Presently, roughly 200 magnolia species are primarily used for the purpose of landscaping. These trees are loved for their attractive flowers that blossom in spring. In addition, they are also quite hardy and can resist pests as well as diseases to some extent. Nevertheless, this does not indicate that these trees are completely immune to diseases and pests. In fact, magnolia trees may also develop some diseases, especially those spread by bacteria and fungus. They are vulnerable to fungal and bacterial leaf spots, which may deteriorate to leaf blight, sometimes verticillium wilt and other diseases.
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Symptoms: The leaves have grey/ purple spots and sometimes the entire leaf may be discoloured.
Potential causes: Fungal leaf spot disease may be responsible for this condition.
Identification tips: While various types of fungi can infect magnolia trees, Phyllosticta, Septoria and Coniothyrium are the most common among others. The symptoms of this disease on the trees may also vary somewhat depending on the type of fungi that invades the plant. This condition may be recognized by the leaf spots that can occur in different shades including yellow, grey, red, brown, black or purple. Some fungal infections result in the formation of leaf spots having dark margins, while there are others that develop in the form of concentric rings. Provided the infestation is heavy, the tiny leaf spots often merge and lead to large dead areas - a condition that is known as leaf blight. In a number of instances, the leaf spots may have a white center. Alternatively, the center of the leaf spots may also have tiny holes. Eventually, the color of the affected leaves may change from bluish-green to yellow or even brown and then drop from the branches. In the case of some magnolia trees, the stems, petioles, flowers as well as the fruits may also have fungal spots.
Causes: Fungal infections are said to be the most widespread reason behind these conditions. These fungi usually thrive in the wreckage of the infected trees. These fungi generate spores throughout spring as well as summer and these spores are transported by wind, rainwater and even several insects. When the spores end up on a magnolia tree, they start germinating and growing whenever the prevailing conditions are favourable. Therefore, magnolia trees that grow in damp, shady and humid places are more susceptible to being infected by these fungi and develop leaf spots. At the same time, plants growing in a crowded location may facilitate the development of these fungal diseases. While mild fungal infections are unlikely to cause much harm to the affected magnolia trees, when the infestation is heavy and occurs often accompanied by untimely shedding of leaves, it may adversely affect the growth of the trees.
Control and prevention: In order to avoid these diseases from affecting your magnolias, it is important to select plant varieties that are disease resistant. At the same time, never water the magnolia foliage. Ensure that you cut back the weak as well as affected branches as soon as you notice them. Spraying fungicidal may help to prevent the occurrence of these diseases. In addition, they may also help to avoid secondary infections in magnolia trees that are already infected by these fungi. Before using the fungicides, it is important to read the instructions on the product label carefully and strictly abide by them. In case the foliage of your magnolia tree is dense, it is advisable that you prune the branches with a view to ensure proper air circulation. When the air circulation is appropriate, it will help in preventing the magnolia trees from being infected.
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Symptoms: Leaves developing brown spots, leaf dropping and dieback of the stem tips.
Potential causes: These conditions are generally attributed to the disease called bacterial leaf spot.
Identification tips: When you notice brownish, angular spots that build up the length of the veins of your magnolia leaves. In a number of instances, you may also notice a yellow halo encircling the brown leaf spots. When the bacterial infection spreads, the brown spots merge to create large lacerations that cause the leaf to dry up from its center. As a result, the affected leaves become curly, distorted, develop holes and are even tattered and worn out in places. In many cases, the trees may also shed these affected leaves prematurely. There are some other symptoms associated with the bacterial leaf spot disease and they include stem tip or flower dieback, veins becoming darker, and even stem cankers that emit a gum-like fluid. When the tiny leaf spots fuse to develop into large noxious and destructive areas, the condition is known as leaf blight - as in the case of fungal leaf spot disease.
Causes: While fungal leaf spots are quite widespread in magnolias, bacterial leaf spots are not so prevalent in magnolia trees. The bacterium, for instance Pseudomonas syringae, which is responsible for leaf spots as well as leaf blight, is generally present in the rubble of the affected trees. Such bacteria are carried from one place to another by wind, water and even polluted soils. They enter the plants through their wounds, abrasions or scratches. Therefore, it is important to ensure that your magnolias are not hurt while pruning or otherwise. In addition, these bacteria may also get into the trees through the pores that occur naturally on the trees. When the bacteria have entered the tree, they start multiplying as well as spreading rapidly. Over a period of time, the bacteria affect the entire tree and eventually causing the tree to wilt and die.
