Can't you trade with the natives to get mushrooms?
No, there is no such option, and if you think about it for half a second, you realize that would defeat the whole purpose of the objective.
Read a guide:
DESTROYING THE YELLOWKNIVES: If you destroy the yellowknives before the epidemic
you will fail the mission. If you destroy them AFTER the epidemic, you will
complete the mission, but they are well defended, so it's much easier to gather
the mushrooms to win.
What guide am I supposed to read? That doesn't make much sense that the objective is failed but the mission succeeds? The defenses are trivial. I already have troops and fortifications far in excess of what is needed with a proper strategy. I guess it is in keeping with the Eurocentric theme of genocide though. The only thing that matters is that all non-white cultures are exterminated, right? I thought this game might offer more. Oh well.
So you are probably at mission 4 vineland now since that one requires 30 mushrooms
Vineland, correct. I said Markland by accident.