Answer:
This is because a single species of potato plant was predominantly cultivated at a time by farmers on the same farmland year in-year out in ireland in the 1800.This is monoculture.
One of the disadvantages of this practice is lack of genetic variation, which reduces the resistance of these plant to potato blight.
Lack of genetic diversity made the same gene to be inherited by all the potatoes and therefore no room for a resistant gene which might have provided resistance gene, through variation thus all the potatoes with the same weak resistant genes were exposed to the same phytophtora infestant which caused rotten .
This is an example of genetic diversity and its importance in diseases protection.
