> But why would a superbug that takes out all the bananas have any effect on the corn, wheat, etc growing down the road?

Simply, copies.

If a superbug is able to destroy all the world's bananas, it would grow in population 1 million fold to do so. A bacteria or virus with a million times as many members will evolve a million times as quickly.

Given such, a species jump wouldn't be hard to do. And after the first couple, the superbug would likely start to tailer itself to genes that many plant species share, making the next jumps even easier.