Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answered 2 months ago

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.