Plum tree problems

I have 2 plum trees I planted bareroot 3 years ago they are in dame area and conditions one is lovely and healthy the other has lime/yellow leaves with a few red dots on it , leaves dont fall of any new growth is yellow leaved , is this a fertilizer issue our ph issue our something more sinister , its on a south facing slope in workable clay soil ground normally isn't to wet as it all runs off , any advice please? I bould it in lidl so it was cheap so if its a kill it option I won't be to worried , I think it's a Stanley plum tree

 

If it's in a south east facing garden in clay soil, I'm leaning towards it not getting enough water.

 

Pictures?