Posted: 27/04/2013 at 12:49
Pentillie, Black spot is a fungus which affects the leaves and is transmitted by spores, so watering anything on the roots won't help. All you can really do in addition to the things you have already mentioned is to more regularly spray the leaves and keep them really well fed. On my clay soil, it doesn't matter what I do so just have to put up with it. One rose "Peace" is about 40 years old and gets it really bad, but is still growing as strongly as ever. If you see signs of black spot on the actual stems, cut them back really hard early next spring and it will lessen the effect for a couple of years. Try and keep the roses as open in structure as possible - wine goblet shaped is recommended, as the more light and air that can get to the middle of the bushes, the better.
The JF might work if watered on roots of plants affected with vine weevils though.