Prior to the arrival in Ireland of the disease Phytophthora infestans, commonly known as "blight", only two main potato plant diseases had been identified.
One was called "dry rot" or "taint", and the other was a virus known popularly as "curl".
Phytophthora infestans is an oomycete (a variety of parasitic, non-photosynthetic organisms closely related to brown algae, and not a fungus).

In 1851, the Census of Ireland Commissioners recorded 24 failures of the potato crop going back to 1728, of varying severity.
General crop failures, through disease or frost, were recorded in 1739, 1740, 1770, 1800, and 1807.
In 1821 and 1822, the potato crop failed in Munster and Connaught.
In 1830 and 1831, Mayo, Donegal, and Galway suffered likewise.
In 1832, 1833, 1834, and 1836, dry rot and curl caused serious losses, and in 1835 the potato failed in Ulster.
Widespread failures throughout Ireland occurred in 1836, 1837, 1839, 1841, and 1844.
According to Woodham-Smith, "the unreliability of the potato was an accepted fact in Ireland".

How and when the blight Phytophthora infestans arrived in Europe is still uncertain; however, it almost certainly was not present prior to 1842, and probably arrived in 1844.
The origin of the pathogen has been traced to the Toluca Valley in Mexico, whence it spread first within North America and then to Europe.
The 1845–46 blight was caused by the HERB-1 strain of the blight.

In 1844, Irish newspapers carried reports concerning a disease which for two years had attacked the potato crops in America.
In 1843 and 1844, blight largely destroyed the potato crops in the Eastern United States.
Ships from Baltimore, Philadelphia, or New York City could have carried diseased potatoes from these areas to European ports.
American plant pathologist William C. Paddock posited that the blight was transported via potatoes being carried to feed passengers on clipper ships sailing from America to Ireland.
Once introduced in Ireland and Europe, blight spread rapidly.
By mid-August 1845, it had reached much of northern and central Europe; Belgium, The Netherlands, northern France, and southern England had all already been affected.

On 16 August 1845, The Gardeners' Chronicle and Horticultural Gazette reported "a blight of unusual character" on the Isle of Wight.
A week later, on 23 August, it reported that "A fearful malady has broken out among the potato crop ... In Belgium the fields are said to be completely desolated.
There is hardly a sound sample in Covent Garden market ... As for cure for this distemper, there is none."
These reports were extensively covered in Irish newspapers.
On 11 September, the Freeman's Journal reported on "the appearance of what is called 'cholera' in potatoes in Ireland, especially in the north".
On 13 September, The Gardeners' Chronicle announced: "We stop the Press with very great regret to announce that the potato Murrain has unequivocally declared itself in Ireland."

Nevertheless, the British government remained optimistic over the next few weeks, as it received conflicting reports.
Only when the crop was lifted (harvested) in October, did the scale of destruction become apparent.
Prime Minister Sir Robert Peel wrote to Sir James Graham in mid-October that he found the reports "very alarming", but reminded him that there was, according to Woodham-Smith, "always a tendency to exaggeration in Irish news".

Crop loss in 1845 has been estimated at anywhere from one third to as high as one half of cultivated acreage.
The Mansion House Committee in Dublin, to which hundreds of letters were directed from all over Ireland, claimed on 19 November 1845 to have ascertained beyond the shadow of doubt that "considerably more than one-third of the entire of the potato crop ... has been already destroyed".

In 1846, three-quarters of the harvest was lost to blight.
By December, a third of a million destitute people were employed in public works.
According to Cormac Ó Gráda, the first attack of potato blight caused considerable hardship in rural Ireland, from the autumn of 1846, when the first deaths fro
Prior to the arrival in Ireland of the disease Phytophthora infestans, commonly known as "blight", only two main potato plant diseases had been identified. One was called "dry rot" or "taint", and the other was a virus known popularly as "curl". Phytophthora infestans is an oomycete (a variety of parasitic, non-photosynthetic organisms closely related to brown algae, and not a fungus).
In 1851, the Census of Ireland Commissioners recorded 24 failures of the potato crop going back to 1728, of varying severity. General crop failures, through disease or frost, were recorded in 1739, 1740, 1770, 1800, and 1807. In 1821 and 1822, the potato crop failed in Munster and Connaught. In 1830 and 1831, Mayo, Donegal, and Galway suffered likewise. In 1832, 1833, 1834, and 1836, dry rot and curl caused serious losses, and in 1835 the potato failed in Ulster. Widespread failures throughout Ireland occurred in 1836, 1837, 1839, 1841, and 1844. According to Woodham-Smith, "the unreliability of the potato was an accepted fact in Ireland".
How and when the blight Phytophthora infestans arrived in Europe is still uncertain; however, it almost certainly was not present prior to 1842, and probably arrived in 1844. The origin of the pathogen has been traced to the Toluca Valley in Mexico, whence it spread first within North America and then to Europe. The 1845–46 blight was caused by the HERB-1 strain of the blight.
In 1844, Irish newspapers carried reports concerning a disease which for two years had attacked the potato crops in America. In 1843 and 1844, blight largely destroyed the potato crops in the Eastern United States. Ships from Baltimore, Philadelphia, or New York City could have carried diseased potatoes from these areas to European ports. American plant pathologist William C. Paddock posited that the blight was transported via potatoes being carried to feed passengers on clipper ships sailing from America to Ireland. Once introduced in Ireland and Europe, blight spread rapidly. By mid-August 1845, it had reached much of northern and central Europe; Belgium, The Netherlands, northern France, and southern England had all already been affected.
On 16 August 1845, The Gardeners' Chronicle and Horticultural Gazette reported "a blight of unusual character" on the Isle of Wight. A week later, on 23 August, it reported that "A fearful malady has broken out among the potato crop ... In Belgium the fields are said to be completely desolated. There is hardly a sound sample in Covent Garden market ... As for cure for this distemper, there is none." These reports were extensively covered in Irish newspapers. On 11 September, the Freeman's Journal reported on "the appearance of what is called 'cholera' in potatoes in Ireland, especially in the north". On 13 September, The Gardeners' Chronicle announced: "We stop the Press with very great regret to announce that the potato Murrain has unequivocally declared itself in Ireland."
Nevertheless, the British government remained optimistic over the next few weeks, as it received conflicting reports. Only when the crop was lifted (harvested) in October, did the scale of destruction become apparent. Prime Minister Sir Robert Peel wrote to Sir James Graham in mid-October that he found the reports "very alarming", but reminded him that there was, according to Woodham-Smith, "always a tendency to exaggeration in Irish news".
Crop loss in 1845 has been estimated at anywhere from one third to as high as one half of cultivated acreage. The Mansion House Committee in Dublin, to which hundreds of letters were directed from all over Ireland, claimed on 19 November 1845 to have ascertained beyond the shadow of doubt that "considerably more than one-third of the entire of the potato crop ... has been already destroyed".
In 1846, three-quarters of the harvest was lost to blight. By December, a third of a million destitute people were employed in public works. According to Cormac Ó Gráda, the first attack of potato blight caused considerable hardship in rural Ireland, from the autumn of 1846, when the first deaths fro
Great Famine (Ireland)