thin the limbs to let better air move through....cut other things back for more sun exposure and then call county Extension office for information in your area. they are very helpful.There is no charge for this service as your tax dollars pay for it.
It may be white flies. I would spray the trees with Sevin, which is made for fruits & veggies. It will take multiple applications, if the neighborhood is full of this, as mine was.
Definitely call your County Extension Center for information, they are great.