In Unfinished Tales Part One II, Narn i Hîn Húrin, earth-bread is described as being '...white and fleshy within their skins, and when boiled they were good to eat, somewhat like bread...' This certainly sounds like a description of a potato, or at least of something very similar.

This raises the question of how potatoes could have been found east of the Great Sea during the First Age, when long, long afterwards they would have to be imported from the New World. Mîm gives a hint about this, saying that the Petty-dwarves kept earth-bread a secret because Men '...would not spare till all the plants had perished...' (ibid). Perhaps we're to take it from this that earth-bread was indeed something like a potato, but became extinct in Middle-earth at some point, so that it had to be re-introduced from the Americas millennia later.

This would also help to explain how the Shire-hobbits (long after Mîm's time) could have been familiar with potatoes, which theoretically should not have existed in Middle-earth during their period of history. The implication seems to be that potatoes, or at least earth-bread, disappeared from Middle-earth at some point during the Fourth Age.