Matt has a great new post up on The Scientist Gardener called The Myth of Monocultures. He gives talks about the PBS special on Michael Pollen’s Botany of Desire (which I still need to watch myself), addresses some misconceptions about diversity and monoculture and talks about the best way to ensure crops aren’t wiped out by pathogens that can evolved to overcome a single resistance gene:
Theoretically, it would be useful to maintain crop populations with diverse resistance genes. The industrial application of this (multilines) invovles breeding many different versions of a favorite crop variety that are identical except for their resistance genes. As appealing as this idea is, it hasn’t really worked out in the real world. The alternate approach (pyramiding) seems to be more effective. Here, many different resistance genes are combined into a single crop variety. Pests and pathogens may be able to overcome a single gene at a time, but it’s usually almost impossible to simultaneously overcome several.
Think of it like the story of the three little pigs. [spoilers ahead ] Each pig builds his house of a different material: straw, wood, and stone. Then a wolf comes along, managed to break into the straw and wood houses and, depending on your version, the two pigs who didn’t build their houses out of stone either have to escape to the third pigs house or get eaten up by the wolf.
Now imagine there are a bunch of different predators that can get into different kinds of houses. One can get into wood and stone houses but is defeated by straw. Another can get into straw and stone but is defeated by wood. Diversity means building three houses one of each material, and letting two of the three pigs be killed and eaten but knowing one will be safe regardless of which predator attacks.
Pyramiding is breeding all the resistance genes breeders can find into the crop. Instead of one house of stone, one of wood and one of straw, each pig is protected by walls of all three materials. This solution is less diverse than the previous one (each pig lives in the same style of house), but still a safer one. Regardless of which predator attacks, all the pigs are protected by a barrier it can’t penetrate.