Annie ...
From what I've read and from my own experience, once a leaf that has black spot on it drops, the spores on that leaf die with the leaf. BS spores are in the air and when conditions are right, depending upon which strain of BS is in your garden, you are going to get BS on your roses. Doing that extra clean up won't make a difference, but you can help the rose by spending your time pulling off infected leaves.
There are a lot of right ways to grow roses. Since I don't spray, the roses I grow have to be able to recover from an infestation of BS. Also, I don't get summer rains, so my climate isn't BS friendly and I have less disease pressure.
Studies have shown that there are five active strains, or races, of BS active in the US and not all roses are susceptible to all of the different strains of BS. So, it depends on the rose and the strain of BS you have in your garden as to whether or not a rose gets BS.
In my climate, which can be wet during the spring, but very hot and dry in the summer, under-planting has little or no impact. The BS goes away when temps reach 85F because the spores are not active above those temps unless they are on a moist/wet leaf for more than 6 hours at night.
Smiles,
Lyn
