How can I treat my small lemon tree that has what appears to be a fungus on the leaves?

Question

Many of the leaves have a grey splatter on the top or underside and soon fall off.

Answer 1

What you describe may be "greasy spot disease".
http://edis.ifas.ufl.edu/hs263

Photos would help.

Or go through this list of possible citrus diseases from U Florida an see if you can determine a more likely disease. http://edis.ifas.ufl.edu/topic_citrus_pests_diseases

Also look under citrus pest management. Mealybugs are common, but they are usually white fussy, sticky masses, not grey.