Please ignore all jargon about soil sampling, tissue sampling and having to use so much science for such a simple matter. If you notice the roof gutter drain spout in the picture. It is feeding this corner with tons of water when ever it rains and if you have had lots of rain and its been irrigated it has been over watered to the point of root rot. Even if irrigation has been minimal and rain has been abundant like most of the country. You have what looks like rocks pyramiding up to the crown of this plant and if you have water retaining soil along with any heavy type of mulching then add the roof gutter downspout aimed to flood this corner of the bed then water becomes the worst enemy. Don't over think this. 4" or more of rock or any mulch piled up to the crown along with allot water will drown any root system unless the plant is an aquatic type. You also have plastic bed liner 4" high wrapped around this corner. I'll bet when it rains heavy there is 4" pool of standing water on/in this corner every time. It's been flooded! I'll even go so far as to say if you pull this plant out you will have standing water in the hole. That's BAD!