can be used to describe a nasty woman's genitalia, as in mold found in a nasty bathroom or kitchen,
can also be used as a question to let others around know the person is being moody for reasons not caused or involved with the much like how one may ask if you have sand in your vagina. using the phrase this way works on both male and female partys
Guy one: No way I'd hit that!
Guy two: I know right I bet she has mold on her Petunia
The man at the post office was a real jerk today; I asked him whats your deal do you have Mold on your petunia?