Question
I have a couple of Myer lemon trees and the underside of the leaves is yellowing.  One tree has no fruit  and the fruit on another is covered in a scale from the stem join to half way down the fruit.  I have used a citrus fertiliser but can't fix it. We live in Hawaii and the plants get plenty of water.

Answer
Libby:

There may be more than one issue here.  I am not sure if the fruit and leaf symptoms are related. There may be a nutritional problem or a pathological one here at play.  Can you send a closeup photo of the leaf yellowing and fruit "scale"? Try to get as close as you can with your camera and make sure the photo is in focus by reviewing it before sending.  In the meantime, you may consider carrying some of the affected portions to your local nursery/garden center for their comments. Examination of a physical sample may reveal something that could be previously overlooked.  Seeing the tissue in "real life" can be useful, plus these personnel have often seen a lot of maladies and may help shed some light on it for you.

Regards
Steve