Potato Blight

From 1845 through 1852, over one million residents of Ireland starved to death, and another two million were forced to emigrate elsewhere, all because of potato blight. Before you lose your crop to potato blight, let’s learn more about this tiny water mold.

Potato infected with late blight (USDA) Public Domain

Water mold

Water molds are mostly land-dwelling organisms called oomycetes. Oomycetes fall between fungi and algae. These pathogens attack stems, roots, and tubers and frequently kill host plants. Common water mold diseases include phytophthora tentaculata, crown rot, damping off, sudden oak death, and potato blight. Potato blight, also known as late blight, is caused by a specific oomycete called Phytophthora infestans. The word phytophthora means plant killer, and rightfully so.


The Great Potato Famine

Botanists identified the potato blight pathogen in 1843. Initially, it was limited to New York and Philadelphia. But local winds spread spores into neighboring regions. 


Potatoes were not grown in North America until the 1500s. And they did not become a popular crop until the 1700s. At first, potato blight wasn’t considered a serious threat to anyone. Then, seed potatoes sent to Belgium in 1845 spread this devastating disease across Europe. Since monoculture of a single potato species was common practice, it wasn’t difficult for this disease to take hold. It spread like wildfire. Ireland suffered the most in what became known as the Great Famine, or the Great Starvation. 


Potato blight lifecycle

The potato blight pathogen prefers cool temperatures and moist environments, which Ireland has in abundance. Spores are produced from 54°F to 65°F, while lesions develop when temperatures are between 64°F and 75°F. And it takes surprisingly little moisture to create a water mold habitat. Morning dew on a leaf is all it takes, though more water is preferable. These pathogens can also attack other plants in the nightshade family, such as tomatoes. Early blight (Alternaria solani) is more often the culprit on tomatoes.

Life cycle of Phytophthora infestans on potato (M. Piepenbring) CC BY-SA 3.0

Water mold reproduction is odd. [Remember, oomycetes fall somewhere between algae and fungi.] It starts with an asexual phase during which branching structures, called hypha, grow, followed by spore development. Then, the receptacle where spores develop, called sporangia, begin to germinate, much the way pollen granules germinate in fertilization. Then, our tiny water mold grows more hypha, and the process continues. Sexual reproduction occurs when two mating types meet.


Symptoms of potato blight

Potato blight symptoms start as small, dark green, irregularly shaped, water-soaked spots on leaves, stems, petioles, and tubers. These spots have a yellowish halo. These lesions rapidly expand when moisture is present, turning purplish brown. Grayish-white fuzz can also be seen on the underside of leaves as spores develop.

Lesion on underside of leaf caused by potato blight (Howard F. Schwartz) CC BY 3.0

Scientists have crafted a group of genetically modified potatoes resistant to potato blight. These cisgenic potatoes appear unable to catch the disease. If you prefer not to grow genetically modified plants, there are other ways to prevent potato blight from taking hold.

Potatoes exposed to Phytophthora infestans. Normal potatoes have blight but the cisgenic potatoes are healthy. (Ronald Hutton) CC BY 3.0

How to prevent potato blight

Fixed copper sprays are the best preventative measures against potato blight. In fact, during WWII, when copper was collected to make artillery shells, farmers faced new threats from potato blight because they could not spray their fields. 


Potato blight can find its way into your potato bed through contaminated potatoes, visitors, materials, and rain or irrigation water splashing from infected plants to healthy plants. These are excellent reasons for quarantining new plants and not planting grocery store produce. [Just because a plant is healthy enough to eat now does not mean it isn’t carrying diseases that may stay in your soil for years.]


Allow the soil to dry out between waterings, pruning for good airflow, and add organic material to your potato bed to improve drainage.


A healthy potato bed is a thing of beauty. Let’s keep it that way!