Black spot is not a virus.
Certain fungi can produce diseases that mark the plant with black lesions.

More likely, "summer drought" causes damage due to a water shortage.

Any chance of posting pictures? Ifs o, it's like we can clarify the issue.

Plants that show signs of "disease" every year are trying to tell you that the conditions they are growing in are not suitable for them.
What is the soil like they are growing in?
What is the pH of that soil?
Are the nutrients in balance?
What kind of life is in that soil?
Does the soil drain well?
Does that soil retain moisture well?
What does that soil smell like?

i would ask.. do you mulch ...

some of these problems are soil borne.. and by covering the soil.. and not letting it get on your plants.. you can reduce the level of the problem ...

also ... late fall cleanup.. and removal of all previously damaged plant parts helps ...

that said.. i have never seen a peony.. in my various gardens.. over the years.. that did not spot.. later in the summer and fall.. i always presumed it was a function of the plant closing down for the year ...

ken

Thanks for all the responses. I'll try to answer them all. I do think drought is the likely culprit but in the past 3-4 years, I have noticed that a number of my plants get spots on them at the end of the season. I have very good amended soil, adequate mulch with pine bark nuggets, and only water with soaker hoses under the mulch. But sometimes in the hottest times, I don't go out and turn on the water like I should. I do remove the foilage when its dead. When it has the black spots on it, should I remove it then or wait till it dies back. I have taken pictures but can't find them. I'll look more. I have included a picture I took last week of a damaged flower. All the other flowers (profuse) were lovely but one was rotten in the center.

If only one bud I'd not worry about it. That may have been breached by a bug making it susceptible to other maladies.

tj