Plume, sometimes called bloom, is a fine white or pale grey crystalline dust that can appear on the wrapper of long-aged cigars. It is widely believed to be exuded oils from the tobacco that have crystallised on the wrapper surface during years of slow aging at stable humidity, although the exact chemistry remains debated even among industry experts. Plume is harmless. It brushes off cleanly with a soft cloth, leaves no stain, and the cigar underneath smokes normally.
The critical distinction is plume versus mould, because they look superficially similar at a glance and the consequences are very different.
Plume is fine, dry, evenly distributed across the wrapper, white or pale grey, and brushes off completely with no residue. The wrapper underneath is unblemished.
Mould is fuzzy, raised, often clustered in spots rather than evenly distributed, may carry colour (green, blue, or black tints), and stains the wrapper underneath. Mould on the foot of a cigar is particularly serious because it may have penetrated the filler. Mould-contaminated cigars should be discarded; the affected ones risk spreading to others in the same humidor.
If you are unsure, use a soft brush. Plume disappears; mould smears or leaves a stain.
For more on long-term aging conditions, see How to Rest a Cigar.