willow dieing back/canker

[Dean O]

can anyone tell me what is causing this small Kilmarnock willow to die back?

One side of the graft appears to be very cankerous, but it now seems to be spreading to the other side.

 

I'd like to positively identify the cause if possible, and would like to know whether its just bad luck or whether the causal agent sticks around in the soil -

 

this tree is a replacement that has been in situ for 2 years and replaced an old Kilmarnock that died - the client suggests that the old tree lived happily in the same spot for approx. 15 years.

 

many thanks

Dean