Driving from Canada to the US we've had customs tell us that we can't bring in citrus fruit that's not peeled. As long as it's peeled we're fine. Even the border officers say it in a way that makes it clear they just enforce the rules and don't necessarily agree that they make sense, but that's the rule we've encountered. I would think the same rules apply to all Canada to US border crossings whether they're by land or air. We've never had a problem with grapes, bananas, apples, pears or plums which we frequently have in our cooler which border agents have opened and seen.