Observations & pre-amble...
In the research and writing of this very very touchy subject it would be my humble belief that the vast majority of cigar websites claiming they have "plume" or "bloom" on their cigars is total BS; 99.9% of the photographic evidence clearly shows nothing more than mould.
Over the last 30 odd years enjoying cigars I have seen absolutely shocking photos of mould and even some retailers whose humidors could be classed as a bio-hazard but were trying their best to sell unsuspecting cigar aficionados nothing other than rotten, mouldy cigars.
The most famous one was many years ago when Pete Johnson of Tatuaje publicly outed some low-life retailer in the USA and he withdrew his cigars and in fact got ever major cigar family to remove their cigars from said dodgy retailer.
Then again, I've probably witnessed 100x as many cigar retailers whose humidors were bone dry with no humidity - it could be easily argued that that is the bigger crime in our industry...
Anyway, I digress... please read on for my take on this touchy subject.
This is an example of mould; patchy, splotchy; fuzzy mould...
Cigar Mould (Mold)
Cigar mould occurs when the tobacco (cigar) gets too much moisture and becomes damp as well as higher than normal temperatures.
Mould is not good, it means something horrific has gone wrong inside your humidor; generally the two-parts already mentioned (moisture & heat)...
If you suspect your cigar(s) have mould - remove them immediately from your humidor and destroy; check how high the humidity is and also the ambient temperature; as these are the two key elements to create the mould.
In most cases we would recommend you to remove all the cigars from your humidor and use rubbing alcohol to sterilize your humidor and re-season it before re-introducing cigars (some people go as far as to sandpaper back the inside a bit; but only once you've used the rubbing-alcohol to sterilize your humidor).
Pro Tip: Check the open foot of the cigar, often this is an easy spot of mould to begin
Cigar Plume (Bloom)
Plume typically occurs due to natural oils, minerals and sugars rising to the surface of the cigar wrapper and forms a crystalized formation (crystallization).
Plume also does NOT spread from cigar to cigar; that's another sure fire sign of mould (sorry). Plume may be on more than one cigar if you were lucky enough to get the most amazing tobacco that produces plume and also the most optimum storage conditions to provide such an environment for its creation.
But remember, your box of cigars was most likely drawn from 100's and upto a 1,000' different cigars as they are all muddled around and colour-sorted for the most uniform colour finish to make each individual box up.
So your odds of having 20 or 25 of the exact same cigar from the exact same plant are as good as having the winning lotto numbers, sorry. Yes, I am figuring that for "plume" to happen, you'd need the same plant; not just the same crop - it is honestly that bloody rare.
On a handful of times over my 30+ years of being in the cigar world have I seen crystalized behavior on a cigar... but it is beyond rare and given I've probably personally handled 10's of thousands of cigars over my life so far.
Logic (well, my logic) says that an overly mineral rich yield of tobacco from a plant may produce plume in the most perfect conditions; it is not a sign of the holy grail of tobaccos however; unless you really enjoy over-the-top mineral tasting cigars.
How to Tell the Difference?
Plume on a cigar should be easy to brush off and not leave a stain or mark on the tobacco (cigar). It also shouldn't be fuzzy like 99% of the "plume" photographic evidence on the internet...
It should be as I've described it, crystalized and like I've said; it should brush or wipe off without any effort...
In fact some sites now pretty much claim there is absolutely no such thing as plume; their studies (under stupidly strong microscopes) show a mould formation on every single cigar sample they had/have for plume.
After spending days and days on this subject I'm starting to agree with them and I'm quite alarmed by the number of high-profile websites that say their photos show plume, when its bloody obvious that it is splotchy mould...
And that last, obvious question... can I still smoke my mouldy cigar?
No would be my advice; you might see the start of mould on the wrapper or foot of a cigar; but as well know with a mouldy loaf of bread; that mould has already started to form elsewhere - you just cannot see it yet.