China’s agtech is modernizing, and the Americas and Europe is already modern. We divert enough calories for 1 billion people into biofuels. The only realistic thing that will cause a famine is a breakdown in global trade/markets. I think you are being overly dramatic about the disease threat. I am not quite sure why you think genetic diversity within a species is so important. What is important is to have species diversity and trade to avoid potato famines.

That's what trade is for. Trump just sold lots of pork to China. Admittedly not all places are as integrated into the world economy as China though, so not everyone can afford food, but aid programs are usually well funded and effective. Right now, the only places where people are starving are ones where aid programs don't have access e.g. because of war.