Not necessary, because the argument doesn’t rely on any claims about future banana strains.

Necessary, because if it were not true — if large plantations do NOT produce most of the world’s bananas — then the fact most large plantations are in places we can use fungicide would not establish that most of the world’s banana crop is safe from Sigatoka. This raises the possibility that most bananas are actually grown in places where fungicide can’t be used.

Not necessary, because the argument never compares the speed of Sigatoka spread among large plantations and small banana groves. Even if the disease spreads at an equal rate in both areas, that doesn’t undermine the author’s reasoning.

Not necessary, because the argument concerns only Sigatoka disease. The author never argues that bananas aren’t threatened by other kinds of diseases.

Not necessary, because the small banana groves are merely one place mentioned where bananas happen to be grown. We have no reason to think the author believes most of any kind of banana trees are found in small banana groves as opposed to other places bananas might be grown.