Well-named, this fungus is a byproduct of insects feeding on the plant, usually aphids, but on a lemon it’s likely to be scale. The insect excretes excess plant juices onto the leaves below and the sooty mold grows on that. It’s likely that the floor under your plant is sticky from sooty mold as well. You know it’s sooty mold if it rubs off. Scale on lemons is usually brown or tan bumps on the underside of leaves or stems of the plant. You can scratch them off with your thumb. Put the plant in the shower and wash it off as best you can, and spray while it’s there with insecticidal soap or horticultural oil – ask at your garden center for a product for scale. Gently scratch off any scale that you can as well, and wipe leaves with a soft, damp cloth.