Category: Genetic diversity

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Mapping the 1970 corn blight

Here are my 2 maps ((Quick & dirty, without cross checking the numbers, but I think the maps speak for themselves.)) for this discussion. I used linear regression to predict corn yield for each county in the US, using time (year) as the independent variable. I used the years 1950 to 1969 to create the model, and to predict corn yield in 1970. This should be a reasonable estimate of the ‘expected yield’ for 1970 for each county, if it had been a ‘normal year’.

I then computed the difference between the expected yield and the yield obtained by farmers, and expressed that as the percentage of the expected yield. Negative numbers mean that yields were lower than expected in a county, positive numbers mean that they were higher than expected. Counties with data for less than 9 years were excluded.

1970 corn yields were indeed much lower than expected in the southeast. Corn blight hit very hard. But also note that yield was stable or up in the north and in the west, and look were US corn was grown in 1970. The map below expresses corn area as the percentage of the total area of a county.

Most corn is grown in the corn-belt. The southern parts of it were much affected by the disease (The Illinois Secretary of Agriculture’s estimate that, by August, 25 percent of his state’s corn crop had been lost to the blight may have been spot on). But 1970 was a normal or good year for corn yield in the northern and western parts of the corn belt, and that compensated for the losses incurred elsewhere. If you sum it all up, corn production was about 15% lower than what could have been expected. That is whole lot of corn — but perhaps not that exceptional as far as bad years go.

Here is a table of estimated corn yield by state, as percentage of the expected yield for 1970, and the corn area, as percentage of the national area (only for states with more than 1% of the national corn area in the counties data set).

State Yield Area   State Yield Area
Florida -36 1   Minnesota -12 8
Georgia -33 3   Missouri -11 5
Illinois -31 18   Nebraska -9 9
Indiana -27 9   North Carolina -5 2
Iowa -26 18   Ohio -1 5
Kansas -24 2   Pennsylvania 0 2
Kentucky -22 2   South Dakota 6 4
Michigan -12 3   Wisconsin 15 3
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US government blames maize yield losses on Southern Corn Leaf Blight

Our friend Jacob’s Google-fu is stronger than mine. He found this annotated graph of maize yields in the US.

See how they’ve claimed that blight reduced yields by 18% in 1970? That would be the Southern Corn Leaf Blight that wasn’t a problem, and the yield loss wasn’t caused by lack of genetic diversity.

Well, of course, the government would say that, wouldn’t they, after shelling out all that money on plant breeding and stuff …

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Blight is right: genetic uniformity was to blame

The Southern Corn Leaf Blight epidemic that struck the US in 1970 is usually seen as a canonical example of the dangers of genetic uniformity. I use it that way myself, often. Certainly yield losses in 1970 seemed very high, higher than the average 12% “expected from all diseases of corn”. But could we all be wrong? A commenter thinks so.

[W]as it a major problem? Over twenty years ago I gave a seminar at CIMMYT. I had prepared a slide showing the year on year average yield increase ((I don’t know whether he really means year on year increase; somehow, I doubt it. What would be the point?)) in maize in the USA for about 70 years‚ but leaving off the actual years. … I challenged the audience to identify the blight year (1970). Nobody could. … Try this on colleagues and students.

I did, and it is true, 1970 does not look all that extraordinary against the trend.

A more interesting graph is this one, in which the rising trend in average yield is removed from the actual yield each year.

Now 1970 is a little more visible, though I agree it still doesn’t look catastrophic. I mean, compare that with 1988 and 1993. There is one huge difference. In 1988 drought was widespread, while in 1993 floods devastated many farms and yields in the northwest corn belt. Weather in 1970 was just fine, thank you. Weather is clearly a very important factor in annual yields, and it interacts with pests and diseases in complex ways, but it seems pretty clear that the yield loss of 1970, while not as drastic as in other years, was certainly not the result of wayward weather.

The commenter asked “are we making too much of a fuss about the Leaf Blight”? I don’t think so, obviously, so I asked Professor Darrel Good, of the University of Illinois. He knows more about maize yields than almost anyone (and is responsible for the graphs above). He said:

I have not seen any specific analysis of 1970, but am pretty sure that the decline in corn yield was in fact attributed to the outbreak of southern corn leaf blight. Hard to quantify that impact relative to weather. It is a similar phenomenon as the aphid damage to the soybean crop of 2003. ((A pest recently arrived in the US from the soybean’s native China, and a rabbit hole I am not now going to explore.)) These rare events are not captured in our models.

