Some rosebushes will totally defoliate with Black Spot. As long as the canes are green, the foliage will return. Pruning heavily will only add more stress to an already stressful situation for the rosebush. Two things I would do and have done in the past. 1. Purchase a fungicide called Mancozeb. It is available on-line at Amazon.com and some other sites. When you get it, spray the rosebush canes and any foliage well with it. It does leave a yellowish powdery residue on the foliage and canes but that is part of how it works. It is the best fungicide I have found to stop Black Spot in its tracks! If the foliage has started to return when you get the product, spray ASAP and spray again in 5 to 7 days for three to four sprayings.
2. Water the rosebushes effected with some water that has both a product called Super Thrive and a root stimulator in it. Water the bush with a fresh batch of such mix the next 5 to 6 times they need watering. This helps the rosebushes deal with all stresses and gives the root systems a boost to get things growing back nicely. Keep after the black spot, it is a sneaky foe! Once the black spot appears on the foliage, it does not leave even after treatment. However, new foliage should be clean and beautiful again once the fungus has been killed.
