Answer: The ants and the black residue indicate aphids or scale insects or mealybugs feasting upon the leaves of your plant and exuding a sticky substance called honeydew. This substance attracts ants because of its sugar content, which is just enough to get bacteria or fungus to grow, as well. Thus you see the black "sooty mold". So, you need to identify which pests are feeding on the leaves and attracting the ants. Once you've controlled the pests, the rest of the problems will go away, as well. Best wishes with your magnolia tree.
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