As I've noted several times on the thread, foreign farm workers aren't being kept out. They can be employed, legally, withoug limitation.
And as I noted earlier, given the choice between exporting apple growing to Mexico or importing Mexico's abusive economic system, I'd opt for the former. But my guess, American producers will do fine. Most of them.
That's one of the myths in this debate. There aren't a fraction of the visas and quotas available to permit an average Mexican peasant to come here legally to do farm work. It's nearly impossible to do.