The red growths on the leaves are galls. They are caused by an insect called spiny witch-hazel gall aphid. Examine the undersides of leaves for beneficial insects such as ladybird beetles and their larvae, predaceous midge larvae, Syrphid fly larvae, lacewing eggs and larvae, and parasite activity. Descriptions of these beneficial insects may be found in this publication: https://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG62%20IP... IPM: A Common Sense Approach to Managing Problems in Your Landscape. Any combination of these predators and parasites may give sufficient control without having to spray with an insecticide. If damage is heavy, spray with a registered systemic insecticide. Coverage of the lower surfaces of the leaves is critical, as the aphids are fairly protected in the corrugated folds of the leaves.