Identifying and Treating Leaf Spot Fungus on Magnolia Trees

Black or brown leaf spots on magnolia leaves can develop when the tree grows under stress, Like many ornamental trees and shrubs, when the problem is severe, leaves fall off and cause the tree to be virtually empty of foliage. The problem is mostly seen in moist conditions, which is why magnolia, a common tree in humid southeastern regions, is so often affected.

Cause of Leaf Spot on Magnolia Leaves

Insect damage and bacterial, or viral diseases can also cause some types of spots to appear on magnolia leaves. According to the Pacific Northwest Plant Disease Management Handbook, a bacterium can also be the culprit: namely, Pseudomonas syringae pv. syringae. In most cases, though, leaf spots on magnolia are caused by one of four common fungi:

• Cladosporium spp.
• Coniothyrium spp.
• Phyllosticta magnoliae
• Septoria spp.

The actual spots are not always black, by the way. They may be brown, tan, or even red. These same or related fungi may affect a wide range of other plants as well. If you notice leaf spot on your magnolia tree, it's a good bet that other ornamentals in your landscape may also be affected by fungal leaf spots.

Fungus Leaf Spots Thrive in Moist Conditions

Fungi thrive in moist conditions. As the University of Connecticut (UConn) Extension observes, in the presence of fungal spores, all it takes is "a film of water" on the leaf for the spores to germinate and penetrate the leaf. It is at that point that foliage becomes diseased.

Leaf spot is most likely to appear during periods of rainy or humid weather and often clears up again on its own when sunny, dry weather returns. In warm, humid climates, mature healthy trees may experience an annual appearance of some leaf spot, and it is rarely a cause for serious worry.

Treating Leaf Spot

Most experts do not regard leaf spot as something to worry over when it occurs on a mature specimen in this genus, such as a star magnolia tree or saucer magnolia. Their advice is to remove the diseased brown leaves that have dropped and then dispose of them properly to prevent the fungus from spreading. Do not dispose of diseased leaves by composting them, since the fungal spores can linger in the compost and spread to other plants. In trees under stress, which includes young, newly planted trees, leaf spot can be a serious disease. In these situations, chemical control is warranted.

The best option to treat leaf spot on magnolia leaves is a copper-based fungicide. Because fungicides work as a protectant and not a cure, you must apply them upon seeing the first signs of leaf spot. If you notice similar trees in the area beginning to be affected by leaf spot, preemptively begin spraying those trees.

Preventing Leaf Spot Fungus

Healthy magnolia trees tend to resist leaf spot, so the best preventive measure you can take is to care for your magnolia properly. Since moisture promotes the spread of fungus, it is a good idea to enhance air circulation to the degree possible. This can be achieved by:

• Pruning off branches on the magnolia trees that are rubbing against each other.
• Pruning off branches of any surrounding trees or shrubs that may be invading the magnolias' space.