year.
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