In Michigan black spot is common any time! We tend to have a lot of hot days, cooler nights and high humidities. That's perfect BS growing conditions.
I don't like to recommend any kind of home remedies. They seldom work very well and they are often harmful in other ways. At least that one doesn't seem to have cooking oil in it! I literally cooked all the leaves off my roses once spraying them with a home brew that had oil in it. Oh, I didn't have BS for sure but I also didn't have any leaves left, lol! What ever you decide to use, never spray any thing but plain water on your roses if it is over about 75 degrees. Always spray early or late in the day. Never at the hottest, sunniest part of the day and always water your roses the day before and after to keep them hydrated. Personally, I've learned to look for roses that don't spot easily and to just live with the spots I do get.
