The leaves of deciduous magnolia trees naturally turn brown and fall every autumn. However, when this happens in any other season, it could be due to high winds that dry out the foliage, frost damage during spring or inadequate watering during summer. Evergreen magnolia leaves do not fall during autumn.
If brown magnolia leaves fall from the plant immediately after changing color, it could signify an iron deficiency. In this case, a soil test can check the tree's iron levels. If the leaves develop black spots before turning brown, the plant might have been infected with leaf spot, a plant fungus.