I bought a serissa a few months ago. We were told to spray it with
dishwasher solution to kill any potential bugs. We did that few
months ago and about 1/3 of the leaves died. After that we put it
under a full-spectrum incadescent light bulb. Leaves continue to fall
and some of the leaves turn brown, as if they are burning, and some
even turn darker shade of brown, almost black. We only water it when
the soil is dry, every 3-4 days. We give it grow light for about 14
hours from a distance of 16-24" and spray it once a day.

What is causing the leaves to go burn and go brown? Why are so many
leaves dropping? Some branches have no leaves at all now.