Late blight is a fast-moving plant disease that attacks potatoes and tomatoes, capable of destroying an entire crop within two weeks. It’s caused by an organism called Phytophthora infestans, which despite being treated with fungicides isn’t actually a fungus. It belongs to a group called oomycetes, sometimes called water molds, that thrive in cool, wet conditions. Late blight is most famous for triggering the Irish Potato Famine in the 1840s, and it remains one of the most destructive plant diseases worldwide.
What Late Blight Looks Like
The first signs appear on leaves as small, dark, water-soaked spots. In cool, moist weather, these spots expand rapidly into large brown or purplish-black patches. A key identifying feature: white, fuzzy growth on the undersides of leaves, visible in humid conditions. Unlike many leaf diseases, the spots aren’t confined by leaf veins. They spread freely across the leaf surface, growing irregular in shape.
Stems develop similar dark lesions, and the entire plant can brown, shrivel, and defoliate within 14 days of the first visible symptoms. On tomato fruits, the disease produces shiny, dark or olive-colored lesions that can cover large areas and invite secondary soft rots. Potato tubers develop a dry, corky rot that ranges from brown to reddish, though tubers often look perfectly fine at harvest and only show symptoms weeks later in storage.
How It Differs From Early Blight
The names sound similar, but early blight and late blight are caused by completely different organisms and look distinct up close. Early blight (caused by a true fungus, Alternaria solani) produces dry, papery spots with a characteristic bullseye pattern of concentric rings. These spots are typically bordered by leaf veins, giving them an angular shape, and they’re often surrounded by a yellowish ring.
Late blight spots, by contrast, lack that target pattern. They start as pale to dark green irregular blotches, grow across veins rather than being contained by them, and expand far more aggressively. The white fuzzy growth on leaf undersides is a dead giveaway for late blight and won’t appear with early blight. Perhaps the biggest practical difference: early blight progresses slowly over a season, while late blight can level a garden in days.
The Weather It Needs
Late blight is a disease of cool, wet weather. The pathogen develops best between 15°C and 24°C (roughly 59°F to 75°F). Temperatures above 25°C (77°F) slow it down, which is why hot, dry summers offer some natural protection. But nighttime temperatures above 10°C (50°F) are enough for the organism to survive and spread.
Humidity is the other critical factor. Relative humidity above 90% is essentially a green light for the disease. Long stretches of rain, fog, heavy dew, or overcast skies create ideal conditions. A week of cool, rainy weather during the growing season is exactly the scenario that triggers outbreaks. Gardeners in the Pacific Northwest, the Northeast, and other humid regions deal with late blight pressure regularly.
How It Spreads
The pathogen produces spore-containing structures called sporangia on the surfaces of infected plants. In species like P. infestans, these sporangia detach easily and can travel several kilometers on wind currents or splash from plant to plant in rain. This airborne dispersal is what makes late blight so explosive compared to diseases that only spread through direct contact or soil.
The organism also produces thick-walled survival spores through sexual reproduction. These can persist in soil and plant debris for years, surviving freezing temperatures and even some chemical treatments. This means an infected field or garden bed can harbor the pathogen long after the visible disease is gone. Volunteer potato plants, those that sprout from tubers left in the ground, are a common source of new outbreaks the following year.
The Stored Potato Problem
Harvesting potatoes that look healthy doesn’t guarantee they’re free of infection. Tubers can carry the pathogen without visible symptoms at harvest time. In storage, the skin first discolors to brown or purple, then develops into a brownish dry or wet rot. These symptoms typically appear within the first month of storage but can continue emerging throughout the storage season.
The bigger risk is what happens next. Soft rot bacteria colonize the blight-damaged tubers as a secondary infection, turning them to mush. That rot then spreads to neighboring healthy tubers in the pile, potentially ruining an entire harvest in storage. For commercial growers, this makes careful inspection before and during storage essential.
The Irish Famine Connection
Late blight’s most devastating chapter began in 1845, when it swept through Ireland’s potato crop. Between 1845 and 1850, Ireland’s population fell by over one-third. An estimated 1 to 1.5 million people died, and another 1 to 1.5 million emigrated. The 1841 census counted over 8.1 million people in Ireland; by 1851, that number had dropped to about 6.5 million. The worst year was 1847, known as “Black ’47,” when at least 400,000 deaths were reported. The resulting diaspora eventually spread to nearly 80 million descendants, many in the Americas. That a million people starved in what was then part of the wealthiest nation on Earth remains one of history’s starkest examples of how a plant disease can reshape societies.
Managing and Preventing Late Blight
Prevention is the cornerstone of late blight management. Repeated preventative fungicide applications on susceptible crops are the primary control method. The key word is “preventative”: once the disease has taken hold of a plant, it’s extremely difficult to stop. Timing applications before symptoms appear, especially when weather forecasts predict cool, wet conditions, is far more effective than reacting after the fact.
For organic growers, copper-based products have long been the standard recommendation, though their effectiveness varies against current strains. Research from the American Phytopathological Society found that some copper treatments performed poorly against recent North American strains of P. infestans, while certain biopesticides showed better results when applied preventatively. Conventional fungicides generally offer more reliable protection across different strains.
Beyond spraying, several cultural practices reduce risk. Remove and destroy any infected plant material rather than composting it. Eliminate volunteer potato plants in spring. Improve air circulation by spacing plants adequately. Avoid overhead watering, which keeps foliage wet. Choose resistant varieties when available: the tomato variety “Legend,” bred at Oregon State University, carries late blight resistance and works well in containers and gardens alike. For potatoes, check regional extension recommendations, as resistance varies by strain and geography.
Tracking Outbreaks in Real Time
In the United States, a tool called USABlight serves as a national monitoring system specifically for late blight on potatoes and tomatoes. Users can view maps of current and past disease occurrences, report new cases, submit samples for strain identification, and sign up for text alerts when late blight is detected in their area. Knowing that late blight has been confirmed nearby gives you a critical head start on preventative treatment, since spores can travel long distances on wind.