Gnomish
podod
noun. potato
grothorf
noun. potato
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
The word for “potato” in the Gnomish Lexicon of the 1910s was G. grothorf (with a deleted alternate form {grothor >>} grothrof), a combination of G. groth “earth” and G. orf “apple”, similar to French “pomme de terre” (GL/42). This same document had another form G. podod for “potato” (GL/64), perhaps a phonetic adaptation of the European word.
Neo-Sindarin: For purposes of Neo-Sindarin, I would adapt this word as ᴺS. cevorf “potato, (lit.) earth-apple” using the later element ᴹ√KEM for “earth”.