Potato blight is a destructive plant disease that can wipe out an entire crop within days under the right conditions. There are actually two forms: late blight, caused by an organism called Phytophthora infestans, and early blight, caused by a fungus called Alternaria solani. Late blight is by far the more devastating of the two, costing the global potato industry an estimated £20 billion per year in crop losses and treatment costs. Both types are treatable if caught early, and largely preventable with the right approach.
Late Blight vs. Early Blight
Though both diseases attack potato foliage and tubers, they behave differently and show up at different times. Knowing which one you’re dealing with changes how you respond.
Early blight tends to appear on older, lower leaves first and rarely affects young, actively growing plants. It favors warm temperatures and high humidity. The spots start as small, dark, dry, papery flecks that expand into brown-black circles or ovals, often with concentric rings that give it the nickname “target spot.” Leaf veins sometimes border the spots, giving them an angular shape. Early blight progresses relatively slowly and, while it reduces yield, it seldom destroys a crop outright.
Late blight is the one that made history during the Irish Potato Famine, and it remains the single most serious threat to potato production worldwide. It thrives in cool, moist conditions rather than warm ones, and it spreads with alarming speed. Under optimal conditions, infection can establish in as little as two hours. You’ll see large, water-soaked, dark green to brown lesions on leaves, and in moist weather, a white, fluffy fungal growth appears on the undersides of affected foliage. Unlike early blight, late blight can destroy healthy, vigorous plants at any stage of growth.
How Blight Spreads
Late blight’s lifecycle is driven almost entirely by moisture and temperature. The organism produces spores most actively at 90 to 100 percent humidity and temperatures between 54 and 75°F. Those spores land on wet foliage and infect most readily at 64 to 75°F. At cooler temperatures, between 46 and 64°F, the spores release tiny swimming cells called zoospores that can move through water films on leaves for 2 to 10 hours, creating new infections throughout a field or garden.
This is why late blight outbreaks tend to explode after stretches of cool, rainy, or foggy weather. A few days of these conditions can turn a handful of spots into a field-wide disaster. Wind carries spores to neighboring gardens and farms, so one infected plot can seed infections across a wide area. Early blight spreads more slowly through rain splash and contaminated soil, and it overwinters on plant debris left in the garden.
What Blight Looks Like on Tubers
Blight doesn’t just damage leaves. It infects the potatoes themselves, both in the ground and in storage.
Late blight tuber symptoms appear as slightly depressed areas of brown to purplish skin on the outside. Cut one open and you’ll find a tan-brown, dry, granular rot that extends roughly half to three-quarters of an inch into the flesh, with an indistinct, blurry border. If these tubers end up in warm or humid storage, the rot keeps advancing. Secondary organisms often move in and destroy the tubers completely.
Early blight tuber lesions look similar from the outside, with raised edges that may appear purple to dark metallic gray. Inside, the flesh is brown, dry, and leathery or corky. As the disease progresses, affected tissue can turn water-soaked and yellowish green. Early blight lesions are less prone to secondary rot than late blight lesions, so affected tubers hold up somewhat better in storage.
Preventing Blight Before It Starts
The most effective defense against blight is prevention, because once the disease takes hold, you’re playing catch-up. Several practices dramatically reduce your risk.
Start with resistant varieties. Cornell University’s trials identify Elba as the potato variety most resistant to late blight, a late-season white potato that also resists early blight, scab, and several other common problems. Other strong choices include Kennebec (resistant to late blight, blackleg, and certain viruses), Chieftain (a red-skinned variety with late blight resistance), Island Sunshine (highly resistant, with yellow flesh), and Nicola (good blight and scab resistance). Planting resistant varieties won’t make you immune, but it buys significant time and reduces the severity of any outbreak.
Cultural practices matter just as much. Space your plants to allow good air circulation, which helps foliage dry quickly after rain or dew. Avoid overhead watering when possible, and water early in the day so leaves dry before nightfall. Destroy volunteer potato and tomato plants, which can harbor the organism between seasons. After harvest, remove all plant debris from the garden rather than leaving it to decompose in place. Do not compost rotten store-bought potatoes, as they can carry the pathogen.
Treating an Active Infection
If you spot blight symptoms, act immediately. Remove and dispose of visibly diseased plants and any tubers of all sizes from the soil and garden area. Don’t compost them.
For fungicide options, copper-based products are the most widely available organic choice. Copper sulfate and copper octanoate sprays are commonly used, though copper alone offers limited control against late blight and is not recommended as a sole treatment. For stronger protection, chlorothalonil-based products provide broader coverage and can be alternated with copper applications. Apply at the first sign of disease and repeat according to the product label.
Timing is critical. Preventive sprays applied before symptoms appear, particularly during cool and humid weather, are far more effective than reactive treatments. Once late blight is visibly spreading through a planting, fungicides slow the disease but may not save the crop. For late blight specifically, the pathogen has developed resistance to several classes of fungicides over the years, so rotating between different products helps maintain effectiveness.
Protecting Your Harvest in Storage
Even if your plants looked healthy at harvest, infected tubers can rot in storage and spread disease to neighboring potatoes. A few precautions make a significant difference.
Harvest when soil temperatures are between roughly 54 and 64°F. Handle tubers carefully, because bruises and nicks become entry points for post-harvest infection. Before storing, sort out any tubers with discolored skin, soft spots, or visible lesions.
In storage, good air circulation is essential. Free-standing water, high carbon dioxide levels, and warm temperatures all promote disease development. Keep storage conditions cool, dry, and well-ventilated, adjusted to the variety you’re storing. Phosphorous acid applied to tubers going into storage can limit late blight spread during handling and storage, though it won’t reverse damage already present in infected tubers.
Why Blight Remains So Difficult to Eradicate
Late blight’s persistence comes down to biology and economics. The organism reproduces explosively under favorable conditions, produces spores that travel on wind for miles, and has repeatedly evolved resistance to widely used treatments. In the UK alone, the potato industry spends an estimated £50 million per season fighting late blight. For home gardeners, the challenge is simpler but no less frustrating: a week of cool, damp weather can undo months of careful growing.
The most reliable approach combines resistant varieties, good garden hygiene, preventive sprays during high-risk weather, and prompt removal of anything infected. No single measure is foolproof, but together they tilt the odds heavily in your favor.