Each year on March 17^th, the death of Saint Patrick, known as the “Apostle of Ireland” is observed in Ireland and throughout the world wherever there are Irish people. In honor of that commemoration, this article will discuss a plant disease known as “Late Blight” caused by Phytophthora infestans whichhad a profound effect on Ireland in the 1840s, changing its history forever.

Irish Potato Famine

Prior to the 19^th century, Ireland's farmers primarily grew grains such as wheat, oats, and barley. However, by the 1800s, due to a rapidly growing rural population, the Irish began to rely on just one crop – the “Irish Lumper,” a variety of white potato (Solanum tuberosum). It was easy to grow on small farms, even in the worst soil, and provided enough sustenance for impoverished tenant farmers and their families. In the mid-1840s a blight struck potato plants all over Ireland. Leaves would wither with shocking speed and when the tubers were dug up for harvest, they were found to be shrunken, mushy, and inedible.

By late 1845, between one-third and one-half of Ireland's potato fields had been wiped out. The destruction continued the following year with three-quarters of that year's harvest destroyed. People began to starve. Great Britain's blundering relief efforts only made the situation worst, compounded when British landlords evicted tenant farmers who were unable to pay their rent.

Known as the “Great Hunger,” by 1851 one million Irish had died of starvation with another million emigrating to other countries, reducing Ireland's population of 8 million by twenty-five percent. With a current population of about five million, it has not recovered to this day. The huge flood of Irish pouring into the United States in the late 1840s had a profound effect on our country, especially in urban areas. By 1850 it was estimated one-quarter of New York City's population was Irish.

Cause of the Late Blight

For years it was believed the pathogen causing the potato blight was a fungus. In the 20^th century it was determined the cause was an oomycete, Phytophthora infestans. Oomycetes, also known as “water molds,” are a group of several hundred fungi-like microorganisms which include some of the most devastating plant diseases including blights, damping-off, root rots, and downy mildews.

Late blight affects a wide range of plants, including potatoes, tomatoes, peppers, and eggplant. Symptoms include brown-purplish spots and lesions on leaves and stems, with white fuzzy growth underneath the leaves, ultimately causing fruit rot and plant death. Late blight continues to be a destructive disease, causing serious agriculture losses worldwide in tomatoes and potatoes.

Late Blight Management

If you suspect your garden potatoes are infected by late blight, bring in a sample to your local UC Cooperative Extension office. You can also learn more about late blight at: https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/lateblight.html

Saint Patrick's Day

Celebrated for over 1,000 years, immigrants from Ireland brought the custom of observing St. Patrick's Day with them when they came to the USA. Today more than a hundred St. Patrick's Day parades are held across the country, particularly in New York, Boston, and Chicago (which transforms the Chicago River to a green color by adding an environmentally friendly green dye into the water way). By “the wearing of the green” we celebrate the Irish. One can also say we also celebrate our green gardens! Slainte!

Denise Godbout-Avant has been a UCCE Master Gardener with Stanislaus County since 2020.