Hi Gary.

Thank you for reaching out to us at the UNH Extension Infoline!

It is tricky to ID 100% what the main cause is here regarding your Itoh Peony. I will be sharing a few resources with you below for possible causes. I will also share our plant diagnostic lab information where you can submit a sample to get proper diagnostic and management steps.

Here is the link for the UNH Plant Diagnostic Lab homepage, which has the address to send the sample to, as well as information on best practices for submitting a plant sample and other resources: Plant Diagnostic Lab Homepage
And here is the link to the submission form to submit a plant diagnostic sample: UNH Plant Diagnostic Lab

Additional resources:
Peonies frequently experience various forms of bacterial leaf spot, however, your photos show smaller black dots versus the larger, more defined spots. Further down the plant, I am seeing torn leaves. This could be related to the leaf discoloration or an additional pest concern.
https://americanpeonysociety.org/learn/diseases-and-pests/
https://www.torontomastergardeners.ca/askagardener/peony/#:~:text=Brown%20leaves%20may%20also%20be,tissue%20dark%20brown%20or%20black.

It is interesting that only one peony is experiencing this issue with them all being planted nearby! Did they all bloom this year?

I hope this information gets you in the right direction. Please reach out if you have any other questions!

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