Although the holly bush is generally hardy and resistant to many diseases and insects that attack other shrubs, even with the best of cultural practices, insects can attack the holly bush. If you are in doubt as to what is infesting or infecting your holly, take a sample to your county cooperative extension agent for diagnosis. When leaves turn black, it is usually a sign of black sooty mold, which grows on insect excrement. Sooty mold, if allowed to build up on the foliage, can kill the plant by interfering with photosynthesis. It is also a clear indication of an insect problem, most commonly aphids.
Prevent sooty mold by releasing ladybugs on the plant. Ladybugs or lady beetles eat many of the insects that infest the holly.
Use a strong blast of water from the hose to wash the aphids off the holly bush.
Spray the holly bush with a systemic insecticide if the aphid infestation is particularly heavy. Apply the insecticide at the rate suggested on the package and treat both sides of the leaves.
Treat the holly bush while it is dormant with dormant oil spray. This will kill any aphid eggs that may overwinter on the plant. Apply the spray at the rate suggested on the label two times over the winter and again in early spring.
Remove sooty mold from the foliage with a mixture of 4 oz. of dish detergent in 1 gallon of water. Pour the solution in a tank sprayer and drench the foliage with it. Allow the solution to remain on the foliage for three minutes and then wash the foliage with water from the hose.