In some respects, pests and diseases are as unpredictable as weather. In industrialized agriculture, genetic diversity within a crop is unlikely to provide much protection against the vagaries of weather. ((Subsistence agriculture is almost certainly different.)) But genetic diversity definitely can protect against unpredictable pests and diseases, not just in maize, and not just against Southern Corn Blight.

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Brainfood: Pollinator threats, Predicting drought tolerance, Markets and conservation, Groundnut oil composition diversity, European wheat landraces, Dung beetles, Livelihoods, Phenology

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Biodiversity is more than a matter of breeding

One of the great problems in talking about biodiversity is that it has so many different meanings. ((At which point the average lazy English writer would insert the Humpty Dumpty quote and move on. Next would come the Samuel Johnson quote, and then we can get back to the story.)) That makes it easy to misunderstand one another, and to make mistakes.

So, for example, we pointed out that having great diversity in your pedigree does not confer any diversity in the here and now if all the offspring of those highly diversified matings are genetically identical. Our friend Mike Jackson was quick to point this out, and he recently followed up with a link to an IRRI annual report for 1997-98. Delivering Diversity to the Field amplifies the confusion, talking about the huge increase in the number of ancestors in a variety’s pedigree ((Confusingly calling them all “parents”.)) and telling us that “these varieties have also increased the danger of genetic vulnerability to major disease or insect pest outbreaks”. Modern varieties, then, are more diverse because they have many “parents” and more vulnerable because they are more uniform. ((No point, here, in picking more holes in that article; we leave that as an exercise for the reader.))

All this is important because it addresses something that Bruce Chassy brought up in response to our criticism of his effort to debunk an anti-GMO paper by bringing biodiversity into the picture. Chassy wrote:

Biodiversity has taken on an almost spiritual meaning. I do not mean to in any way diminish the importance of biodiversity but sometimes when people start believing they stop thinking.

Amen to that. Chassy goes on to illustrate, as follows:

The American Chestnut comes to mind as species that was diverse and widespread through diverse ecosystems in the US. Three billion trees, 25-30% of all trees in the Eastern US, were nonetheless wiped out by a single disease, Asian bark fungus (Cryphonectria parasitica) in a few short decades. Just a few scattered individuals remain of this once common tree. Biodiversity did not save them.
On the other hand, the Quaking Aspen (Populus tremuloides) is propagated by root and shoots. A grove of these Aspen is essentially a giant monoclone; the specie itself appears to have very little biodiversity. They have survived for perhaps 100s of millions of years and have been called the largest living organisms since all the trees in a grove are connected by roots and can be thought of as part of one distributed organism. Lack of diversity appears not to have hurt this specie.
A number of common crops are propagated clonally as well. Naval oranges, bananas, potatoes, pineapples, apples are examples. Why doesn’t biodiversity matter to these crops? Because they were produced by plant breeders and are propagated commercially before being planted by farmers.

Where to begin?

At the beginning. Biodiversity can exist at several different levels. A landscape contains different species from different kingdoms. Populations of a single species will differ from one another. And individuals in a population will differ genetically from one another. So all those chestnuts may have been “diverse” but not where it counted, in their susceptibility to Chestnut blight, although at least one resistant individual has been found. As for Quaking Aspen, some groves are indeed large clones (as are groves of many other trees) but groves differ greatly from one another, and aspens will reproduce sexually, for example after fire, increasing the genetic diversity among stands. Aspens are also threatened by many pests and diseases.

I could make the same points with regard to vegetatively propagated crops and obligate out breeders. Clubroot, anyone?

One measure of diversity is the probability that two individuals chosen at random will share some specified quality, but it really matters what that quality is. Could be something as simple as a specific sequence of DNA. Could be a specific protein coded by the DNA (which can be identical even if there are differences in the DNA). Could be something as complicated as susceptibility to a disease, or resistance to drought. It’s complicated. But that’s no excuse for wilfully misusing an important concept.