Rotted peony roots
I have a robust hedge of peonies in front of my house, probably 20 to 30 years old. I have been here 4 years. Last year one of the peonies seemed to have dwindled: it was smaller than its mates, slower to sprout, had fewer blossoms. I thought that some ice-melt might have been spilled on it, or some other winter mishap. (It had been a vicious winter). This spring it was very late to come up. The shoots were red and spindly and it kept getting botrytis on those pathetic bits. None of the others have ever shown signs of botrytis; I think this was secondary, because when I realized it was very ill, I dug it up, placing it on a plastic sheet so as not to affect the soil next to it, and found most of the roots were rotten, mushy rotten, with a coating of spidery white fungus. It has gone to the landfill.
What was going on? Is whatever disease it had likely to affect the other peonies? Can I ever plant another peony in that spot again? Or anything? (I cleaned the spade and my gloves and the plastic sheet with alcohol)>
Without a pathogen culture, I can't say for sure if the disease would stay in the soil or not. Maybe you can look at the symptoms in this fact sheet and determine what disease it might have had.
I'm sure you can most likely plant any other plant than a peony there are disease are typically host specific. You might want to just try planting a not too expensive peony and see what happens. It really depends on the disease organism.