I noticed the leaves on our lemon tree are covered in very small black spots. It looks similar to pictures I have seen online but the dots are smaller than e.g. the photos I have seen for the "Guignardia Citricarpa" fungus so I was curious.

The tree is a Meyer lemon tree. We live in San Francisco so temperatures range from low 40's at night to high 70's and humidity is around 35-55 percent.

Thanks for your help, Kevin

Citrus black spot appears on the fruits and the lesions eventually spoil them: that's why it's such a devastating disease for commercial orchards.

Try to see if those dots wipe clean with a damp cloth. If they do you can just forget about them. If not it's most likely melanose: it can be managed by applying a copper fungicide, albeit now it's a little late for eradication and will only keep the disease at bay until flowering season.

They wiped clean! Phew... do you think I should clean them off regularly or are they harmless?