My last house 1/2 hour away: acidic soil mulched with acidic pine park - the worst black spots on roses despite frequent spraying.
My current house: alkaline soil, next to a limestone quarry, with well-water high in lime, mulch with horse manure (has lime). My water is alkaline, pH of 8 and my soil pH is 7.7. Lime is alkaline and also a natural fungicide. I don't spray, but had 10 clean Austin roses last year, and again with 38 clean roses this year (hybrid teas, Austin, Romanticas, and floribundas).

This year I did some experiments: I put Hollytone fertilizer (same as Rosetone, except with sulfur). Four roses applied with Hollytone broke out in blackspot, the rest mulched with horse manure are clean. I also dumped coffee ground and rotten tomatoes around Firefighter rose: it broke out in blackspots immediately. In contrast Paul Neyron rose is 100% clean with neutral potting soil, plus alfalfa meal, lime and 45% COMPOSTED pine-bark (has tannin, a natural fungicide).

Below is my healthy Paul Neyron rose, taken in 70% humidity and prolonged rain, despite rain water being acidic with pH of 5 to 5.6. Paul Neyron is known for very blackspot-prone. I don't spray.

Below is my 2nd-year Golden Celebration, a yellow Austin rose that's very black spot prone. You can see it's base quite healthy, and the yellow wooden post at corner to support the bush, which becomes 5' x 6' in partial shade. This picture is taken today, September 18, after all day & all night rain:

From 1989 through 2007, that was my practice in my old Newhall garden which contained over 1,200 roses of virtually all types and breeding lines. I found the second line of disease defense to be shovel pruning. Those which wanted to live and perform well there were encouraged. Those which proved themselves uncooperative, either went to other gardens or into the trash. You really CAN "cure diseases in your garden". All it really requires is a good, sharp shovel! Kim

NOTE: I don't recommend anyone grow Paul Neyron nor Golden Celebration as posted above. They are the most disease-prone roses, and unless you mulch with horse manure or have specialty-blend soil with composted pine, lime, and gypsum - they won't be healthy. Paul Neyron was bred 1869 and Golden Cel. was bred in 1992 - both are old roses.

One way to grow healthy roses without spraying is to grow recently bred disease-resistant roses, such as Annie Laurie McDowell, bred by Kim Rupert 2001. Annie L. McDowell blooms at the expense of growth, likes heat, free of blackspots, rust, and mildew. It's small in zone 5a and can be grown in a pot. It smells better than my 15 Austin roses and other strong fragrant roses. Having Annie L. McDowell is like smelling lilac and lavender all summer long. See picture below:

Kim Rupert also breeds Lynnie, as tough as Knock-out, but blooms more, and more drought-tolerant. I put Hollytone, or acid fertilizer on both Lynnie and Austin rose "Eglantyne". Eglantyne broke out in horrible blackspots, but Lynnie broke out in blooms instead. She has more blooms than leaves now. Lynnie is a landscape rose that blooms well in partial shade, disease-free, very mildew-resistant, and set beautiful hips for the fall. It's in my no-water garden. See Lynnie rose below, taken after all-night rain.

Besides being disease-free, Lynnie rose is almost thornless (90% smooth), and Annie Laurie McDowell rose is 100% smooth, zero prickles, with pretty leaves like a tropical plant. See picture below:

I think growing roses organically is both easy and fun. Roses get blackspot and drop their leaves here, however they grow well and bloom well. Blackspot does not diminish vigor or hardiness.

Good soil creates an environment where roses make strong garden plants, even with blackspot present in the garden.

I've found that roses thrive on organic fertilizers and topdressings.