Hi,
Rosea are hard to grow without some chemical intervention, at least
the hybrids are. I have read that alliums, such as garlic and onions
planted near roses repel some insects and may reduce the black spot
fungus as well. I have planted some garlic next to mine, but its too
early to tell, whether it makes any difference.
Often black spot won't kill the roses, if they are in a good sunny
location, but it sure wreaks havoc with their appearance.