Lecanium scales (there are about 12 species) are known as “soft” scales and are common pests on many ornamental plants all over North America. Holly, elm, redbud, walnut, citrus, apricot, pear, persimmon, beech, box elder, grape, pecan, rose, and willow are a sample of the diverse range of hosts that Lecanium scales can parasitize.
As these scales feed, they excrete large quantities of honeydew which serves as a substrate for sooty mold fungi.
I would suggest trimming off the most severely infested twig tips. This pruning will be an opportunity to shape the plant and remove many of the unsightly scale-infested twigs. After pruning the entire plant should be sprayed with a horticultural oil/summer oil. This oil spray should be applied so that all plant surfaces are wet to run-off. One thorough application should control the problem. However, from time to time you should check your plant to see that no new infestations are establishing. During these inspections be aware of ant activity on the plant(s). If ants are present, some attention should be given to ant control as the scale problem will not go away as long as ants are present to protect and nurture them.
Use all plant protection products according to label directions for best results.