Probably scale - wash off the white stuff, or have a very close look where the marks are - you should see a sort of small shield like object in amongst the white stuff. Depending how bad it is, you can either touch each one with a cotton bud dipped in methylated spirits and clean them off, or buy a spray treatment made by Provado - make sure it mentions scale insect on the list of things it treats. If the trees are large, some scale infestation is 'normal', you often see it on large trees, but you may want to treat it if it gets bad.