Control and prevention: In case, the bacteria have only infected the leaves and twigs of your magnolia tree, it is advisable that you prune them at the earliest and then get rid of those parts appropriately. When you are pruning the branches, disinfect the tools before every cutting with a view to prevent the infection from spreading to healthy leaves and branches. If you find that the main trunk of your magnolia tree has developed blights or cankers, you should know that this condition has turned systemic and just pruning the tree will not help. When this occurs, you need to cut down the entire tree and dispose it properly. However, you may adopt preventive measures to protect your trees from developing this condition. First of all, you need to select magnolia cultivars that possess the ability to resist diseases. In addition, you should plant the trees in locations where they can get sufficient sunlight and air circulation - both of these are necessary for the healthy growth of the plants. At the same time, ensure that you do not water the plants from above. Mulching the trees is also important, as it will help the soil to retain much moisture and also lessen the need for watering. Also ensure that you only undertake pruning when the conditions are dry.
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Symptoms: Formation of green-grey spots on the leaves of your magnolia tree.
Potential cause: A disease called algal leaf spots may be responsible for this condition.
Identification tips: You can ascertain that your magnolia tree has been affected by algal leaf spots, when you notice the leaves developing small greenish-grey spots. These spots are somewhat round and have curly margins. In summer, the color of these algal leaf spots may change to reddish-brown and their texture may become like velvet because they generate structures that produce spores. When the trees are affected heavily, their leaves may become discoloured and even drop prematurely. In a number of instances, such spots may occur even on the magnolia twigs, fruits and blooms. In case the algae succeed in getting into the branches via cankers, it may eventually result in the death of the trees. There are occasions when particular forms of fungi may also form colonies in these algal leaf spots making the condition even worse.
Causes: A parasitic algae called Cephaleuros virescens is responsible for algal leaf spots that affect magnolia trees. In fact, these algae produce spores in summer and they are transmitted by wind and/ or rains. It has been found that weak magnolia trees and those having a very sluggish growth are usually susceptible to this algal disease. Trees growing in inferior soil conditions are also vulnerable to algal leaf spots. At the same time, magnolia leaves that are too damp or moist also develop this disease easily. It has also been found that humid conditions encourage the growth of this type of algae on magnolias.
Control and prevention: If the algal attack is mild, just get rid of the affected tree parts and dispose them in an appropriate manner. Also sterilize the pruning and cutting tools properly before and after each use. On the other hand, when the infection is severe, use copper fungicides to eliminate the parasites. Before using the fungicides, it is important to read the instructions on the product label and also follow the directives of any experienced horticulturist. It has been found that healthy magnolia trees do not usually develop algal spots easily. As in the case of other plant diseases, ensure that you cut down the dense foliage of the trees with a view to help the plants receive better air circulation. Also ensure that you do not water the plants from above.
Symptoms: The leaves and branches of magnolia trees start wilting, while the sapwood becomes discoloured.
Potential cause: This condition is known as Verticillium wilt and caused by a fungus named Verticillium dahliae as well as Verticillium albo-atrum.
Identification tips: As the name of this plant disease indicates, it causes the leaves as well as the branches of magnolia trees to wilt. This is the most common symptom of this condition. In the initial stage, the color of the magnolia tree leaves start turning yellow having brownish margins, thereby developing a scorched appearance. When the magnolia trees are attacked by this fungus, brownish stains too develop on the sapwood. In a number of instances, the infected trees slowly die after many years. In many cases, the death of the magnolia trees owing to verticillium wilt is much faster.
Causes: The disease called verticillium wilt is among the most widespread diseases that affect magnolia trees and it is attributed to a particular type of fungi called Verticillium dahliae as well as Verticillium albo-atrum. In most cases, the fungus Verticillium dahliae is responsible for this disease in magnolia trees. These fungi possess a unique characteristic - they have the ability to remain dormant in the soil for a prolonged period before becoming active again. These fungi germinate after coming in contact with magnolia roots that are either stressed or weak and subsequently infect the plants. Gradually, the fungal infection spreads to different parts of the tree, including the branches, leaves, flowers and fruits, via the magnolia tree's vascular system. As the infection spreads, it blocks the supply of nutrients and water to the affected tree parts. At the same time, scarcity of some particular nutrients may also make your magnolia trees vulnerable to verticillium wilting. On the other hand, it has also been found that feeding the magnolia trees with excessive nutrients may also make them prone to developing this fungal disease.
Control and prevention: Since both Verticillium dahliae as well as Verticillium albo-atrum possess the aptitude to survive in a dormant stage in the soil for a prolonged period, it is quite difficult to eradicate these fungi and also the disease totally. When a magnolia tree is affected by this condition (verticillium wilting), cut down the infected branches and burn them as soon as you notice the problem, so that the infection does not spread to the healthy parts of the tree. Also destroy the debris of the affected tree. In fact, chances are there that if you don't burn the infected tree, it may re-grow as well as live longer than the disease. Therefore, it is essential to get rid of the infected trees for good and sterilize the soil before planting new plants in the same location. At the same time, before you plant a magnolia, ensure that the cultivar you have selected is hardy and possesses the aptitude to resist this particular disease. Also make sure that you provide the plants with appropriate and sufficient fertilizers.