Spotted Leaves on Magnolia Trees

Fertilize magnolia trees that have algal leaf spot to help restore their health.

Magnolias exhibiting spotted leaves may have contracted a disease called algal leaf spot. Algal leaf spot severely damages magnolia trees that are already suffering from a health decline. Sometimes, improper cultural methods such as overhead watering encourages algal leaf spot. Both cultural and chemical control methods help eradicate this disease to keep your magnolia from dying.

Algal Leaf Spot

Algal leaf spot is caused by an alga called Cephaleuros virescens. The disease overwinters in spots and twig cankers. When wet windy weather conditions favor its spread, magnolia trees become infected from spores blowing in the wind or rainwater. Generally, a fungus grows with the alga that causes leaf spots to turn gray. Alga colonies begin spreading in the summer, but the disease is more evident on magnolias in the fall. Because algal leaf spot survives on plant debris, magnolia trees continually become infected when leaves are left on the ground.

Symptoms

Leaf spots caused by the disease appear raised and velvety. Spots grow up to 1/2 inch in diameter on leaves. When spots first appear, they may be gray, brown, orange or green before turning grayish green. Leaf tissue shrivels underneath the spots. If the disease goes untreated, magnolias display yellow leaves, twig lesions, cracking bark and defoliation. Any cracked or girdled areas on twigs create the perfect entryway for fungal diseases.

Cultural Control

Prune infected leaves to control algal leaf spot's spread. Use a pair of sterilized bypass shears or pruning saw to remove diseased portions of the magnolia. To sterilize your pruning tools, clean them with a mixture of 70 percent denatured alcohol and 30 percent water. Rake up all debris underneath the magnolia. Burn or throw away the debris. Refrain from composting the debris to prevent spreading the disease throughout the yard. Prune any nearby trees that have overhanging branches near the magnolia tree to increase air circulation. Increased air circulation helps keep a dry environment where spores cannot spread.

Chemical Control

Apply a copper-containing fungicide to the magnolia tree such as a Bordeaux mixture. Pour 8 tbsp. of the Bordeaux mixture into 1 gallon of water to spray the tree. Evenly cover the tree with the fungicide. Reapply the Bordeaux mixture every two weeks, especially during wet weather. When using a fungicide, keep up your watering schedule. However, water your magnolia underneath its dripline. The dripline is the area directly underneath the outermost branches. Refrain from watering the tree's foliage.

References

• Mississippi State University Extension; Algal Leaf Spot of Southern Magnolias; Clarissa Balbalian
• Alabama Cooperative Extension; Algal Leaf Spot; Jacqueline Mullen, et al.; November 2003