We would like to have more information.  Please send photos of the whole tree, around the base, and affected foliage so we can see what you are dealing with. 

Marian

Thank you for the photos. Our plant pathologist looked at them this morning and does not see anything that appears abnormal for a mature Magnolia tree. These Magonolias do shed their older leaves. After a season when the leaf growth was very abundant, the loss of older leaves can sometimes appear alarming, but this is not out of the range of normal. 

The bark has some moss, algae, and lichen. None of those organisms are harmful to the tree.

Christa

We recommend that you contact a certified arborist who can evaluate your tree on site. A qualified arborist will have the tools and knowledge to evaluate possible causes of the bark cracking and assess the structural integrity of your tree. You can find a certified arborist near you using the directory provided by the International Society of Arboriculture. https://www.treesaregood.org/

Christa