Perhaps scale?

Mix 40 parts water with 8 parts of rubbing alcohol and one part dish soap.......spray and soak the entire tree. Wait 15 to 20 minutes and wash off with a garden hose (with some water pressure)....you will see alot of the scales come off and the remaining ones on the tree will be dead......
The alcohol in this mixture dissolves the scale's waxy coating, allowing the soap to kill the critter under the shell.....
This works very well, getting rid of magnolia scale.....
Spray on a cloudy day......You may have to repeat this in a couple of days if you missed any.

Looks like some nice scale you've got there!

ians_gardener is right on the money with the rubbing alcohol. That stuff does wonders for getting rid of scale.

My only issue with it is the cost, though - for a whole tree, you'd need a lot of the stuff!

If I had this problem, I'd probably try scraping the scale off or dislodging them with a high pressure hose before I did anything else.
Sometimes manually removing the bulk of scale insects is enough to stop them from causing any noticeable damage (when the plant is outdoors).
Besides, it's practically impossible to eliminate them entirely.