logistically, the easiest explanation is a cargo ship container carrying grain contaminated with a rust fungus, gone overboard and ruptured open, spreading the resulting gooey goodness before port inspectors could go "blech" in person. note that I'm not saying it went overboard accidentally, although it would be hard to implement without a dock loader-- it's a lot easier to write off a container "lost" at sea than "contents consumed by fungus"