Some species of magnolia drop their leaves naturally during certain times of the year, particularly in the spring. It is also possible that a fungus is growing on the leaves.
The color of the leaves that fall from the tree is an easy way to determine whether the leaf loss is healthy or the result of a sick tree. If the leaves change to a yellow color prior to falling from the tree, the loss is likely part of a natural shedding process that takes place, particularly in the spring. Brown or black leaves indicate that the tree is getting either too much or too little water. Check the leaves for spots or other signs of abnormalities. Their presence is a sign of fungal growth. The tree itself is not in danger as long as the leaves with the fungus growing on them are immediately removed. This is includes the leaves on the ground that have fallen from the tree. If the leaves look otherwise normal aside from the color, make sure the tree is getting enough water. Magnolia trees need moist soil that drains well in order to grow. They also need plenty of space for their roots to expand.