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.
If I remember correctly from a grad school field tour of the Salinas Valley, commercial spinach varieties now have 6 or 7 resistance genes to downy mildew stacked up inside them. Each new one only lasts a few years.
I was really interested in multilines and mixed cropping in grad school but was never able to find much evidence of their practical application in large-scale agriculture. The one exception was a study where they mixed different rice varieties in the same fields in China*. A modern blast-resistant variety was mixed with a high-value, traditional (but very susceptible) variety. The mix of plants with different heights made the canopy microclimate less conducive to blast. It’s a fascinating study but the catch is that a mix of completely different varieties can’t be machine harvested.
* I think it’s Zhu et al. Nature 406. 718-722. Genetic diversity and disease control in rice.
Comment by Matt — November 1, 2009 @ 7:35 am
So they’re bringing in a new one as each old resistance gene in spinach fails? You’ve gotta wonder how long spinach would have been protected if they’d been able to introduce all 6 or 7 at once in the same stack.
It’s not quite the same since it’s only one resistance gene , but did you see ERV’s post on using transgenic ring-spot resistant papayas to provide protection for the production of higher value organic papayas? Fun stuff.
http://scienceblogs.com/erv/2009/06/gmo_plants_and_herd_immunity.php
Comment by James — November 1, 2009 @ 10:46 am