mgleason,

Henry Kushka (I hope I've got the name right) posted the following link a while ago I believe. I have used it and it seems to work, especially on keeping the furry type mold from growing. I don't know how it would do on established blackspot. My guess is that, though the plant has blackspot now, it will still root, and that the new leaves may be prone to the existing blackspot. However, once the plant is overwintered, and the canes are cut back to the soil level, all fungii will also be removed with the canes, so one needn't worry.

You would be disappointed to visit my propagation area this year because I never bothered taking any cuttings. But if you ever show up, we can tour the major private gardens in the immediate area, and some of the owners are as good as I am or better at growing from cuttings. One friend has a beds of various favourites grown from one mother plant, hybrid teas like Double Delight, much to the contrary of those who say one can't grow strong HTs on their own roots. It's amazing, so come on along.

Steve.

Here is a link that might be useful: hydrogen peroxide effects on root induction