Question
Our lemon tree has a white powdery substance under all of its leaves.  Tree appears to be healthy otherwise.

Any ideas on what it is and how we should treat it.

Answer
Hi Dan,

From your description, I would first suspect a fungal infestation.  Do you see any crawling white specks or insects where the leaf petiole attaches to the stem?  There are many fungal species which are pathogenic.  Powdery mildew is a common pathogen, but the leaves become yellow (chlorotic) and will eventually desiccate and fall off.  Since your leaves otherwise appear healthy, your lemon may have an early stage of disease where symptoms are not yet apparent.

If you have a powerful magnifying glass, detach one of the affected leaves and examine the white powder closely.  If you can see mats of filamentous strands, that is the mycelium of the fungus.  Let me know what you find upon closer examination, perhaps I can be of more help with added information.