Most likely aphids.

Also looks like ran short of water.

Did it run dry? Has the temperature soared? did you miss a watering or two?

They don't exactly look like aphids but if not active insects, that's a reasonable possibility. If so, they should just wash off easily with a strongish stream of water. You may have to repeat this practice from time to time until the tree loses its leaves and enters dormancy. This is really the best method when dealing with JM's as they resent spraying with most pesticides, even those as benign as insecticidal soap.

I'd also agree that the leaf problem is most likely a sign of watering irregularities rather than any sort of disease.