Southern Magnolia Tree Diseases

Southern magnolias are broadleaf evergreen trees that flourish in the southern United States. They thrive in hot sunlight and acidic, moist peat soil. Southern magnolias are susceptible to several different diseases.

Types

The Verticillium dahliae and Verticillium albo-atrum fungi cause verticillium wilt in southern magnolias, according to the Missouri Botanical Garden website. Nectria fungi cause nectria canker, while the Pseudomonas syringae bacteria cause bacterial blight. The Septoria, Cladosporium, Phyllosticta magnoliae and Coniothyrium fungi cause leaf spot diseases.

Effects

Verticillium fungi infect southern magnolia trees through their roots. The tree wilts because the roots, which become clogged with spores and gums, cannot transport water to the leaves and stems. Some trees die quickly, while others die over several seasons, according to the Missouri Botanical Garden. Nectria canker is characterized by sunken and discolored areas of bark and by the presence of colorful pink, purple or orange fungal bodies. Bacterial blight creates irregular dark spots on magnolia leaves and destroys new shoots, while leaf spot disease appears as purplish or black spots on leaves.

Prevention/Solution

The Missouri Botanical Garden indicates that fungicides are ineffective for verticillium wilt. According to the City of Seattle, verticillium-resistant Kobus magnolia trees are an alternative to southern magnolias. Pruning out infected branches and sanitizing equipment helps to control the symptoms of nectria infections and bacterial blight. Leaf spot infections can be controlled with an appropriate fungicide.

References

• University of Florida IFAS Extension: Magnolia Grandiflora - Southern Magnolia
• Missouri Botanical Garden: Verticillium Wilt
• Missouri Botanical Garden: Nectria Canker and Dieback
• Oregon State University Extension: Magnolia -- Bacterial Blight
• Oregon State University Extension: Magnolia -- Leaf Spots
• City of Seattle: Recommended Trees for Future Planting