Janet Tweedy wrote:
Though quite why hawthorn should be bad for apple
trees isn't in any book I have.

The only caution is not to plant it near apple or pear orchards, since
it is associated with fireblight disease in these trees. The bacterium
Erwinia amylovora hosts include hawthorn (which I have), amelanchier
(which I have) chaenomeles (which I have), cotoneaster (with I have),
quince (which I have), photinia, pyracantha .... All of those that I
have are situated in my front garden and all surround my apple tree.

Amazing bad luck I know.