We have a Merril magnolia (4-5 years old) that is suddenly and rapidly dying. The leaves on one branch turned brown and it quickly spread from there. I am afraid it is verticillium wilt, but just before this happened we also tilled around this flower bed to expand it and wasn't sure if maybe we damaged the roots? We are trying to identify the problem and see if there is anyway to save the tree.
Cecil County Maryland
From your picture, it does appear that one branch has died on the right side, and was probably damaged at the base or on sub-surface roots. I suspect that one lead branch will not come back, and you may need to trim it off. Be careful with pruning, and prune off just outside the swelling at the branch collar, not flat with the trunk. Do not mulch up around the cut place and do not paint with any substance.
If it was mechanical injury, it will not spread to the rest of the tree, but keep in mind that any wound will open the way for pathogenic fungi to get in. Keep an eye on the tree over the next year.
I doubt it is Verticillium wilt, but it is possible. If the dieback spreads, it may not be possible to save the tree.
Thank you for contacting Delaware Cooperative Extension.