Answer: Sticky substances are almost always the result of insect pests that can cover foliage with a clear sticky exudate. Sometimes a dark fungus can grow on the surface, giving some leaves a dark or black appearance. Additionally, sooty mold can develop on the sugary insect secretions in damp conditions.
The insects causing the residue can sometimes be difficult to see. They my be aphid, scale or leafhoppers. They may have left your plant, leaving the residue as evidence they were there.
To solve this problem, look on the underside of the leaves for small eggs or insects. Clean off any sticky spots on stem and leaves on small trees with a q-tip and rubbing alcohol. Outside, hose off the tree thoroughly with water. If insects are still present, spray the tree with a solution of horticultural oil or Safer's soap according to manufacturer's directions.
I would try to keep ants off your tree. They will farm scales or aphids, moving them from place to place, milking their secretions, and protecting them from beneficial insects. Ant baits may be helpful.
Although your orange tree might withstand winter weather in your gardening region, I'd play it safe and take it indoors each winter. Plants in containers can suffer root freeze which is lethal. For the health of your tree, take it in when nighttime temperatures hover around 50F.
Best wishes with your orange tree.
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