Native to isolated woodland pockets across the Southeastern United States, big leaf magnolia (Magnolia macrophylla) grows slowly to reach a mature size between 30 to 45 feet tall and equally as wide. Perhaps not best grown as an ornamental lawn tree, it is no doubt a sight to behold with large light-green leaves with silvery undersides that measure up to 3 feet long and 1 foot wide. Its fragrant, dinner plate-sized ivory flowers occur by early summer. This deciduous magnolia is best cultivated in USDA zones 5 through 8. Risk of disease increases in trees that are grown in less than ideal conditions.
For the best health of a bigleaf magnolia, it needs to be growing in a deep, fertile soil that is rich in organic matter and remains moist. Ample sunlight is also needed for the leaves to manufacture enough sugars to sustain the entire plant.
Adaptable to partially sunny spots, mimicking the woodland habitat, too little sunlight causes uneven branch development and causes stress on big leaf magnolia trees. "Stress" for a plant means a disadvantage that affects normal life processes or metabolism. Drought, flooded and compacted soils also cause stress in this magnolia since its roots are shallow and wide-spreading. When a tree is stressed, it is less capable to defend itself naturally against diseases and/or insect pests.
The Ohio State University considers big leaf magnolia "short-lived and prone to disease" and does not recommend it for casual ornamental garden use. Among the most prevalent of problems with magnolias are fungal diseases, most often the result of occasional inhospitable weather conditions or a weakened plant from stress. Leaf spots occur on foliage and look like yellow and brown lesions on leaves. Root and branch rot is causes limb die-back. Root rot may not be seen until the fungus produces a fruiting body (mushroom conch) from an exposed root or on the trunk flare; branch rots are seen where there are breaks or wounds in branch crotches or severed bark.
Leaf spot is a minor but nuisance fungi-caused disease that is most common in wet spring and summer conditions or when there is poor air circulation around plants and insufficient sunlight. Root root becomes a problem in poorly drained soils, such as after a seasonal flood, according to Alan Windham of the University of Tennessee Extension. Improper pruning, infected cutting tools or storm damage also exposes living tree tissues to airborne fungal spores and other pathogens.
If the big leaf magnolia is severely threatened by disease, it is impractical and cost-prohibitive to apply chemicals to large-sized specimens. Therefore prevention is the key to avoiding diseases on this slow-growing species. Diminishing any effects of stress is key: plant in fertile soils that are moist, irrigate in severe droughts and situate the tree where it receives at least eight hours of sunlight daily. Proper, restrained pruning is needed, according to Texas A&M University. Always sterilize pruning blades with rubbing alcohol.