For the story of General Sumner, you can read the JP Historical Society article here. As far as the story below, it seems to have been common during the time, and a New York Times article of 1903 debunks it. Seeds can last quite a while if dried properly and protected from pests, but apparently the Mummy Corn story was a favorite of the 19th Century. File under "Cool if it was true."
The curse? I guess it's the curse of being scammed.
The Daily Evening Bulletin December 24, 1856
Egyptian, Or Mummy Corn. --- Perhaps the most wonderful and interesting specimens of the fruits of the earth in the Horticultural Exhibition recently closed was some Egyptian Corn, raised in the gardens of Gen. William H. Sumner, of Jamaica Plain, and kindly sent by him for exhibition, thus giving thousands an opportunity of seeing one of the greatest curiosities within our knowledge. The seed from which this corn was raised, was taken from the folds of cloth wrapped round a mummy three or four thousand years ago, and, wonderful as it may seem after being entombed for so many centuries, like a resurrection from the dead, it springs up in new life and vigor. It is undoubtedly the kind of grain for which Joseph's brethren went into the land of Egypt - the same "corn" of which the Bible speaks. It is luxuriant in its growth, and the heads resemble wheat, but are very much larger, forming in inverted conical clusters as large as the closed hand; the kernels are large and very sweet to the taste, and the stock and leaves are similar to our Indian corn. There seems to be no reason why it may not become a valuable addition to our cereal productions, and thanks are due to the gentlemen who are multiplying it and bringing it into notice. --- Boston Journal.