sunfish

I am no expert that is for sure. But looking at the picture of your plant the first thing I notice the plant looks a little deep. I would pull some of the dirt from around the plant and use it to make a basin. I would also dig down and check to see how wet the soil is. Maybe the water is sitting in the bottom of the hole and not draining . If it is rotting it has to be from to much moisture. Sometimes with a drip system the water goes straight down and it can be dry on the top and wet down below.