They are already infected. The spray just brings everything out quickly. That is why I spray every week during the growing season. If I miss a couple of weeks, my roses that do get BS are infected even if they look clean.

Oh, i don't know but mine do the exact same thing! It drives me nuts! I think, aha!, I'll catch it quick but then it seems like I just made it worse. So frustrating!

I'm sure you're probably right, Ken. It was already there but it just hadn't shown symptoms yet. But it's still maddening!

Me too. I think I will try to start spraying weekly too.

As I said on the other thread, the Bayer Natria ready-mixed is not the Bayer product commonly recommended here (Disease Control for Roses, tebuconazole). The former is probably only slightly effective under severe conditions. The latter will prevent black spot almost completely.

If you are spraying in the evening so the leaves stay wet, using the organic product could make blackspot worse, but it takes around a week from germination to visible spots. Otherwise getting spots after spraying is just coincidence. There were already germinated spores or yet-invisible infections on the leaves.

Sometimes, though there is not evidence of that here, it is really spray damage. Where I see it most is in the spring, when people have been told they have to spray to stop blackspot from starting, but it is too cold. The result is a lot of little black dots.

Thank you all - at least I know I am not alone