These could be overwintering scale insects or their egg cases.
They can be controlled with a dormant oil spray such as Bonide All Seasons Oil. When temperatures are above 40oF, spray with Bonide All Seasons Oil at the dilution rate recommended for a dormant spray. This will smother the eggs or insects. Apply according to the label directions.
Dormant oil sprays are one of the most important sprays of the season for fruit trees as they help kill many overwintering pests on your trees.