The state tree of Mississippi, the magnolia tree is commonly associated with southern states. The magnolia is a large tree; depending upon the variety, a mature tree ranges from 20 to 90 feet tall. An evergreen, the tree has dark green broad leaves, with rusty undersides and large cone-like seeds. Throughout the spring and summer, it produces fragrant creamy-white blossoms. Although easy to grow, several diseases attack the magnolia tree.
Algal Leaf Spot
Algal leaf spot is a common disease on magnolia trees according to the University of Arkansas. The algae look like grayish green, brown or orange raised patches, up to ½ inch in diameter on the magnolia leaves. The area beneath these algae spots will shrivel and die and--if severe--will cause a premature leaf drop. This can cause the tree to weaken, leaving it susceptible to other diseases.
Removing the affected leaves will significantly help in controlling with low infection. Keep the area under the tree clean to reduce reinfestation. Prune any surrounding trees or shrubs that are crowding the magnolia to allow greater airflow, which reduces conditions for disease. Algal leaf spot can be treated with a fungicidal spray that has copper. Contact your local extension agent for more information on what product to use.
Another disease of magnolia trees is sooty mold, caused the certain sucking insects. This gray-black fungus coats the top of the leaves and can be scraped away. Although it generally does not harm the tree, the University of Ohio Extension explains that, in rare cases, if enough of the leaves are covered, it can affect photosynthesis in the tree.
The disease is treated by chemically controlling the insect that is attacking it. Take a sample of affected leaves to the local extension agent or garden center for identification and advice for treatment.
One of the serious diseases that attack magnolia trees is verticillium wilt, which can severely damage or even kill the tree. Verticillium wilt is a soil-borne fungus that enters the root system through naturally occurring wounds and spreads throughout the tree. Verticillium can appear at any time during the growing season, but most often appears in July and August. Symptoms of verticillium include slow growth with small yellow leaves, browning--or scorch--of the leaves, unusually heavy production of seed. Leaf symptoms include curling, abnormal coloring--red or yellow--of the leaves or complete dieback of leaves or branches.
Managing trees infected with verticillium wilt will take time and knowledge, according to the University of Minnesota. Contact your local extension agent to determine whether it is verticillium wilt and for advice on treatment. With immediate treatment, verticillium is not a death sentence; the trees can survive. Trees that are not stressed by low watering are less susceptible to verticillium wilt.