can this tree peony be saved?

I planted a chinese tree peony last year. I live in zone 4, Minneapolis, Minnesota. I protected it and it came through the winter with no damage apparent whatsoever. It leafed out well but produced no flowers. A few weeks ago, it started to show signs that a whole branch was in trouble, the leaves turning brown and crispy. I have removed dead leaves but now I am concerned that the more of the plant is starting to die.
I haven't over watered it. I check with a water meter and have hardly watered at all since we have had sufficient rain. Since it started looking ill, I have only fertilized it once, with fish emulsion mixed with water.
Our soil is bad, very heavy clay so I excavated a hole for it about 3 feet square and filled it with compost, peat and black dirt. I made a hill for it to ensure good drainage.I mixed clay soil conditioner with the backfill to make the surrounding soil less container like. This spring, when I removed the winter mulch, I incorporated some compost and perlite around the base. The tree gets full sun most of the day and faces the south.
So far, the damage seems to be limited to one large branch but I am concerned if it is spreading. Could it be rust? As I said, it has been damp and cool so far this summer. Any other ideas? I will move the plant to save it, but right now I can't think of a more ideal spot. Maybe a pot? Any suggestions would be welcome.
mm
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You might have botrytis. Remove the dead branches and leaves. If you compost the leaves and branches, don't use it on your peony. Sometimes compost hasn't aged enough and botrytis gets on your peony that way. When adding compost or manure in future, put it about a foot around the shrub so it's not in direct contact with the shrub. Sunlight is the best thing for the healthy branches.
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I don't think it is a problem with compost. It isn't homemade, it is commercial, mostly rotted manure. I will keep that in mind though for my homemade stuff which is cooking right now.
mm

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A foot of manure seem a bit extravagant to me.
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