Identifying and Treating Leaf Spot Fungus on Magnolia Trees

Magnolia sieboldii tree with white flowers.
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Like many ornamental trees and shrubs, magnolias may develop black or brown leaf spots. 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. 

Leaf Spot on Magnolia Is Usually Caused by Fungus

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, as well.

Fungus Thrives 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 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 be 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, 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, not a 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: