If the leaves on your holly bush begin turning black, suspect a fungal disease called "black sooty mould," also known as Capnodium. Black sooty mould doesn't harm the plant directly. The mould, however, blocks sunlight from reaching the leaves, disrupting photosynthesis. If your holly has aphids, it is susceptible to black sooty mould. Aphids secrete a substance known as "honeydew," creating an ideal breeding ground for fungus. The only way to rid the holly of black sooty mould is to eliminate the aphids first.
Spray the holly foliage using a strong spray from the hose. This should knock off any aphids.
Apply insecticide to the holly bush for large aphid infestations. Apply it at the rate listed on the label, to both sides of the leaves.
Apply dormant oil spray to kill the aphid eggs when the plant goes dormant. Use the amount listed on the package and apply it twice during the winter and once in early spring.
Treat foliage in the summer with insecticidal soap. This can be purchased at a gardening centre or you can make your own by combining 118ml. dish washing soap in 1 gallon of water. Pour the soap mixture into a spray bottle or tank sprayer and spray the entire holly bush until the foliage is dripping. Allow the soap to remain on the holly bush for five minutes and then rinse it well with clear water.