What was the tree that Allah ordered Prophet Adam (AS) to not eat the fruit of or go near to it? Was the forbidden food an apple or wheat?
Narrations say that the tree was a grape "tree", a wheat/grain "tree", or the tree of envy of Ahl al-Bayt (A), which Adam and Eve ate from and then became envious of blessings and position given to the Ahl al-Bayt (A) and their closeness to God; for that reason, God cast them to earth, and they had to repent. [For instance, see Uyun Akhbar al-Rida (A)].
It is related from Imam al-Rida (A) that all these views are correct in that it was a tree that bore more than one type of produce, since it was not a normal tree.
"Apple" is not present in narrations but to my knowledge the idea of it being an apple tree was also a later addition to Christianity.