Pawpaw Paw-paw Paw Paw Papaw? What is the proper spelling?

There is no standardized spelling for pawpaw, which just goes to show how little known is the fruit. “Paw paw” as two separate words is common, including in dictionaries. However, I prefer “pawpaw.” I reason that it is a single word, not two. And if it were two words then ‘paw’ should be an adjective modifying a noun ‘paw’; which makes no sense. After all, we do not spell straw berry or pine apple or water melon.