Magnolias are very tough plants and are seldom seriously troubled by insects and diseases. However, some times slugs may attack very young magnolia plants, but you can use modern baits to control these pests easily. Occasionally, leaf miners also attack magnolia leaves - these pests not only gnaw the leaves, but the flowers too. Most growers face this problem in the southern regions of the United States. Employing systemic insecticides will help to keep these insects under control. However, these insecticides are extremely toxic. Moreover, as these pests do not kill the plant they inhabit, you may opt for other means to get rid of them or even decide to continue with the problem.
Apart from slugs and leaf miners, the magnolia species that bloom during the summer may also be troubled by the Japanese beetle. In fact, Magnolia grandiflora is commonly affected by this problem. You may, however, pick these insects by hand. However, this is not possible if the magnolia trees are large or you are dealing with a vast population of these pests.
People residing in places having cold climatic conditions have an advantage vis-à-vis these pests. In fact, these pests are killed naturally when the temperatures drop drastically during the winter. In addition, you can also use any good insecticide to get rid of the pests. However, prior to using any insecticide, you need to consider the benefits of using them and the damage that the chemicals can cause to the plants.
Magnolias are also affected by various other diseases like leaf spots that include powdery mildew and magnolia scab. In fact, usually controls are neither required nor feasibly in dealing with this problem, especially if your tree is large. This is because you can hardly reach all the areas of the tree that may be affected by this condition. However, you may spray a sulfur compound on the affected leaves that are reachable. On the other hand, you can deal with a variety of cankers like dieback, nectria canker and trunk decay by straight away cutting down the diseased or dead branches. However, none of these diseases will lead to the death of the magnolia.
On the other hand, the condition called magnolia scale (scientific name Neolecanium cornuparvum) is a more serious problem that affects magnolia trees. These scales have a brown color and round shape, measuring roughly 1/4 inch (0.50 cm) across. Their body is often coated with a white, dust-like wax. Similar to other scales, magnolia scales also extract the plant's sap and when this occurs continuously, the tree may eventually die. The plants excrete the surplus sap in the form of honeydew, which forms the base for the development of fungus sooty molds. This causes the leaves as well as the branches of magnolias to appear blackish. It is advisable that before you purchase any magnolia plant, examine it carefully to see if there are any scales present on it.
When you notice magnolia scales on the trees in your garden, you may spray horticultural oils (also called summer oils) on the insects as well as the entire stages of their life cycle. These chemicals will effectively kill the scales by choking them. However, it will be more effective if you remove as many scales as possible by hand before applying the horticultural spray oils. In fact, these horticultural oils prove to be more effective when they are applied to crawlers (the young scales) that are settled some time during August end. Subsequently, you should apply a dormant oil between October and November and once again in March. Ensure that the leaves as well as stems of the plant are completely wet when you apply the dormant oil.
The trees that are attacked most commonly include the star, saucer, cucumber and lily tree magnolias. In addition, magnolia scales also feed on Virginia creeper and Daphne.
Magnolia scales possess effective sucking mouthparts and when they invade a plant in large numbers, they usually cover the branches completely, eventually causing the branches to die. In the initial stages of their infestation, they suck out the life-giving sap from the magnolia branches and twigs making them weak and also retarding their growth. Even the leaves on the affected branches and twigs are affected, as they tend to be under-developed. When a magnolia tree is attacked continuously, it often leads to the death of the tree. Similar to majority of the other soft scales, the surplus sap of the plant is expelled in the form of honeydew - a sweet and sticky substance. This excess sap eventually drips on the branches and foliage of the magnolia. The honeydew that drops on the magnolia branches and foliage give rise to a dark-hued fungus known as black sooty mold that affects the leaves changing their color to black. This actually destroys the leaves' ornamental worth. What is worse is that ants, flies, wasps and bees are drawn by honeydew, which forms their food. These insects also harm the trees to some extent.
During the winter months, magnolia scale inhabits the twigs of young plants - usually those that are in existence for one or two years. During this phase, the scales appear as dark-hued nymphs. When the temperatures begin to warm up with the onset of spring, the magnolia scales start sucking the sap from the plants and by the time it is early May, they would have normally molted (shed their coating) once. In early May, you will usually find two different forms of magnolia scales - male and female scales. The male magnolia scales continue to be small; measuring just 1/8 inch and their color soon changes to lucid white. After the color of the male scales become white, they appear as minute gnats whose color may vary from pink to yellow and they appear as other insects that have two elongated wax-like threads emerging from their abdomen's tip.
On the other hand, the body of the female magnolia scales keeps on growing and by the beginning of June, their color changes to brownish-purple. In fact, this is the time when the scales suck out excess sap from the plants forming large quantities of honeydew. By the time it is July, the body of the female magnolia scales is swathed with white, powdery, wax-like substance and this makes their color look more yellowish than tan. Some days later - from the end of July to August, the adult female scales start giving birth to their offsprings called crawlers. These minute, moving crawlers keep moving till they discover a proper feeding site on the tree, where they settle, feed and stay put throughout the winter months.
You can also control scale by employing contact insecticides. However, the success of using these insecticides depends on the timing of their use. You can obtain the best results if the sprays are applied to scales that are in their crawler stage and during the period between the end of August and the beginning of September. While the adult magnolia scales are brown in color, those in the crawler stage have different hues - brown, orange and purple, and when they crawl they appear as moving dusty spots. It is, however, important to go through the manufacturer's instructions that come along the product, before using any horticultural oil or insecticide on your magnolias. There are a number of other scales that attack magnolias and these include the tulip tree scales and wax scales. You may follow the same treatment for these insects as those that are used for magnolia scales.
Among other diseases that affect plants of this species are leaf spots. In fact, magnolia trees are susceptible to leaf spots by the bacterium Pseudomonas syringae. In addition, a great number of fungi may also damage the magnolia leaves. You will seldom require using chemical controls to get rid of this problem. Instead, just identify the affected leaves, pick them manually and destroy them to prevent the problem from spreading to healthy leaves.
A fungal disease called canker disease actually kills healthy branches. Fungi that cause this disease inhibit the branches and cause the bark in the area to die. It is very difficult to control this disease and once it affects the branches of your magnolia tree, simply prune the branch. However, providing the trees with sufficient fertilizers and watering them regularly during the dry weather will not only keep the trees healthy, but also help to prevent the occurrence of this disease.
Another condition called verticillum wilt may also kill some branches of your magnolia. Sometimes, this condition may even kill the entire magnolia tree. Therefore, as soon as you notice any dead branch, prune it out. At the same time provide the tree with fertilizers containing high amounts of nitrogen to keep the tree healthy.
In addition to the pests and diseases discussed above, magnolia trees are also affected by a number of other diseases, including powdery mildew, black sooty mold, and wood rot. A brief discussion regarding these diseases, their causes, symptoms and the means to control them is presented below.
