Answer: I'd look for insect pests such as aphids, scales or mealy bugs. The sticky, sappy substance is probably 'honeydew' excreted from sucking insects. Inspect the undersides of the leaves, the areas where the stems join the branches, and the areas where the branches join the trunk. If you find insects, wash or wipe them off, or dab them off with a cotton swab dipped in plain water. Once the insects are gone, the sticky residue should disappear, too.
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