How Can I Help My Beloved Magnolia?

I have a few problems with a beloved Magnolia tree. New leaves never seem to develop. They emerge looking healthy, and within a couple of days have browned and withered. The same goes for the buds, which form but never develop beyond tiny balls, before rotting. The tree is well watered and drained, and not in shade. The two mains stems have mottled white bark. Any advice gratefully received.

 

It sounds a bit like verticillium wilt. If you chop a bit off and see if there is a brown stain under the bark it might confirm this