Answer: Even though the seed you planted was from a sweet orange, the orange tree has become a rootstock rather than a sweet orange tree. The oranges from your tree will never be sweet so don't grow it for its fruit but for its flowers and evergreen leaves. What you describe on the undersides of the leaves are insects. Either mealybugs or scales. As they feed they excrete a sticky substance called honeydew. It eventually turns black (called sooty mold). It won't hurt the leaves but it doesn't look very attractive. If you control the insects, the sooty mold will weather away and no more will appear. Use insecticidal soap to kill off the insects. Best wishes with your citrus tree!
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