How do you treat black spots on citrus fruit?

[#1]

Sap-sucking scale insects can cause black spots on the fruit of citrus trees like lemon and lime.

The mainstay of scale insect control is the application of horticultural oils. Oil sprays suffocate small insects such as scale Insects.

I would recommend applying Multicrop 1L EcoPest Oil Insect Spray. As this is a hard scale, the spray won't remove the scale that is already on the fruit, but with continued use, it should help prevent future fruit from having the same issue.

It is also important to note that as long as the plant is healthy, this hard scale is really an aesthetic issue, and the fruit will be perfectly edible. 

For more information, check out the article How to get rid of scale insects. - MitchellMc