wrote
What evidence is there for this?

http://www.telegraph.co.uk/earth/365...ust-warns.html

Or is it the National Trust just scaremongering to get rid of plants
it dislikes?

If oaks have the disease should they not be culled?

Oaks are very much a British native and Rhododendron ponticum isn't.
It does invade woodland and stops any undergrowth as light is excluded from
the ground permanently, they being evergreens. Even forest fire does not
kill them completely as they come back from the roots, seen that myself. For
that reason alone they should be removed but when you add in the fact that
they incubate/carry the dreaded disease that potentially will decimate our
oaks then IMO they must be removed ASAP.
This is one case where we can remove the invader and should.

--
Regards
Bob Hobden