The stately Southern magnolia (Magnolia grandiflora) and its many cultivars grow few molds and fungal diseases in the Southeast and other climates with sporadic rain. Magnolias will grow U.S. Department of Agriculture plant hardiness zones 7 to 9, but if you live in a climate with prolonged spring rain, your tree may suffer from fungal diseases. In many cases, there is little you can do to combat them other than to remove and destroy infected parts of your tree.
Magnolia Leaf Spot
Numerous species of Cladosporium, Coniothyrium, Phyllosticta magnolia and Septoria fungi cause leaf spots on magnolia. These fungi all grow in wet, rainy spring weather. Phyllosticta causes tiny purple or black spots on the tops of the leaves, developing an off-white center as the spots grow, sometimes surrounded by a faint yellow halo. Avoid overhead watering and remove and destroy fallen leaves. Numerous fungicides are available for application when shoots are young.
Magnolia blight (Pseudomonas syringae pv. syringae) appears in cool, wet spring weather. Multiplying bacteria destroy plant tissues and produce a protein around which ice can form, increasing frost damage. New shoots rot and die back in spring and leaves may have dark brown spots sometimes surrounded by a yellow halo. Bacterial blight is countered with copper-based bactericides applied once in autumn and twice in spring as the buds break. Prune infected branches and shoots at the end of their dormant season.
Magnolias are among numerous species of trees attacked by the soil-borne fungus Verticillium dahliae that infects the xylem, plant cells that move water up from the roots. Verticillium fungi partly block the flow of water through the xylem, and its toxins cause the tree to wilt. It may kill the cambium, the growing tissue on the inside of the bark, forming long cankers that other plant pathogens may infect. Leaves on a single branch or on one side of the tree, yellowish and smaller than normal, may suddenly wilt and die. Dead leaves may linger on dead limbs as other branches die. An infected magnolia may die in weeks or live for years. Verticillium infection may not show symptoms in the leaves but still cause buds to fail or branches to die. A few preventive fungicides are registered to help prevent verticillium wilt, but none will suppress active infections.
The fungus Botrytis, commonly called gray mold, can cause blight on magnolia stems, buds and flowers. Gray mold grows on dead parts of a tree or enters through wounds. A thick web of gray Botrytis spores can grow on diseased tissue in humid conditions. Spores spread by wind and rain. The wetter the magnolia, the more likely it is to develop gray mold. Gray mold fungi can blow in from a distance, which means that they are difficult to prevent. Remove and destroy infected tissue and flowers. There are fungicides to help prevent or reduce infections of stems and leaves, but none will eliminate them entirely.
Scales, aphides and other sucking insects often excrete a sticky substance called honeydew on magnolia leaves. Sooty mold fungi, including Antennariella, Aureobasidium, Capnodium, Cladosporium, Limacinula and Scorias, form colonies on the honeydew. Sooty mold on leaves blocks light necessary for photosynthesis. The fungi typically multiply during drought when trees are stressed. One way to control sooty mold is to eliminate insects that secrete honeydew. There are pesticides that will kill aphids, but they also kill beneficial insects. University of California entomologists recommend releasing predatory insects that feed on aphids.