Answer: A sticky substance on the leaves usually indicates an insect infestation. Pests like scale insects, aphids, whiteflies and mealybugs all suck the juices from the plant tissues and exude a sticky substance called honeydew. Once you've controlled the pest population and rinsed off the sticky residue, your tree should perk right up.
You'll need to do some detective work to determine which pest is attacking your tree. Look on the undersides of the leaves, along the stems where they meet the trunk, and on all the branches. Once you've found the pest you can control it - if you can't identify it yourself, take several in a plastic bag to your local nursery for identification. Nursery personnel can recommend a control strategy. Best wishes with your lemon tree.
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