I have 6 iceberg roses growing in pots on my terrace. Last summer a number of them had black spot. Do I respray when the leaves come out? How often? What about the soil?
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.
Black spot is difficult to eliminate, but several cultural practices can be used to manage this disease, as outlined in these Toronto Master Gardener webpage.