I was under the impression that Monsanto has a stranglehold on seed corn in the Midwest.
Fumonisin is caused primarily by Fusarium verticillioides. Aflatoxin is caused primarily by Aspergillus flavus. Both fungi are common contaminants of corn - not just GMO corn. They tend to produce toxins when the corn is undergoing drought stress. The problems of these toxins near Mexico is not that surprising - its a stressful growing environment.
Imagine what is going on in Africa where corn cultivation is being widely promoted in areas it has no business growing. Corn is displacing the traditional cereals (sorghum and pearl millet) which are not prone to these toxins. I suspect we will see widespread health consequences in societies that are already struggling with widespread poverty, AIDS, malaria, and malnutrition.
First of all, the Rio Grand valley is not generally considered "The Midwest". Secondly there are lots of seed corn producers in the corn growing regions. I don't think Monsanto owns all or even most of them. There are several others listed as producing bioengineered seed corn. But not all corn is bioengineered, other than the way everything else you eat is, that is by selective breeding. AFAIK, the latter is still the most used seed corn, not the genetically modified stuff, although the use of GM strains has been growing steadily.