The idiom "melon patch and plum tree" comes from "Ballad of the Gentleman" and means you should keep away from situations that invite suspicion.
Tips
culture
It opens 君子防未然，不处嫌疑间。瓜田不纳履，李下不正冠。 'A gentleman guards against what has not yet happened and does not stand where suspicion falls: in a melon patch he does not stoop to fix his shoe, under a plum tree he does not straighten his hat.' It is the source of 防患未然 and 瓜田李下.
history
The poem is an anonymous old song kept in the 乐府诗集; some editions credit it to 曹植. 行 in the title means a ballad.
