The soot is probably sooty mold growing on the honeydew secreted by certain pesky insects, including whiteflies, scale, or aphids. Depending on which insect is the cause you can control those mentioned above with sprays of a lightweight, horticultural oil, or if they are aphids another option is insecticidal soap sprays. The castile soap should work too. Test it on a branch first to see if it is going to burn the foliage. Direct sprays upward from beneath the plant making sure to wet the undersides of all leaves and branches. Repeat spraying will likely be necessary to get the pests under control.
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