These trade routes existed for the best part of 1000 years by the time of the black death. During the Peloponnesian War (434-404BC) the Athenians relied on grain from the Black sea to feed the city and when they lost naval supremacy, they lost the war. At the time of the black death outbreak, Genoa had control of the southern coast of Crimea for the grain trade with Byzantium. Venice and Genoa fought a war over control of the Black Sea. The Venitians diverted the 4th Crusade (1202–1204) to attack Byzantium because they had just done a trade deal with the Egyptians. The Bordeaux Bristol wine trade was origin of the ton as a weight,. The tun was the largest barrel of wine size and ships were measured in how many they carried. So these trade routes were moving significant tonnage annually and not a one off thing caused by a famine. I think its stretching a point to say that the grain trade caused the black death when the trade routes already existed and had existed for significant periods of time.
