Potato blight is a plant disease that destroys the leaves, stems, and tubers of potato plants, sometimes wiping out an entire crop within days. There are two forms: early blight, caused by a fungus, and late blight, caused by a water mold. Late blight is the more destructive of the two and was responsible for the Irish Potato Famine of the 1840s, which reduced Ireland’s population by roughly 25% through death and emigration in just six years.
Early Blight vs. Late Blight
Though both diseases attack potato foliage and tubers, they’re caused by different organisms and behave differently in the field.
Early blight is a true fungal infection. It typically appears on older, lower leaves first and progresses upward. The telltale sign is dark brown or black spots with a distinctive “target” or “bullseye” pattern created by concentric rings of dead tissue. These spots tend to follow the leaf veins, giving them an angular shape, and they’re often surrounded by a yellowish-green border. Early blight usually shows up in warmer weather and weakens the plant over time rather than killing it outright.
Late blight is caused by a water mold, not a true fungus, and it’s far more aggressive. Leaf spots start as small, pale green patches that quickly expand into large, dark brown or purplish-black areas. Unlike early blight, these spots aren’t confined by leaf veins and can spread freely across the leaf surface. The signature feature of late blight is a white, fuzzy growth that appears along the edges of dead tissue, usually on the undersides of leaves during cool, damp weather. In the right conditions, late blight can destroy an entire field in a matter of days.
How It Spreads
Late blight thrives in cool, wet weather. The organism produces microscopic spore capsules on the undersides of infected leaves and along stems when humidity is high. These capsules spread through wind and rain splash, sometimes traveling miles from the original infection.
Temperature determines how the organism infects new plants. Below about 18°C (65°F), each spore capsule releases six to eight tiny swimming spores that need a film of water on the leaf surface to reach the plant’s cells. This process takes roughly two hours. Above that temperature, the capsule can germinate directly without releasing swimmers, though it still needs moisture. The sweet spot for spore production is 18 to 22°C (64 to 72°F), which is why late blight tends to explode during stretches of cool, overcast, humid weather rather than during hot, dry spells.
Early blight, by contrast, favors slightly warmer conditions and stressed plants. Older leaves, nutrient-deficient plants, and those carrying a heavy crop load are most vulnerable.
What Infected Plants Look Like
On leaves, late blight creates large, dark brown blotches with a grayish-green edge. These patches look water-soaked at first before darkening. If you flip the leaf over during humid weather, you’ll see the thin, powdery white growth that confirms the diagnosis.
Stem infections appear as firm, dark brown lesions with rounded edges. Once the stems are infected, the disease can move rapidly down into the tubers through the soil.
Infected tubers develop discolored patches ranging from brown to reddish-purple on their skin. Cut one open and you’ll find the rot extends inward. These damaged tubers are often invaded by secondary bacteria that turn them soft and foul-smelling in storage. Even tubers that look healthy at harvest can develop symptoms weeks later if they were exposed to spores during the growing season.
Conditions That Trigger Blight Warnings
Agricultural agencies in many countries issue blight warnings based on weather forecasting models. The general conditions that favor an outbreak are temperatures between roughly 7°C and 27°C combined with extended periods of high humidity (above 75%) and cloudy or foggy weather. In the UK, the system known as the Hutton Criteria analyzes hourly weather data to predict when conditions are ripe for infection. When a warning is issued, growers know they have a narrow window to apply protective treatments before spores begin spreading.
If you’re growing potatoes at home, the practical takeaway is simple: watch for stretches of cool, damp, overcast weather lasting two or more days. That’s when you should inspect your plants closely and consider preventive action.
Preventing Blight in Your Garden
No single step eliminates blight risk, but several practices working together make a significant difference.
- Choose resistant varieties. Some potato cultivars have been bred specifically for blight resistance. Varieties from the Sarpo breeding program (developed by the Sarvari Research Trust) are among the most widely available options with strong late blight resistance. Alouette is another variety gaining popularity for its resistance.
- Plant certified seed potatoes. Using certified disease-free seed tubers prevents introducing the pathogen into your soil from the start.
- Space plants for airflow. Good air circulation helps leaves dry faster after rain, reducing the moisture window that spores need to infect.
- Avoid overhead watering. Watering at the base of the plant keeps foliage dry. If you use sprinklers, water in the morning so leaves dry quickly.
- Earth up tubers well. Mounding soil around the base of the plant creates a physical barrier that makes it harder for spores washed off the leaves by rain to reach the tubers underground.
Managing an Active Infection
Once you spot blight symptoms, speed matters. For home gardeners, the most effective immediate response is to cut off all infected foliage and remove it from the garden. If the infection is widespread, cut the entire plant down to ground level. Leave the tubers in the soil for two to three weeks after removing the tops. This gives the tuber skins time to toughen up and reduces the chance of spores on the soil surface infecting them during harvest.
For organic growers, several biological and mineral-based products can help prevent or slow infection when applied before symptoms appear. Copper-based sprays have been used for over a century, though their use is increasingly restricted in some regions due to copper accumulation in soil. Newer options include products based on beneficial bacteria, potassium bicarbonate, hydrogen peroxide compounds, and plant-derived oils like garlic and clove. These biological products work best as preventives or for suppression rather than as cures for an active infection. Applying them on a regular schedule during high-risk weather periods gives the best results.
Conventional fungicide programs are widely used in commercial potato production and can be highly effective when timed correctly. These are typically applied on a preventive schedule guided by blight forecasting models.
Disposing of Infected Plants Safely
How you handle blighted plant material matters more than most gardeners realize. The organism produces tough, resilient survival structures called oospores that can persist in soil and compost for extended periods. A typical home compost bin does not reach temperatures high enough to kill them.
The Royal Horticultural Society recommends three safe options: bury infected material deeply below the depth you’d normally dig or till, send it out with your local council’s green waste collection (municipal composting systems reach the extreme temperatures needed to destroy oospores), or burn it. Do not add blighted plants to your home compost pile, as this can reintroduce the pathogen to your garden the following season.
For the same reason, any volunteer potato plants that sprout from tubers left in the ground should be removed promptly. These “groundkeepers” can harbor the disease over winter and serve as a source of infection for next year’s crop.