How can I treat my small lemon tree that has what appears to be a fungus on the leaves?
Many of the leaves have a grey splatter on the top or underside and soon fall off.
What you describe may be "greasy spot disease".
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.