There is no natural (as in wild) corn anywhere in the world. The wild ancestor of corn, teosinte, can still be found in some places, but you would not have any luck trying to eat its seeds the way you would corn kernels. The beautiful photos illustration the vast genetic diversity of corn are all breeds of corn that have been under artificial selection for thousands of years by farmers from Chile to Canada.
You are correct that "The wheat and corn from 50 years ago is NOT genetically modified in the modern sense of the word" however I believe the point the GP was making is that the changes made by artificial selection were
equivalent to, if not greater than, those that are now being produced with genetic modification "in the modern sense of the word."
The genome of B73, a completely un-genetically modified variety of corn, was published back in 2009 and I've had my head buried in it ever since. I've seen broken genes, moved genes, genes missing the sequences that should control when and where they are turned on, even frankenstein genes assembled from the pieces of other genes. All these changes occurred naturally in individual corn plants and are found today in B73 as the result of either artificial or natural selection.
For example, and yes, this is real, they make crops that have weaknesses so that you need to buy more pesticides of the kind they sell.
Citation needed. I know there are GM crops resistant to certain herbicides, but in the absence of those herbicides they grow identically to their unmodified siblings. I don't even know how an effect like the one you describe could be produced. But if you can back it up I will certainly look into it.