I double-checked this, as the fruit/fruits dilemma is a common problem for English learners. You can use jackfruit, grapefruit and kiwifruit to mean the plural form. No need to add "s", so it's correct to write, "Jackfruit are cultivated..." If you just say "kiwi", then the plural is "kiwis". Remember "kiwi" also refers to the bird, as well as a nickname for New Zealanders.

Jackfruits, Grapefruits, Kiwis