[baroffoos]

I doubt its that dire. There are a lot of things currently that can destroy crops but they are mostly prevented from spreading by border controls on things like fruit not being able to cross over.

[liability]

You're correct. Rice and corn are almost tied as staple crops. Wheat, potatoes and bananas trail those two relatively closely. There may be little genetic diversity inside any particular staple food, but there are nevertheless several unrelated staple foods that feed the world.

If blight were to wipe out one or two, things would get bad but I'm confident we'd pull through as a species.