It looks like some leaf miner has been there, an insect larva. Probably nothing to worry too much about

are those suckering from the bigger trunk ... ???

if so.. the bigger is probably dying...

if not.. then the bigger may be stealing the water the babes need ..

i am loathe to jump to disease in august ... there is usually a big difference between the cause and the effect of leaf damage ...

if drought is the cause.. then fixing a disease on a leaf.. doesnt really solve the real issue ... eh???

ken

It's damage from a water shortage.

How long have they been in the ground? If planted during recent months, likely the original rootball is too dry.

Further, how far are they from the fence? (Looks way to close from here.)

I don't believe in water shortage. When that happen all leaves would be affected to some degre but this tree has only completely healthy and completely brown leaves. Also, those are probably suckers from the roots, european aspen sends those up all the time even if the mother tree is just fine.

There are som white traces on the dead leves wich I think are from leaf miners.

Google Bronze leaf disease.