Magnolia trees are popular because they are fairly easy to grow and don't draw a large number of insect pests. However, these trees, common to the eastern and southern United States, do occasionally suffer from insect infestations. Magnolias also prefer warm weather and sunlight and are susceptible to cold weather damage.
Lack of blooms is a problem that will concern many who grow magnolias, as the tree's showy leaves are one reason it's so popular. If your magnolia tree has stopped blooming, you may have frost damage. This damage should be temporary and new buds should show before long. You also should check the tree's location to make sure it's getting enough sun. Check the soil. Magnolias tend to thrive in acidic soil. One sign indicating a lack of nutrients in the soil is the colour of the leaves. If they are dark green or yellow, your soil requires a nutrient boost.
A common magnolia tree pest is the magnolia scale. These insects have sucking mouth parts and heavy infestations can cause branches to die. If branches are infested, growth is slowed and leaves may be underdeveloped. If not stopped, the pest can kill the tree. Scales begin to suck sap on the tree in the spring. Scales do have some natural predators, but these predators don't usually do an effective job of controlling infestations.
Make sure the magnolia is pest-free when you buy it. Most infestations arrive with the plant. Scale exoskeletons from a previous season can sometimes be seen. You also can try horticultural oils. Applying these oils in fall after scales have settled on a tree can help reduce the number of insects. Thoroughly wet down stems and leaves. You can use oils in October and November and again in March.
Another option is to use an insecticide. These have proven effective if applied when scales are in the freshly settled stage in late August or early September. Spraying at another time won't be as effective.
Another pest, called thrips, attacks the flower buds on magnolia trees. If your tree had flower buds that never opened, check for thrips. Look for brown trails on flower buds that the insects leave as they eat through unopened petals.
If a tree keeps losing foliage and leaves have brownish spots, it could be a soil problem. Magnolias need a lot of iron and nitrogen to thrive so a lack of these nutrients could cause the leaves to fall off. Brown spots could be caused by cold weather. Other causes of brown leaves could be fungi, such as leaf blight or black mildew. These diseases usually occur during a wet spring. Take a leaf specimen to a garden store. They will be able to tell you if it is a fungal issue.