Scales
Many species
These small insects are mostly a problem on woody plants and look like small bumps, which often causes them to be mistaken for a disease. They feed by sucking sap from the plant and when present in sufficient numbers they can cause leaves to yellow and die, which can seriously weaken the plant. Some types have a hard armored coating and others are soft with a waxy coating (they are related to mealybugs). They are most effectively controlled with dormant oil which smothers them. You should also encourage predatory insects
Image: John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org