Question
what is the black soot-like substance covering my lemon tree leaves and how do I cure it? thanks1

Answer
James:
This is probably sooty mold, which is a fungus that develops and grows on the sugary excretement of various insects that are on your plant.  Management: control the insect that is producing the excretement (called honeydew).. On your computer, look for images of sooty mold to see if this is a correct "guess".

There are various insecticides available from your local garden center.  Try first to get the "critter" identified correctly before you buy an insecticide. Look the plant's leaves and stems over for bugs, then carry a few leaves to your nursery/garden center to find out the bug's ID.

Regards
Steve