Answer: Unfortunately based on your description I am not certain what exactly is affecting your plant. Sometimes asters are attacked by mildew if weather conditions are conducive to it. Thinning the plant in the spring and locating it in a spot with good air circulation can help, as can making sure it is well watered during dry spells. Your local county extension may be able to give you a more precise diagnosis and based on that suggest the most appropriate control. If it is something that requires a chemical control they will have the most up to date information on what to use and how/when specifically to apply it for the best results. I'm sorry I can't be more specific for you long distance.
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