agraham

Tyr,

It's hard to say.In general your tree looks healthy enough.It could just be the natural replacement of old leaves with new ones.If your tree didn't go dormant and lose all the leaves in winter,it will drop the old leaves over a longer period of time.

Looking at the green leaf in your third picture,it looks slightly mottled..perhaps you have spider mites.You would want to check for those and treat if necessary.

The best thing for you to do is to ask someone who has experience in your area and with elms.A up close personal inspection is always superior to a guess while looking at photos on the internet.

andy