I wrote:
I saw some product sold as a sort of horticultural disinfectant,
with a mention that it could be sprayed on roses with black spot
as it would kill any live spores on contact. It turned out that it
was "hydrogen dioxide" at a concentration of 27% or so.

See http://www.biosafesystems.com/labels...ec%20Label.pdf
for information on ZeroTol. It's labeled for black spot, 1:100
concentration for
the initial treatment if I'm reading the label right. Contact fungicide
only, of course.
That'd be more like 1:10 for store-bought cheap 3% peroxide.

I'm going to try it. I'll let you-all know what happens.

Mark., zone 8b