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... Answer updated 6 years ago
Narrations say that the tree was a grape "tree", a wheat/grain "tree", or the tree of envy of Ahl al-Bayt (A). [For instance, see Uyun Akhbar al-Rida (A)]. Maybe there is some truth to all of them since it wasn't 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.