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Why Did My Magnolia Foliage Get Black Spots? Its (Brown) Leaves Fell Off

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Question: Why Did My Magnolia Foliage Get Black Spots? Its (Brown) Leaves Fell Off

A reader reported buying two magnolia trees and bringing them home, only to discover that the leaves had black spots all over them. So the reader returned to the garden center, scrutinized the rest of the crop, and learned that all the other magnolias this business had for sale displayed the same black spots. When confronted, their response was: moisture is to blame. Back home, the leaves eventually turned brown and fell off.

Here's my response:

Answer:

Your problem sounds like a case of leaf spot. Since it's a fungus, "moisture" (which was cited as the culprit at the store) is, indeed, likely to blame. Fungi thrive in moist conditions. As the University of Connecticut (UConn) remarks, 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's at that point that foliage becomes diseased.

The actual spots of "leaf spot" aren't always black, by the way. They're commonly brown, as well. But don't assume that, just because the color of the spot isn't dark, the disease isn't leaf spot: the color can be tannish, too or even red.

Most experts do not regard leaf spot as something to worry much over when it occurs on a mature specimen in this genus, be it a star magnolia tree, saucer magnolia, etc. Their advice typically is to remove the diseased brown leaves that have dropped and dispose of them properly. The idea is to prevent the fungus from spreading.

But, as UConn notes, leaf spot can be a serious disease for trees under stress, which includes newly-planted trees (as in your case). Already weakened by the stress, young magnolia trees infested with leaf spot can die from the infestation. It's in these situations that chemical control is warranted. Seek a copper-based fungicide to battle the leaf spot.

Problem is, as the same source observes, you have to spray the fungicide on at the right time in order for it to do its job. When is the right time? Well, you must understand that the fungicide works as a protectant, not a cure. This means, unhappily, that the right application time is before the fungus attacks a leaf.

Do you see the challenge implicit in this treatment? You must be vigilant enough to detect the leaf spot when it first attacks your magnolia tree. If you catch it and spray before too much foliage has been infected, you may be able to halt its advance. Otherwise, treatment is unlikely to be effective.

In terms of prevention (for future reference), healthy magnolia trees tend to resist leaf spot. So learn as much as you can about proper care to keep your specimen in tip-top condition and, thereby, more resistant to this disease. Since moisture promotes the spread of fungus, enhance air circulation by:

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

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