Answer: Rather than a fungus, I think your plant has an infestation of mealybugs or cottony scale insects (as evidenced by the sticky substance you've discovered). These sucking insects excrete a sticky substance called honeydew. I'd either painstakingly peel each pest off or use an insecticide marketed for houseplants to kill the pests. You can then wash them and the sticky substance off the plant.
Best wishes with your lemon tree!
Q&A Library Searching Tips