I have an old established mahonia in my new garden which appears to have
a very bad case of mildew (or similar fungal disease). It has quite a
few dead branches which look like the result of problems untreated in
the previous 5 years. Any suggestions how to make if healthy again ?

Or is the effect of 5 years neglect going to prove terminal ?

The gooseberries haven't been pruned for a similar length of time and
also have bad mildew but it isn't causing any die back I can see. I am
worried that the mahonia has something nasty fungal and potentially
infectious.

Thanks for any suggestions.

Regards,
--
Martin Brown