First, you have to divide remedies into two groups -- systemic curatives, and contact preventatives. This article explains them:
https://www.rose.org/single-po...
For a good list of available fungicides used on roses, see below. You can then do individual searches for each to learn more, and decide if they are things you're comfortable using.
http://rosemania.com/page5.htm...
I've found that many products -- and their generic equivalents -- are available at either site below:
https://www.domyown.com/
https://doyourownpestcontrol.c...
If you're already seeing blackspot, you'll want to start with a systemic curative to kill the infection. After that, you'll use one of the contact preventatives to keep them from getting reinfected. There are several organic fungicides out there, but aside from those utilizing beneficial bacteria that kill the fungi -- such as those containing Bacillus subtilis, like Cease -- the rest are basically contact preventatives. This means that if your roses are already infected, using things like potassium bicarbonate, Neem oil, etc. will not fix the problem. You'd be better using them AFTER using a systemic curative to prevent new infection.
Some products containing Bacillus subtilis:
https://www.arbico-organics.co...
So if you want my recommendation at this point, I'd say to use a chemical systemic curative now, possibly repeating it a second or third time, seven to ten days apart (whatever the label says). Then switch to something as a preventative like potassium bicarbonate, perhaps with some kind of sticker-spreader that will prevent or reduce wash-off from rain. Avoid anything with oil (like Neem) when temperatures will be going above the low 80sF. This would also be a good time to try one of the beneficial bacteria products, since (supposedly -- I haven't tried them) they'll eventually colonize your garden and step in on their own to kill a future infection without regular reapplication.
Another spray product to consider is actually listed as a fertilizer, but seems to offer some preventative protection -- DynaGro Pro-TeKt. That might be something worth mixing with the potassium bicarbonate.
https://dyna-gro.com/product/p...
available here:
https://www.greenhousemegastor...
By the way, I found a cheap source for potassium bicarbonate:
https://labelpeelers.com/potas...
Personally, I prefer not using the "big guns" chemical stuff. But for the safer organic stuff to really work best, you need to begin with a fresh start. If you were just starting your garden and used them from the beginning, you'd have things more under control. But using limited amounts of the "big guns" at the start, you'll be better able to achieve a compromise between keeping things looking good and not having to don protective chemical gear regularly. Think of the "big guns" like antibiotics or strong medication -- if you're already sick, you'll need the intervention to get healthy. Then you can STAY healthy without the "big guns" by sticking to more "natural" methods.
:-)
~Christopher
