You can control many magnolia diseases with basic precautionary measures and timely disease management. There are more than 200 kinds of magnolias, but the majority of them are vulnerable to a variety of leaf diseases. Among these, leaf spot and leaf blight are the two most common.
Magnolia Leaf Blight
Leaf blight is a common garden disease caused by the fungus anthracnose. In some plants, it doesn't cause widespread damage, but it can be extremely damaging to young magnolias. It can appear as a leaf spot, or on the branch of the tree. When it is on the branch, it forms a canker which may girdle the branch and cause dieback.
The first symptoms include the appearance of thread-like formation on the underside of leaves. Once the blight spreads, the leaves develop a defining, matted look. These leaves also tend to stick together, creating a cluster.
You should prune all affected branches. Even if only a few leaves appear to be blighted on a branch, prune away the entire branch. Do not use these branches for composting. Instead, dispose of them immediately. Even the residue shouldn’t be used for any gardening activity. Before pruning and after each cut, ensure that your pruning tools are clean by dipping them in a 10 percent bleach solution. This helps to ensure that the infection is not spread through the tree by the pruning equipment.
Magnolia Leaf Spots
Leaf spots in magnolias are caused by a variety of fungi. Among these, the most common include the Cladosporium and the Septoria. They are associated with magnolia spotting in household gardens. The Pseudomonas bacteria can also cause magnolia leaf spotting, but its occurrence is rare.
Gardens in perennially wet conditions or those exposed to heavy irrigation are more prone to being attacked by leaf-spotting organisms. The initial symptoms include development of tiny black or purplish spots. Mature spots have a white center and an outer edge that's dark purple. Spotting that has been present for more than five months often gives rise to fruiting bodies that appear near the spotted leaves.
You should not use overhead irrigation. This is known to be a common cause for creating excessively moist conditions in the upper foliage. Prune away any kind of weathered vegetation, sort out young, intertwining branches and, if your tree is small enough, remove diseased leaves from the tree. This helps to control spotting by reducing contact with diseased foliage and increasing ventilation.
If your tree is suffering from leaf spot, rake up and dispose of all the fallen leaves as well.
Magnolia Disease Caused by Insects
The Neolecanium, or the Magnolia Scale, is the most destructive of magnolia pests. These insects grow within the thick foliage spread of magnolias without displaying any signs of their early development. Once the crawlers emerge, they feed intensively on the young buds and branches. Magnolia scale excretes honeydew after feeding on the tree. This provides an ideal spot for black sooty mold to grow, and it also attracts ants and wasps.
Magnolia Scale feeds on the plants and reduces their vigor. You will see reduced foliage and flower production, as well as smaller leaves and black sooty mold. You may notice large areas of the young branches completely covered with scale.
Plants sold at garden supply stores are often the carriers of magnolia scale. Check the magnolia plants you plan to use for growing purposes in your garden. If the tree shows signs of scale feeding, using horticultural oils when the scale is in the crawler stage; late fall or early spring is recommended. These are available at garden supply stores.
Tip: When using pesticides, be sure to read and follow all the manufacturer's instructions.