Very common problem. One reason I stopped growing asters; that and the rabbits. I was again tempted by theNY aster series "showtime" at a farmstand so I googled it on the spot. The Chicago Botanic garden had trials and in the report it said that many asters are disfigured by brown foliage at the bottom.

I see that in other late bloomers.often cimigifugas...so I have wondered whether the stress of the growing season and variables in weather and water causes or contributes to browning.

I think you need to pay attention to watering evenly....don't stress them with dry and wet.

Thank you, that was very helpful.