By the late eighteenth century agricultural thinkers had realized the importance of crop rotation, the principle of which is that you operate a three or fourfold succession of crops: potatoes, peas and beans, brassicas, onions and leeks, and maybe others in a longer rotation. This technique obviates the possibiity of the build up of plant diseases, which occur if the same crops are grown on land year after year.
But the Irish peasants were unable to rotate crops, for their poverty forced them to grow cash crops for the English market, but the crops were not high quality and the same were grown year after year on the same piece of land. This meant that plant diseases accumulated, and there was a really nasty one, which to make matters worse was completely new to science: Phytophthera infestans, a nasty fungus that ravaged certain varieties of potato. So not only did the Irish meet a problem that they had never previously met, but they were growing a variety of potato that was susceptible to blight, the Lumper. The Lumper was a large, poorly flavoured, cheap variety which appealed to the cheapest end of the market, to people already short of corn and for whom the humble potato was a staple food. Moreover, it was the variety that was grown across the country,so when one person was hit, many were hit by the blight.
Blight is technically known as late blight to distinguish it from early blight, which is less harmful. So the Irish spent much of summer expecting a decent crop and then when funds were low found that the crop had rotted, leaving them financially devastated. Disaster!
The trouble is that blight likes conditions when light is low from dull skies and the air damp with high humidity. Conditions like these prevailed for three years from 1845 to 1848, so not only did one year's crop fail, but three years failed, but the Conservative Government persisted in its defence of the corn laws to defend the landowners' interests until the bitter end in 1846 when conditions became so dire that some of them actually developed a conscience.
At that time no one knew that the condition was caused by a fungus and that there were chemical treatments for the blight, one of which was the later developed Bordeaux mixture, which was based on copper, which is toxic to fungi. This ignorance gave rise to the misconception common among some Irish nationalists that the English had invented the disease to commit genocide against the Irish, which was never true, for they had not the technical or scientific skill to invent the disease.