My Oliver had a similar problem and it was blackheads on his tail.  It
was really easily remedied by just washing his tail once in awhile.
Yes, a balding tail can be a symptom of adrenal disease but what you're
describing sounds exactly like what was going on with Ollie.
 
Here's a thread I found with someone asking the same question and what
they were told to do to resolve it:
 
http://www.ferretuniverse.com/discus/messages/1263/4934.html?1103657125
 
You may want to have your vet check it out just to be sure.
 
If you want a great place to come and get instant answers to questions
about ferret care come join my forum -
http://ferretocious.hyperboards.com.
We have a lot of really knowledgeable ferret folk as well as the owners
of two different ferret shelters - so always someone on that can help :)
 
- Laura and The Seattle Seven at www.ferretocious.com
[Posted in FML issue 5238]