Magnolias (Magnolia sp.) are desirable for their large, showy flowers and for their hardy nature. These trees rarely suffer from insect pests or diseases, according to Edward F. Gilman and Dennis G. Watson, horticulturists with the University of Florida. Still, rarely does not mean never. There are some diseases that magnolias can suffer from, including bacterial and fungal diseases. Most are only unsightly, but one can be deadly.
Leaf spot is the appearance of unsightly spots on the leaves of the magnolia tree. The spots can be tiny specks, or large, dark-colored asymmetrical blotches. In some cases, the spots merge to cover part or all of the leaf. In other cases, the spots have a lighter colored "halo" or border. Most leaf spot is causes by various fungi that travel on water, such as splashing rain or rain blown by the wind. For this reason, leaf spot often occurs during warm, rainy spring weather. Leaf spot can also be caused by the bacteria Pseudomonas syringae.
The only treatment for leaf spot is to prune off the affected branches to prevent the disease from spreading. Leaf spot will not harm the magnolia tree, however, so many home gardeners prefer to just leave the tree alone. When the leaves fall, they should be raked up and disposed of to prevent the fungus from overwintering in the soil. Leaf spot can be prevented by an application of fungicide in early spring.
Canker disease is fungal disease that enters a tree through a wound in the wood. The wound can be caused by pruning tools, gardening tools (such as a nick from a lawn mower) or boring insects. The disease causes the wood of the magnolia tree to discolor and crack. Branches with cankers should be pruned off about 4 inches below the canker. Otherwise, the canker will eventually spread and kill the entire branch anyway.
Cankers can be prevented with a yearly application of fungicide in the spring.
Verticillium wilt is a deadly disease caused by a fungus that often attacks shade trees. This includes magnolia trees, which are known to be susceptible to the disease, according to James Schuster, a horticulturist with the University of Illinois.
Verticillium wilt attacks the roots of the tree, cutting off the supply of nutrients and causing an overall decline in the growth of the tree. Healthy trees can resist the attack much more effectively than stressed trees, so it is usually trees that are already unhealthy that succumb to this disease. Once infected, the tree will die usually within two years. There is no way to cure or even control the disease, so infected trees should be removed as soon as they are diagnosed, and no other plants should be planted in that soil for several years.