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.
This apple of wrath then becomes temptation to the enemy:
And my foe beheld it shine.
The persona telling the story has disguised his anger so well by pretending everything is fine that he deceives his enemy, who is lured in to his trap. Mistaking the persona for a friend, the persona's enemy enters his "garden." Here, instead of encountering friendship, he is poisoned by the "wrath" of his enemy.
The poem conjures the story of Eve tempted by Satan to eat the shiny fruit, often depicted as an apple, on the Tree of the Knowledge of Good and Evil. Satan does not have Eve's best wishes at heart, but full of wrath, like the persona in this story, wants to bring about her death. Tempted by Satan's lies, Eve eats the apple, just as the persona's "enemy" does. Because she and Adam (who also eats the apple) are driven from Paradise, they become mortal and die.
Blake's poem warns us not to plant the "apple" of wrath or anger, for that leads to poisoned relationships and death. We need instead to deal with our anger instead of nursing resentments.