My point wasn't that genetic diversity isn't useful — if you see above, I give an example of how having a genetically diverse germplasm is quite useful for breeding. But having a field full of different crops is not useful. It would lower yields, be difficult to harvest (mechanical harvesters rely on uniformity), and consumers like uniform crops. It also doesn't make sense to take genetic diversity in a crop and plant it in a field because it's completely ignoring our ability to design local crops — ones that are more drought tolerant given a region's rainfall, or ones that could handle colder temperatures. If your field of genetically different crops are all planted together, half may die if there's a cold spell. Uniform genetics allow us to really understand what a single field is going to do, and this is why it's so common. It's also why there are vast field trials —sometimes decades long— for all crop varieties.

Take the wheat example I gave above. Even if you were to plant every variety of hexaploid wheat in a field, despite different flowering times, milling characteristics, best environments, etc. one rust would still kill them all. There is no resistance allele in this entire species of wheat. This is why researchers had to go find Sr35 in an ancient relative.

You could make the claim, well, we should be growing that ancient relative, but this is silly too — it would be risky given it hasn't undergone the same selection for agricultural hardiness as modern varieties.

As I said, there's nothing magical about having a field of diverse crops. A field's population will always be smaller, and encapsulate less diversity than that all of the world (and in other relatives). If your so called black death came along and killed all but a few plants, why would this help? There's still too little to feed or sell. This disease already decimated the population, and our best approach is to breed an entirely resistant population the next year — not throw in more diversity and hope for the best. We can intelligently leverage diversity in breeding, which is more optimal than just planting diverse fields.

> Well, firstly you wouldn't have the disease problem on such a scale if you had less centralized farming / genetic monoculture.

You may very well might. It's possible no plant in the population has resistance alleles and they might all die given a nasty pathogen. Pathogens killing all by certain plants with certain resistance alleles is just selection. Actually, if this number of plants with resistance is small, this natural process could lead to quite a genetic bottleneck and actual remove vast amounts diversity. These bottlenecks occur in the wild, and so does extinction (if say, no individual had resistance).

> Third, your backcrossing line is dubious. Biology is good at survival, we don't need to tend to it ourselves. If we reach a state where we do, then gee golly: we've clearly buggered up hundreds of millions of years of success in a right jiffy!

Sure, but then go eat teosinte rather than maize. I think you're forgetting that we domesticated varieties to survive with our help, and to feed us. Through breeding we can grow maize in Wisconsin rather than Mexico. If backcrossing is dubious to you, you should stop eating all crops because I guarantee all have had parents that have been extensively backcrossed.

> I agree with your premise but not your conclusion. Growing suitable crops closer to where they are consumed, in greater variety, and removing external dependencies such as artificial fertilizer, unnatural water consumption for unsuitable crops, technology and energy for industrial farming, packaging and transport will also go a long way.

Please if you have some magic way to do this and feed met the world's food demands, become a plant scientist! But I think you will find that it's just not this easy.