Scaly Growths on Leaves and Branches of Lemon Tree

The leaves on my lemon tree are falling off. There are tiny scaly things on the branches and undersides of the leaves that leave a residue on the plant. I have been spraying it with insecticidal soap without much success.

Scale insects can cause the damage you describe. These insects suck plant juices causing leaves to yellow, sometimes brown, and eventually drop. They secrete a clear sticky substance called honeydew that is often the first clue gardeners notice. The scale’s hard shell protects them and their eggs from insecticides making control difficult. Start by using an old toothbrush or your thumbnail to gently scrape off the scales. Be careful not to damage the plant. Then treat the whole plant with insecticidal soap. This will kill the immature translucent scales that are not readily visible but are susceptible to insecticides. Continue checking and treating, as needed, on a weekly basis. It takes persistence but you can get these pests under control.