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

Dummer. ゛☀
Why does magnolia foliage get black spots on it? Have you ever had the leaves on such a tree turn brown and then fall off? Then you will find the following exchange between the author and a reader interesting -- and, hopefully, helpful.
The reader reported buying two magnolia trees and bringing them home, only to discover that the leaves had black spots all over them. So he returned to the garden center, scrutinized the rest of the crop, and learned that all of the other magnolias that 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. The reader wondered, "What type of disease is this?"
Here was the response to the reader; this information may be able to help many of you out there who have similar problems:
Since Leaf Spot Is a Fungus, It Thrives in Moist Conditions
Your problem sounds like a case of leaf spot. Since it is usually caused by 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) Extension 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 is at that point that foliage becomes diseased.
The actual spots of "leaf spot" are not always black, by the way. They are commonly brown, as well.
But do not assume that, just because the color of the spot is not dark, the disease is not leaf spot: The color can be tan, too or even red.
Most experts do not regard leaf spot as something to worry over very much 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 then dispose of them properly. The idea is to prevent the fungus from spreading.
But, as the UConn Extension also 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 is 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 as 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.
Preventive Control Measures You Can Take
In terms of prevention (for future reference), healthy magnolia trees tend to resist leaf spot.
So care for your magnolia properly to keep your specimen in tip-top condition and, thereby, more resistant to this disease. Since moisture promotes the spread of fungus, it is a good idea to enhance air circulation. 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.
The Fungi (and One Bacterium) That May Be the Culprits
Above, it was noted that the leaf-spot disease is caused by fungi. But precisely which fungi? And can anything else cause it? According to the Pacific Northwest Plant Disease Management Handbook, a bacterium can also be the culprit: namely, Pseudomonas syringae pv. syringae. The same source lists the following as the main fungi responsible for the problem:
Cladosporium spp.
Coniothyrium spp.
Phyllosticta magnoliae
Septoria spp.
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