Once black spot has taken a firm hold, the best product I have found to kill it is called Mancozeb fungicide. It is available on amazon.com. The spray will leave a yellowish powdery residue on the foliage but that is part of how it works. Once the black spot fungus has been killed, the new foliage will be free of the black spots. The foliage already effected by the black spot will never totally heal of its marks but will remain even after the fungus itself has been killed. Which is why it is also called the Dreaded Black Spot. For prevention, I spray my roses every 10 to 14 days with an earth friendly fungicide called Green Cure. I mix it using the Cure Rate listed on the label as it lasts longer on the foliage and seems to work even better as a prevention tool. Green Cure is available at a website named Planet Natural and at amazon.com.
Answered on April 28, 2017
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