I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
It is normal for a magnolia to drop older leaves when the new growth emerges. The extent to which this happens depends on the genetics of the tree and the growing conditions. Drought and other stresses tend to increase the effect. The plant will be fine and will releaf completely in time. The white spots are an algal leaf spot that is not anything to worry about and which are not practical to try to spray to prevent.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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