Prevent black spots on leaves by taking steps to remove plant debris from the ground, removing diseased foliage, avoiding overhead watering and removing any diseased leaves. Limit the use of fungicides to times when other methods do not work.
As a plant enters its dormant phase, rake any leaves or other debris from the previous growing season away from the base and dispose of them. These plant materials provide a growing media that often harbors the mold and mildew responsible for black spots.
If black spots appear on leaves of the plant, use pruning shears to remove them. Destroy any diseased materials you remove from the plant. Disinfect the pruning shears before moving to another plant or when you finish pruning. Change your gardening gloves often, or wash your hands so you do not transfer the disease to other plants.
Provide plenty of water, and feed the plant regularly to encourage healthy growth, but avoid overhead watering. Overhead watering leaves standing water on the foliage that encourages mold growth. Add mulch to the base of the plant to conserve water, prevent weeds and protect the plant from accidental injury.
If other efforts to prevent black spots do not work, consider a fungicide. Begin with low-toxicity fungicides to protect beneficial organisms in the garden, and only use high-toxicity fungicides if absolutely necessary.