Scale on my Flowering Cherry Tree
Scale is not a disease but a tiny insect that settles into one place and froms a thick waxy scale to protect it. If insect numbers are high enough they will weaken the tree and might even kill it. There is little to be done during the growing season unless you are scouting for the crawlers and can target them for treatment. The crawler stage has probably already passed in your area.
The most effective way to control scale is with dormant oil sprays. These sprays are applied in the early spring as growth begins. Here is a link to an article
Dormant oils are available at garden stores or in the garden sections of many large stores. These are highly refined oils that mix with the water carrier and are applied at 1% to 2% oil in water. You need to completely cover the tree with the oil spray so that all the cracks and crevices are covered. The oil suffocates the scale. Commercial growers often mix in an insecticide for better control. If you do a good job you only need to apply this spray every few years but I would suggest you do it for two years to be sure and knock the scale down hard.
Now you need to figure out how you are going to spray to the of the tree.