Question
I have a potted lemon tree that the leaves crinkle and then they fall off. Is it a virus and is there any treatment.

Answer
Good Morning Charles:

There might be several reasons for the distorted leaves, but a virus disease is pretty far down the list of possibilities. Consider carrying a few of these affected leaves to your local garden center or nursery for someone to take a closer look for such things as insects that may be present on either surface.  It may take a bit of magnification to see these critters.  If this happened rather suddenly, I might wonder if there had been any chemicals (pesticides etc.)that may have been sprayed either directly or in the vicinity of this plant.

FYI. Unfortunately, there are no cures for virus diseases on any plants.  These agents, eventhough simple in their structure, are very "sophisticated" in the way they infect plants. They need a wound to get into plants, then they hijack the plant cells, taking control.

Again, it will be best for someone to see your leaves in person to make the call.

Regards
Steve