We have an Apollo Magnolia that is about 12 years old, and has thrived until this spring. The flower buds have turned brown, with a few exceptions, and the tree appears about half dead, although some branches are showing good leaf. The tree lost a large branch from wind this winter. The main stem and some branches seem to be oozing black sap. I've attached pictures of a) typical flower browning and dying and b) the stem oozing sap. Any ideas what causes this? I can't see any signs of bugs and no growths or canker on the branches, just a general deterioration in the flowering and leaves.