Black spot disease, if left untreated, can cause your shrub to lose all of its leaves and flowers. Black spot disease is characterized by unsightly black or purple leaf spots with fringed edges. It most often attacks after a period of warm, humid weather in spring. Once established on a plant, black spot spores spread easily--a contaminated splash of water can easily infect neighboring plants. So it is important to treat black spot as soon as you notice the symptoms of a suffering plant.
Clear all of the fallen leaves at the base of your shrub. Bag them and throw them away. Do not compost them.
Prune all leaves that show symptoms of black spot using clean, disinfected bypass pruning shears. Keep an alcohol swab nearby and wipe down your pruning shears after each cut to avoid accidentally spreading the disease.
Water your shrub at its base. Do not water it overhead. Black spot spores need wet conditions to thrive.
Spray the shrub with a fungicide prescribed for use on black spot fungus and listed as safe to use on the type of shrub affected. Follow the manufacturer's instructions for application methods and rates, and coat the top and underside of all of your shrub's leaves. For the best results, spray on a windless day with no rain and none forecast for the next 48 hours. For a list of appropriate fungicides, see the Resources section.
Re-spray the plants at the intervals dictated by the fungicide's manufacturer or after every rain during warm weather. Once the weather is consistently hot and dry or cool (below 70 degrees Fahrenheit) you may discontinue the spray.