I have an old citrus tree that has been in decline for a couple of years,
but has no obvious pest.

Extremities are dying off and there is a dead wood in the middle of the
tree.

The bark is discoloured, but not with spot like areas. THe fruit on this
tree is not palitable even when it does produce.

It is very close to a callistomen - but has been for over 30 years without
harmful side effects.

We want to preserve the tree for privacy rather than any other reason, so
fruit crops are not important.

Any ideas on possible reasons for its decline?

Joanne in Perth