The apple has come to be a symbol of evil, and immorality. Everyone recognizes the story of Eve, eating the forbidden apple! But was that the fruit she ate?
Probably not, although there is no way to know for sure. It's likely that the idea of an apple was introduced to Christian Scripture through the fact that the Latin words for evil and apple are differentiated by only a vowel sound. We still use words with those roots. "Malus" is the genus for apples, and "mal" is a prefix often connoting bad things: malaise, malady, malefactor, etc.
"Apple," before the days of systematic botany was a general term for any unknown fruit. Potatoes in several languages are "earth apples," and tomatoes were called "love apples." There are numerous other examples.
So, in Christian art, the apple almost always is a symbol of the original sin.
There is one big exception. When Jesus is shown holding an apple, as in this painting by Lucas Cranach the Elder, "Virgin with Child," it symbolizes his role as the new Adam (see Romans 5). Sin came into the world through one man (Adam), but through one man (Jesus) many will be made righteous. Jesus Christ has overcome evil.