Daylilies: Daylilies dwindling, 1 by gardengirlinmd

gardengirlinmd wrote:
These daylilies have been in the ground for about 6 years. I don't remember where I got them, but they are quality hybrids from a good quality nursery. They have never really thrived. Some of the plants actually just disappeared. I wonder if anyone can tell me what might be wrong with these plants. My soil is heavy clay and I heavily amended with mushroom compost and mulch with pine bark mulch. I always water well in the very hot parts of the summer. The leaves are quite yellow and remind me of discolored German iris leaves. Is this some sort of bacterial blight? The clumps just don't get bigger each year. I thought they would be very hardy and healthy and they never have been.
Other plants in the bed are fine. Some are really doing well. However, I have had to remove an aster because it had persistent fungus which defoliated the plant. Also I removed a 'David' Phlox because stems kept wilting and dying. They were near the daylilies. But there is a Korean lilac next to the daylilies which is incredibly healthy. I'm interested to hear what you all think.