What Causes Potato Blight?

Written by jamie sziklay
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What Causes Potato Blight?
Potatoes are one of the most economically important food crops worldwide. (Hemera Technologies/AbleStock.com/Getty Images)

Late blight is a leaf disease of potatoes. The potato (Solanum tuberosum) ranks as one of the five most important food crops in the world, making it highly important economically. Late blight (LB) is the most prominent and threatening pest of potatoes in North America today. Without crop protection from pests such as LB, attainable potato production would reduce by nearly 75 per cent, due to pathogens, viruses, animal pests and weeds.

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The oomycete (funguslike organism) pathogen Phytophthora infestans causes late blight. P. infestans survives in living, infected potatoes in the field, storage or cull piles. LB spores are transported many kilometres by wind and rain and can destroy an entire crop field overnight. Highly favourable conditions for LB include high moisture, high humidity and moderate temperatures (approximately 50 to 70F). LB spores need water to germinate, after which they can penetrate the potato skin and infect the plant.


The greatest natural catastrophe caused by potato late blight is referred to as the Great Famine in Ireland (1845-1852). During this time period the population of Ireland was reduced by 20 to 25 per cent when approximately one million people died and two million people emigrated.

Although potatoes were initially introduced as a garden crop, by the early eighteenth century one-third of the population of Ireland depended on potatoes for their entire sustenance. To support this staple food product, crop acreage expanded without diversifying the potato species. Four to five consecutive years of successive late blight blasts occurred in Ireland, devastating the entire potato population. Actions and inactions of the Whig government during this time only intensified the magnitude of the situation through food security and housing regulations. The effects of the Great Famine have permanently shaped the demographic, political and cultural landscape in Ireland.

To this day, the exact origins of late blight in Ireland remain unknown. However, most people believe LB was first introduced to Europe by cargo ships arriving from either Peru or the northeastern United States. Although hundreds of other widespread potato crop failures have been reported, the Great Famine remains the most devastating occurrence of potato late blight in history.


The symptoms of late blight develop on infected potato leaves and tubers in the field 4 to 8 days after initial infection. Common LB signs appear as water-soaked lesions on leaf edges and tips. These lesions enlarge daily, becoming brown with a light green border, as moisture (rain, dew or irrigation) increases. White growth and spores develop on the undersides of leaves, which is most recognisable in the early morning. In addition, symptoms of LB can occur on tubers as irregularly shaped and depressed brown or purple spots.


To control late blight in potatoes farmers can use resistant cultivars, chemical controls such as pesticides or an integrated pest management (IPM) program. IPM for late blight on potatoes encourages the use of cultural and biological controls, along with a crop monitoring system.

Cultural controls aim to prevent the introduction of late blight by destroying infected tubers and properly managing cull piles in one of the following ways: burying, composting, winter spreading on the field (thereby freezing and killing the fungus) or summer covering in plastic (thereby heating up and killing the fungus). Biological control refers to using more disease-resistant potato varieties and creating an undesirable environment for fungus growth. Continual monitoring efforts ensure reduced risk of disease through early detection of infection.

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