Dieback on Sorbus

HI
We have 4 Sorbus Aucuparia (Mountain Ash) trees which we planted 2 years ago. They are all losing their leaves - as if its autumn. One of them has very shrivelled leaves and is losing its berries as well. The berries on the other 3 are fine. Our soil is heavy clay which is supposed to be OK for these trees. There is no sign of any pest or fungus. Do you think it is just down to all the rain?
Regards
Treehugger

 

Are the leaves turning brown before falling?

 

Hi
Yes, full autumn colours orange, red and brown.
Rgds

 

I don't think it's down to the rain, as the leaves are changing colour. I think they would just turn brown if it were waterlogged, though I'm not sure.
Have you fed them at all over the time you've had the trees?