Q: Some of our magnolias have leaf “clusters”, that are turning brown . Could you advise us about possible causes?
A: I think the flagging is caused by black twig borer. If it was a twig girdler, you would have complained about lots of twigs on the ground and few in the tree….plus the twig ends would be more conical.
The female borer enters a twig through a tiny hole on the underside of the branch. She makes a home for her eggs in the central pith.
This is not something that will kill your tree. Simply prune out the brown leaf clusters and remove them from your property.
see Black Twig Borer