。
You can't grow eggplants on a melon vine.
Literal
Melon [possessive-の] vine [on-に] eggplant [topic-は] not-bear [classical-ぬ].
A proverb (ことわざ) meaning you can't expect something to produce what it's not suited for — comparable to 'the apple doesn't fall far from the tree' in reverse. 瓜 is a general term for melons/gourds. ならぬ is the classical negative of なる ('to bear fruit, to become'), giving the proverb its timeless register.