Black Spots on a Magnolia
Magnolia plants are well-known ornamental trees or large shrubs prized for attractive, dense foliage and extremely showy flowers. Although these trees are enjoyed for their low maintenance requirements and few pest problems, they are susceptible to a number of fungal and bacterial pathogens that can cause black spots to appear on leaves or flowers. Does this Spark an idea?
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Fungal Leaf Spots
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Several common fungi can attack magnolia trees, including Alternaria and Cercospora phyllosticta. Fungal infections generally appear first as small, scattered spots on leaves that vary in color and shape depending on the specific fungus. Spots usually increase in number and size and coalesce as the season progresses. Fungal fruiting bodies may appear in the centers of spots. Heavily infected leaves may yellow, wither and drop early. Several fungi can overwinter in fallen leaves or other debris, so remove and dispose of dead leaves. Properly fertilize trees, prune out branches selectively to improve air circulation and, if necessary, apply preventive fungicides.
Bacterial Leaf Spots
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Black spots on a magnolia may be caused by a bacterial pathogen. A general indication of a bacterial infection is the presence of yellowish halos around dark spots. Bacteria enter the plant through any wounds, such as abrasions or pruning cuts, or even through natural openings in the leaves. These pathogens persist in plant debris and can be transferred by wind, water, soil or insects. Control and prevent bacterial infections by removing and destroying diseased debris. Prune out infected plant parts, disinfect tools between cuts, mulch the area around the tree with clean leaf mold or compost, avoid overhead watering and prevent bacterial infections with a bactericide.
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Algal Leaf Spot
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Algal leaf spot is a disease that occurs primarily in the humid, warm climate of the southeastern United States. A parasitic alga, Cephaleuros virescens, causes raised blotches or patches on leaves that range from a grayish green to brown or orange. A heavily infected plant may suffer from leaf yellowing and premature leaf drop. The algae spread most quickly during warm, rainy and windy weather, and growth usually becomes apparent in fall. Address algal leaf spot, if the infection is small, by removing and destroying spotted leaves. Selectively prune branches to encourage air circulation and faster drying and use appropriate fertilization. If necessary, you can treat trees under a manageable size with a copper-containing fungicide.
Scales
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Scales, particularly the magnolia scale, can indirectly cause black spots to develop on magnolia plants. Magnolia scales themselves are up to one-half inch wide and shiny brown. During part of the nymph stage, a scale secretes a white wax that covers its body. The magnolia scale feeds on sap from the tree and excretes large quantities of a sticky, sweet substance known as honeydew, which is a growing medium for sooty mold fungus. This fungus gives branches, leaves and items around the tree a spotty, black appearance. Correctly timed applications of horticultural oil, insecticidal soap or insecticides can control magnolia scale.
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References
- University of Wisconsin Cooperative Extension; Magnolia Scale; Phil Pellitteri; Feb. 21, 2006
- Missouri Botanical Garden; Bacterial Spots, Blights and Rots
- University of Arkansas Cooperative Extension Service; Algal Leaf Spot of Magnolia; Stephen Vann
- University of Illinois Extension; Fungal Leaf Spot Disease of Shade and Ornamental Trees in the Midwest; July 1998
- Photo Credit Jupiterimages/liquidlibrary/Getty Images
