My magnolia tree seems sick.

Black spots have appeared on the leaves.

Some of the leaves now have holes.

Other leaves have turned yellow.

No flowers have yet appeared by mid-April. (As far as I am aware, there should be flowers by now.)

The plant gets plenty of sunlight and water. I haven't made any changes to its surrounding environment.

Could someone please diagnose the problem and advise on the treatment?

This is my favourite plant -- I would be very upset if it died.

Thanks!