Q: What is wrong with my patio lemon tree? I see no bugs.

A: The two pictures together give me what I need to see. The tiny “spots” on the leaf indicate a sap-sucking insect like scale, whitefly or maybe aphids. The black sooty appearance of lower leaves confirms this. Sucking insects excrete honeydew which falls onto lower surfaces and serves as a food source for sooty mold.

Since you might be considering eating the lemons, I recommend you use an organic spray like neem (click for sources), thoroughly sprayed onto the back of the leaves. This will suffocate the insects.