Why would my bonsai lose its leaves? The tree also has a light cover of black dust.
Belly Fig Bonsai
Certified GKH Gardening Expert
The black dust could be sooty mold:
If so, your plant likely has an infestation of small insects such as aphids or scale bugs, which are the underlying cause of sooty mold growth. Check your plant carefully for insects, and follow the links in this article for insect control options:
Insect feeding could cause the loss of leaves that you see. Or, the leaf drop could be caused by overwatering, underwatering, lack of light, or the other problems discussed here: