Black spot fungus can totally defoliate a rosebush and thus weaken it such that it will not have the strength to survive the winter months. However, it is rare. Over the years I have found that a fungicide called Mancozeb will stop the black spot fungus in its tracks, so if the attack is severe, I recommend buying some and spraying all of your rosebushes with it at 5 to 7 day intervals. It does leave a yellowish powdery residue upon the foliage which is part of how it works. So do not be concerned about that. The black spots will not disappear. However new foliage coming out should be free of the black spots if the fungus has been killed. If more black spot appears, do the treatment sprays again and spray the roses a few more times than you previously did. If only a few black spots are seen currently, and after gaining control of the black spot, spray the roses every 7 to 10 days with an Earth Friendly fungicide called Green Cure. Both are available at amazon.com. If you live in a high humidity area the black spot can be a very tough foe to eliminate.
