Look at the branch to see if there is some mechanical damage to the branch that is restricting the flow of nutrients to the leaves.

Thanks Kimmsr, I looked & don't see any signs of injury; the branch appears perfectly normal! I thought it must be a fungus or virus? The leaves become quite large and 3 lobed (actually looking rather like sassafras leaves)--like a limb of a different tree has been grafted on--but the leaves close to the trunk are normal. Weird!

http://hort.ifas.ufl.edu/nutdef/report8-Fe-D.htm

Could be iron deficiency, as that sometimes can start with a branch or two before progressing. If veins in the leaves are larger instead of finer, then suspect manganese.
Can't rule out virus though you may want to go through the Extension Service to at least rule that out. Soil analysis would reveal more about possible nutrient problems.

Also, a few species of crabs do have lobed leaves, most notably M. transitoria, however I've seen "regular" leaf types occasionally get an almost lobe.

Thanks for the link stompede. It has been very wet here this year, that could be contributing. I will send the pictures to our OSU extension person & see what they say. I need to talk to them anyway about controlling cedar-quince rust on my hawthornes. I pulled out my sickly junipers 2 years ago & last year thought I had the problem licked, but this year, worse than ever.

It may not necessarily be because of branch damage, the damage could be where you can't see it. In most trees, you will find that damage to a root on one side of the tree will effect the health of the branches..........on that side. That is until the damage is large enough it compromises the the ability of the tree to sustain itself. IOW the vascular system doesn't just dump into a central point like in an animal body, certain roots feed certain area of branches.

Calliope, that is true,however I would expect to see more wilting, stunting, or dieback if that was the case. While there are diseases that may manifest itself that way (Black root rot is one), think of balled-and-burlapped trees surviving a loss of more than 85% of their root systems.

As for cedar-quince rust, good luck. It can be absolute hell trying to keep it off hawthorns, serviceberries, pears in a bad year, fruiting and flowering quince (Cydonia and Chaenomeles), and apples. Sanitation is a huge part of the control. The spores are everywhere, so check your clothes, hair, etc. as well as cleaning tools properly. This fall/winter prune out all galls that may have formed and pick off all infected fruit (they will be dark and shrunken (a few may be swollen) at that time of year). In the spring, when buds start expanding, start treating with Daconil or Mancozeb to protect the new growth and retreat every two weeks, every week if rainy. Every third or fourth spray, use a systemic such as Infuse or Immunox. Repeat the process over. Any leaves that develop spots should be removed and galls pruned out. I would pick out infected fruit. Just a thought on this though, but it may help to wet down the leaves/fruit before messing with the plant as that seems to control the spread of the spores ( I wouldn't worry about infection as the plant will have been sprayed). When dry and if disturbed, the "horns" will release the lightweight spores and they will drift easily.

I had heard about removing cedar trees within a certain distance from the infected plant, however that doesn't work with CQR since it can occur on a host plant year after year.

Let me know what your Extension service recommends.