Fungus on juneberry tree
It sounds like powdery mildew might be a possibility http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=253 amelanchier (June Berry) is quite susceptible. Some people are happy to use chemicals to control it. I prefer a 50:50 solution of skimmed milk and water sprayed on. It usually works well and doesn't harm the beneficial insects and birds.