Appears to be Lecanium scale, soft body insects that can create an ugly situation but often are not harmful to the tree, unless you have large numbers of them for years in succession.  They do have a number of natural enemies that can keep them in check and they can be treated with insecticides, but treating them in this way will also take out the beneficial insects that can keep them in check.   I’ve included three links in which you can read more, including treatment options:    

 http://msue.anr.msu.edu/news/lecanium_scale_and_other_soft_scale_insects

 http://entomology.osu.edu/bugdoc/Shetlar/factsheet/ornamental/Lecaniumscales.PDF

 http://www.uvm.edu/~pmrc/lecanium_scale_info_sheet.pdf

 Be advised also that tree problems of any sort can be addressed effectively by a Certified Arborist, one who specializes in everything woody.  You can find one or more Certified Arborists in your zip code at www.treesaregood.com    Click on Find an Arborist.  Be sure to get an ISA Certified Arborist and not a tree trimmer.  Anyone can call themselves an arborist but certified’s go through a much more rigorous education and testing program.  Arborist evaluations are not expensive and where you can learn about everything else in your landscape as well.

Good luck!