To look at a magnolia tree is to look at the ancient past, for magnolias have been around for more than 50 million years, according to fossil records. When the ice age came, ice floes crushed its forest habitat in Europe, but spared places in Asia and the Americas so that magnolias are native to these far-flung locales, surviving into the modern age. The tree's survival is likely because magnolias don't suffer much with pests and disease. When afflicted, magnolias tend to weather the infestation without much need of intervention.
Gray mold is a fungal disease that attacks many kinds of flowering trees and does so regularly. The fungus doesn't kill, but does cause a blight on flowers, fruit and foliage. In high humidity, a gray web of the mold develops. Magnolia leaves and flowers turn brown; on flowers, this browning can merely look like aging. With aging, the browning of the leaves starts at the outside of the flower and along its tips and edges. If the browning starts somewhere else on the flower, the culprit is likely gray mold. One of the best ways to prevent it is to keep the tree healthy and free of plant debris. Wet plants are more susceptible to the mold.
Verticillium wilt is another fungal disease. It occurs less frequently than gray mold, but is more serious. The fungus starts in the roots of the tree and makes its way up, plugging the magnolia's vascular system along the way. With the vascular system compromised, the tree begins showing symptoms, with leaves dying and falling. If you suspect verticillium wilt, you can cut off a suspect stem or twig and look at the exposed cross-section. The fungus shows itself as a light green to dark green ring of discoloration. Magnolias can rebound from the fungus, but tree death is a risk. If your magnolia dies, try to take out as much of the root as possible. The fungus could live in the soil for years. If you plant in the same location, choose a plant resistant to the fungus.
Black Sooty Mold
Black sooty mold almost always appears on a magnolia tree after the tree has been infested with the magnolia scale insect. The scale lives on the underside of twigs, with severe infestations taking over whole branches. The scales suck the juices from foliage, then secrete a substance called honeydew, which allows the black sooty mold to develop. Thick with mold, the magnolia turns black. You can wash off the mold, but it will recur unless you control the insects. Remove heavily infested branches. Introduce predatory species like lady bugs. Horticultural oil might also be used to control scale. A broad-spectrum pesticide will kill beneficial, predatory insects.