I have a Mahonia x Media 'Lionel Fortescue' in my garden. Its a young plant
and is only approx 2 feet tall at the moment. It has developed some kind of
disease on its leaves and, as I'm an inexperienced gardener, I've no idea
what it is.

Symptoms a many leaves with black patches which seem to be growing.
Leaves eventually drop off.

As I said, its only small and doesn't have too many leaves to lose!

Can anyone diagnose and suggest a cure for this?

Andrew Thomson