It looks very like that your plants have finished flowering, as those look like seed pods on the flower stalks!
Also on mine the foliage always look like yours after flowering, so I quickly chop them down and let fresh leaves grow!
"AND" they need lots of water when flowering or the flower stalks droop!
I lost three large strongly growing clumps of Aster frikartii monch's growing in my allotment last year, they all died at the same time with no reason!
Tonight the lady speaker said that hers had rotted without reason earlier this year, and had anyone else experienced the same, she was looking for answers but noone was able to help!