I have a friend that has never had black spot before on their roses, and this year they have Black Spot. What can you suggest — to treat this condition.
Rose Black Spot is one of the most common diseases of roses. It is caused by a fungus, Diplocarpon rosae, which infects the leaves and greatly reduces plant vigour. This fungal disease causes purple-black spots on leaves that usually yellow and drop.
Here are informative Toronto Master Gardeners posts that offer good advice:
For more information and references: