I wonder if your tree is taking all the water, Sergeant. Could you flood your rose with a hose periodically to make up a water deficit?

What Sheila said.

https://www.gardenweb.com/discussions/6003039/biochar-and-horse-manure-for-roses#n=7

Mildew came up in this discussion, might be worth a look.

Hi Sheila and Jeri - I do not give this rose supplemental water as it always looks green and healthy, putting out lots of fresh growth. It only mildews at bloom time. Does dryness contribute to powdery mildew? That would be an easy fix. Would it just need extra water in the Spring, a few months before bloom?

You could give extra water starting in Spring and through the time you wanted it healthy. Yes, water deficiency can cause mildew in roses.

Yes, water deficiency easily contributes to both mildew and rust outbreaks. I read it in Jack Harkness' and E.B. LeGrice's books and didn't believe it until I DID it. When a rose exhibits either fungal issue for me, I increase the water first to determine if that is the issue. Very often, it is.

Very interesting. Thank you both. I will try that next year before giving up on it. It’s hard to wrangle but I’d lose a pint of blood getting rid of it. Here’s a picture of it (at right) when it was a young’un.

That is beautiful. I’d sacrifice TWO pints of blood!