Like many ornamental trees and shrubs, magnolias may develop black or brown leaf spots when growing under stress. When the problem is severe, it may cause the leaves to fall off and can lead to a tree that is virtually empty of foliage. The problem is most likely to occur in moist conditions, which is perhaps why magnolia, a common tree in humid southeastern regions, is so often affected.
Cause of Leaf Spot on Magnolia Leaves
Insect damage, 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 the 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 very much when it occurs on a mature specimen in this genus, such as a star magnolia tree or saucer magnolia. Their advice typically is to remove the diseased brown leaves that have dropped and then dispose of them properly. The idea is 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.
But, as the UConn Extension also notes, leaf spot can be a serious disease for trees under stress, which includes young, newly planted trees. Already weakened by the stress, young magnolia trees infested with leaf spot can die from the infestation. It is in these situations that chemical control is warranted.
When you choose to battle leaf spot on magnolia leaves, the best option is a copper-based fungicide. To be effective, though, the fungicide must be sprayed at the right moment. Because fungicides work as a protectant and not a cure, the right application time is before the fungus attacks the leaves. This requires that you be very vigilant and spray the tree immediately upon seeing the first signs of leaf spot.
Or, if you notice similar trees in the area beginning to be affected by leaf spot, preemptively begin spraying your own trees. If you catch the fungus at the very start of infection or before it has appeared on your tree, you stand a good chance of halting its spread. If applied too late, treatment won't be effective.
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.