Thanks for writing. Asters are susceptible to a number of diseases that can kill much of the foliage by fall when they flower.
Water your asters with a soaker hose and divide or transplant as necessary to keep good airflow --- both of these techniques will reduce the threat of disease.
You can also try spraying your asters with a fungicide during periods of humid, wet weather to prevent the diseases from taking hold on your plants.
Otherwise, if the disease really bothers you, replace your asters with some of the newer varieties that have better disease resistance.
---Justin, Senior Garden Editor, BHG.com