I have a camperdown elm about 11 years old. A few years ago, I notice it has some kind of condition. There are little round white things adhered to the limbs which seem to be killing the tree. The limbs in these areas are turning black and the tree has fewer blooms and leaves each year and no new limb growth.

I'm not very knowledgeable about gardening and have no idea what this is. Is there something I can do to treat this to save the tree? Thank you.