Magnolia's Pests And Diseases
Magnolias rarely suffer serious damage from diseases and insects. Very young plants can be attacked by slugs, but these pests are easy to control with modern baits. Sometimes, the leaves are attacked by leaf miners that chew through both leaves and flowers. This is especially a problem in the southern United States. Systemic insecticides will control these insects, but they are very toxic and, as the insects rarely kill the host plant, you may choose to live with the problem.
Another insect that might trouble your summer flowering magnolia species and Magnolia grandiflora is the Japanese beetle. These can be picked off by hand, but in big trees and for large populations this is not feasible. If you live in a cold climate, winter will usually kill off these pests for you. Again, an insecticide: can be used but you will want to weigh up the damage versus the use of chemicals to control the problem.
Other diseases that attack magnolias are leaf spots, including magnolia scab, and powdery mildew. Where you can reach the problem, spraying with a sulfur compound may help but often control is not needed or not practicable, especially on very large trees where good coverage is not possible. Various cankers, such as nectria canker, dieback and trunk decay can all be dealt with by cutting out the dead or diseased wood. Rarely will any of these diseases cause the death of the plant.
A far more serious problem is magnolia scale (Neolecanium cornuparvum). The scales are brown, round and about 1/4 in (0.50 cm) in diameter. A white, dusty-looking wax often covers them. Like other scales, they suck sap from the plant and under continuous attack a tree may die. Excess plant sap is excreted as honeydew, on which the fungus sooty mold develops, giving the leaves and branches a telltale black appearance. Check for magnolia scale on any plant you are planning to purchase.
If you find magnolia scale on your trees, you can apply horticultural spray oils (also known as summer oils) at all stages of the insect's life cycle. They kill primarily by smothering the scales and will be more effective if you first remove as many as you can by hand. These oils are most effective if you apply a horticultural oil to settled crawlers (the young of the insect) in late August. Then apply a dormant oil in October to November and again in March. Make sure that the stems and leaves are thoroughly wet.
Contact insecticides can also control scale, but the key to their success is timing. You need to apply sprays when the scale is at the crawler stage-usually late August to early September. (Crawlers are often orange, brown or purple and look like moving specks of dust.) Be sure to read the manufacturer's instructions carefully before using any oil or insecticide. Other scale insects that attack magnolias include wax scales and tulip tree scales. Treat as for magnolia scale.