Asters always seem to grow very well for a while then suddenly the plant dies.
By Penney
United Kingdom 
3 years running I have grown asters fromm seed but round about flowering time at least half of them fall over and die. Some flower for a few days then die for no apparent reason. Mostley in large pots but some in ground
15 Aug, 2009
Answers
It may be this but I would also check the soil for vine weevil grubs too just to be sure.
17 Aug, 2009
Previous question
Hi Penney,welcome to Goy :)
I think your asters may have fusarium wilt-a fungus which likes cool damp conditions and we've had plenty of those !!
Unfortunately it stays in the soil so you shouldn't grow them in it again for possibly 5 years.There is no chemical cure either. Sorry for such bad news.
15 Aug, 2009