I am sure that wars will be started if this threat goes on for any significant amount of time. In the meantime, people who want fries with that will find the fries costing more than the that, and will make appropriate economic decisions, such as seeking substitutes. For others, the increased price of spuds will be only a minor inconvenience, simply costing them more, which because they're rich, will not result in any sensible economic choices.
It sounds like there might be some kind of underground black market in potatoes, headed up by some shadowy figure known only as "Mr. Potato Head."