It's hard to diagnose the symptoms, but they could be due to a stem-boring insect, stem infection (fungal or bacterial), web blight (a fungus, Rhizoctonia), or crown rot from another common fungal pathogen like Southern Blight. You can look for indications of spores of the latter issue using the information on the linked page. Wet weather can increase the spread or severity of many plant diseases, as can frequent watering or irrigation which wets leaf surfaces for long periods; the latter might not be relevant right now if your area has been getting regular rains and you haven't needed to water, but it might arise in the future. Stokes Aster also requires good drainage to thrive, so make sure the planting area isn't getting extra sources of water, like from a nearby roof downspout outlet.

Unfortunately, there is no treatment (fungicide, insecticide, etc.) for any of the above conditions once symptoms manifest, but if you cut open a wilted stem or two lengthwise, you can look for indications of chewed-out centers (borers) or dark-stained points of infection to help narrow-down what might have begun the damage. All you can do for now is to just remove the dying, wilted, and/or blighted foliage and stems. Time will tell if it's a disease that impacts roots as well, killing the whole plant, or just the above-ground portions, where regrowth might be possible. If you are wary of the problem worsening, you can discard the plant(s) and put in new ones.

Miri