Potato blight is a plant disease that destroys the leaves, stems, and tubers of potato plants, often killing them within two weeks under the right conditions. It costs an estimated $6.7 billion annually worldwide in crop losses and control measures, affecting both commercial farms and home gardens. The term usually refers to two distinct diseases: early blight and late blight. Late blight is by far the more destructive of the two and was responsible for the Irish Potato Famine of the 1840s.
Early Blight vs. Late Blight
Despite their names, early blight and late blight are caused by completely different organisms and behave differently in the field. The names refer to when they typically appear during the growing season, though both can show up at the same time.
Early blight is caused by a fungus called Alternaria solani. It thrives in warm, humid weather and produces distinctive “target spot” lesions on leaves: brown-black circular spots with concentric rings, often surrounded by a yellowish border. The spots tend to follow the veins of the leaf, giving them an angular shape. Early blight weakens plants and reduces yields but rarely kills them outright.
Late blight is caused by Phytophthora infestans, a water mold that favors cool, moist conditions. It’s far more aggressive. Leaf spots start as small, pale-to-dark green irregular patches that grow rapidly into large brown or purplish-black areas. Unlike early blight spots, these lesions spread freely across leaf veins rather than being confined by them. In moist weather, a white fluffy growth appears on the undersides of leaves, and infected plants often give off a noticeable decaying smell. Left unchecked, late blight can destroy an entire planting in under two weeks.
How Late Blight Spreads
The pathogen reproduces by producing tiny sac-like spore structures on the surface of infected tissue. These spores sit on stalk-like structures that help them catch the wind, and in humid conditions they form continuously. That white fuzz visible on the undersides of infected leaves is the spore-producing layer itself, actively releasing new spores into the air.
Wind and rain carry the spores to nearby plants, where they germinate and start new infections. The pathogen can also travel through stems and, critically, through infected transplants. A major U.S. epidemic in 2009 was traced to late blight spreading on tomato transplants sold at garden centers, a reminder that the disease affects tomatoes just as readily as potatoes. Infected seed potatoes and tubers left in the ground from the previous season are another common source, giving the pathogen a place to survive over winter.
What Infected Tubers Look Like
Above ground, the signs of late blight are dramatic enough to spot from a distance. Below ground, they’re subtler but equally damaging. Infected potato tubers develop shallow, discolored lesions on the surface. Cut one open and the inner flesh is mottled with reddish-brown patches. These tubers rot quickly in storage, and if they’re planted the following year, they introduce the pathogen right back into the soil.
How to Prevent and Manage Blight
Prevention matters far more than treatment with late blight, because once it takes hold it moves fast. The most effective steps for home gardeners start before planting: use certified disease-free seed potatoes, choose resistant varieties when available, and avoid planting in the same spot year after year. Good airflow between plants helps leaves dry quickly after rain, which slows spore germination. Watering at the base of plants rather than overhead keeps foliage drier.
For home gardeners who want chemical protection, products containing copper or chlorothalonil (sold under the trade name Daconil, among others) are the most effective options available at garden centers. These work as preventive sprays, meaning you need to apply them before symptoms appear or at the very first sign of disease. They won’t cure plants that are already heavily infected. Organic growers are more limited: fixed copper formulations are the only organically approved option with meaningful effectiveness against late blight.
Commercial growers rotate between several fungicide classes to prevent the pathogen from developing resistance. Before disease appears, they spray on a seven-day schedule. Once blight is detected, the interval tightens to every five days with more potent products.
Disposing of Infected Plants Safely
How you remove blighted plants matters as much as spotting them early. The goal is to avoid launching spores into the air where wind can carry them to neighboring gardens or fields.
- Cut, don’t pull. Snip the stem at soil level and place the plant directly into a garbage bag. Pulling the plant up dislodges spores into the air.
- Seal and dispose. Tie the bag shut and put it in the trash. Do not add blighted plants to compost bins or municipal green waste programs. Late blight spores can survive composting that doesn’t reach high enough temperatures.
Any tubers left in the ground can harbor the pathogen through winter, so clearing the bed thoroughly at the end of the season reduces the risk of reinfection the following year.
Blight-Resistant Potato Varieties
Breeding for resistance is the longest-term solution to late blight, and progress has been significant. Scientists at the International Potato Center in Peru transferred three resistance genes from wild potato species native to Mexico and Argentina into Victoria, a widely grown variety in East Africa. Five years of field trials across multiple locations in Uganda showed the bioengineered Victoria was virtually 100% resistant to late blight, surviving exposure that destroyed conventional plants growing in adjacent rows, all without any chemical spraying.
For home gardeners in North America and Europe, several commercially available varieties carry partial resistance to late blight, including Defender, Sarpo Mira, and Elba. Partial resistance doesn’t make a plant immune, but it slows the disease enough to make management practical, buying you time to harvest before an outbreak becomes catastrophic. Pairing a resistant variety with basic preventive practices is the most reliable strategy for any gardener in a blight-prone region.