@Oculunus True that. It's a merchant's method of attack. However, it's not the only reason. What Young Merchant just gained as profit for his "purchase" is "destabilization". Since Union is buying all the wheat, they are leading Central to ruin by forcing people to go hungry. When that happens, people will become discontent, unruly, and desperate. If it's not resolved, unrest will occur, and an emigration and revolts will happen naturally after.
Of course, this plan only works in aiding Southern by the nature of potatoes being transitioned as a staple. With a different, secure foodsource with measures to prevent the rapid depletion via the butterfly effect, this won't hurt Southern so much.
It is a seriously nasty attack...and that's not yet adding adding what you said to the mix.