In 1842 the Irish potato crop was destroyed by the fungus Phytophthora Infestans, starving more than a million people to death.

Recent research by Jean Ristaino at North Carolina State University has shown that this strain of P. Infestans is in fact not related to the modern, '1b' strain. By comparing the DNA from 150 year old samples of Irish potato famine struck potatoes from Kew Gardens with that of the 1b strain, it was found that the old strain lacked a sequence found in all modern 1b strains; proving that 1b had a different ancestor.
The old strain is believed to have originated from Mexico, which in fact has a great many strains of P. Infestans naturally occurring there, but the modern strain comes from somewhere else. If the location of the modern strain's 'home' can be found, there will probably be some potatoes there that have some natural resistance to the fungus, which could prove invaluable in protecting modern day crops from this infection.

Source :- New Scientist magazine, 9 June 2001, Nature (vol. 411, p 695)