Posted: 17/06/2014 at 21:20
Hi Alan, looks like a fungal spot problem, probably caused by wet weather. Try giving it a spray with Westland Plant Rescue Fungus Control every couple of weeks and remove any fallen leaves (and perhaps the worst affected ones, too.) Magnolias prefer slightly acid soil, so an ericaceous feed might help a bit, too. Older trees would probably not be too bothered by it, but as it looks like a young one, it might need a bit of help if, as you say, the whole tree is affected.