The statuesque Southern magnolia can reach mature heights up to 80 feet with a spread that spans about half of its height. This slow-growing evergreen has rich, dark green leaves with brownish-colored undersides. It produces showy white flowers in the summer and large, bright red seeds in the fall. The Southern magnolia, or magnolia grandiflora, is susceptible to several diseases but is seldom killed by them, though disfigurement can result.
Verticillium wilt is a fungal disease that infects the tree through its root system. This soil-borne disease can thrive in the soil for several years without a host. The disease damages the veins and tissue of the vascular system, inhibiting the transport of nutrients and water throughout the tree. Infected trees will show signs of wilting, die back and loss of vigor. Infected wood will display browning and streaking near the phloem area. There is no fungicidal spray to control or eliminate this disease. Southern magnolia trees should be kept healthy through careful irrigation, pruning and fertilization of the tree.
Sooty mold is a fungal disease that infects the surface of the Southern magnolia's foliage and not the tree itself. The fungus develops on excreted honeydew that is released from the foliage after being fed upon by mealy insects. As the fungus develops, it covers the foliage in a black mold. Though the tree is not infected, the mold prevents the penetration of light and air circulation throughout the tree. This lack of sunlight and air eventually suffocates and deprives the tree which results in loss of vigor, wilting and growth stunt. The mold can be easily removed from the tree with a strong spray of water. However, controlling and eliminating the disease requires the use of an insecticide to eliminate the mealy bugs that cause the release of honeydew.
Leaf spot develops on Southern magnolia trees after they have been exposed to long periods of wet soils and standing water. This fungal disease causes the foliage to develop dark-colored fungal spots with off-white centers and yellow halos. The foliage of infected trees will also develop small, black fungal bodies. As the disease progresses, the infected leaves will wilt and die. Though most will fall from the tree, some may remain and should be pruned. The disease can be controlled with a combination of pruning and chemical control.