I lost a few plants that looked exactly like yours, also several others were also sick and on their way out. When I pulled the dead ones, much of the root system was missing and there were a lot of small black insects present. They were eating the roots and chewing on the stem just an inch or so below the surface. I never did identify the insect, I just drenched the soil around the stem with a light dose of sprectricin and that solved the problem...the ones that I saved really never did grow properly, the bug stunted them..
From then on, when I prepare the next spring planting sites in the fall, I use a dose of the sprectricin granular, worked into the soil along with the mulch and fertilizer. By spring, all that has been incorporated into the soil. Haven't had any soil bug problems since.