In "The Poison Tree," the apple has multiple meanings, representing "wrath," temptation and deception.
The narrator of the poem tells the story of nursing an angry grudge against an enemy who has injured him. Rather than confronting his enemy, he buries the grudge and nurses it.
And I waterd it in fears,Night & morning with my tears:And I sunned it with smiles,And with soft deceitful wiles.
This wrath then grows into an "apple:"
Till it bore an apple bright.
And my foe beheld it shine.
Do you remember the story of Adam and Eve in the Garden of Eden? When they ate the fruit from the forbidden tree, the tree of the knowledge of good and evil, their eyes were opened, they realized they were naked, they hid from God and they were eventually banished from the Garden of Eden. In Blake's poem we read:
And it grew both day and night,
Till it bore an apple bright;
And my foe beheld it shine,
And he knew that it was mine,
The tree represents anger, which Blake believes is a type of deadly poison. If you take a bite from the poisoned apple, you will die. In this poem, the speaker did not resolve his anger with his foe, as he did with his friend. Resolved anger ended the anger between speaker and friend. The anger with the foe was not resolved (I told it not, and it did grow), so it grew into a shiny apple which when consumed by the foe, resulted in his death. The apple, then, represents the anger which was not resolved and then grew into something that caused death.
Eating fruit from a poisonous tree is a famous theme in literature, even fairy tales. Remember what happened to Snow White?
There is a deeper discussion of the religious theme and application of this poem here on eNotes. See if you agree with this analysis.