This is a residue called 'honey-dew' which is the sticky secretions from sap sucking insects like aphids, leaf hoppers or scale insects. 

If the tree is mature and doesn't show other signs of stress or decay, it is usually fine and natural predators take care of the insects. Sometimes it takes a season for them to find the insects and the population can be higher one year than the next. This year, probably due to drought conditions this spring, aphids have been seen on everything so it is likely just aphids and with time birds, lady beetles, praying mantids, parasitic wasps, etc will find them and help reduce their numbers. 

If your tree is newly planted or looks to be distressed in other ways you may want to send us some photos of the canopy and we could recommend further help. If it is a young tree, and you can reach the leaves on the canopy, you could just use a strong blast from the hose to help wash off some of the residue and get the aphids to fall off. 

Let us know if you have further questions. 

Emily