Blight is a rapid, destructive plant disease that kills leaves, stems, flowers, or entire branches faster than most other infections. Unlike a simple leaf spot, which stays contained in small circles on individual leaves, blight spreads aggressively across large sections of tissue, turning them brown or black and causing widespread dieback. It can be caused by fungi, bacteria, or water molds, and it thrives in warm, humid conditions. Blight affects everything from backyard tomatoes to commercial orchards, and it’s responsible for one of the most devastating agricultural disasters in history.
How Blight Damages Plants
Blight pathogens are necrotrophs, meaning they kill plant cells outright rather than quietly feeding off living tissue. They do this by releasing enzymes that break down cell walls, along with toxins and reactive oxygen species that overwhelm the plant’s defenses. The result is rapid tissue death: leaves darken, wilt, and collapse, sometimes within days of infection. Stems can develop dark, water-soaked lesions that girdle the branch and cut off nutrient flow.
Some blight pathogens work systemically, entering the plant’s vascular system and spreading through its water-conducting channels. This disrupts the plant’s ability to move water from roots to leaves, causing wilting even when soil moisture is adequate. Other types stay localized, destroying the soft tissue of individual organs like leaves or fruit but not traveling through the whole plant.
Blight vs. Leaf Spot and Mildew
One of the most common points of confusion for gardeners is telling blight apart from leaf spots or powdery mildew. Leaf spots are localized lesions, small dead zones on a leaf where cells have collapsed. They stay relatively contained and spread slowly. Blight, by contrast, is general and extremely rapid browning of leaves, branches, or flowers that quickly kills large areas of tissue. If your plant has a few speckled leaves, that’s likely a leaf spot disease. If whole branches are turning brown and dying back over the course of a week, you’re probably dealing with blight.
Powdery mildew looks completely different: a white or grayish powder coating the leaf surface. It rarely kills plants outright the way blight does, though it weakens them over time.
Late Blight
Late blight is the most infamous type, caused by a water mold called Phytophthora infestans. It primarily attacks potatoes and tomatoes. The first sign is small, dark green or brown spots on lower leaves. These expand into large, water-soaked lesions that turn blackish-brown and necrotic. In humid weather, a white, fuzzy growth appears on the undersides of leaves. This is the pathogen producing spores. In severe cases, affected areas turn completely black and give off a noticeable odor.
On potato tubers, late blight shows up as brown or purple patches on the skin that feel firm or leathery. Cut open an infected potato and you’ll find reddish-brown rot extending inward. Secondary bacteria often move in afterward, turning the flesh soft and foul-smelling.
Late blight spreads fastest in cool, moist weather between 50°F and 70°F with high humidity or rain. Spores can germinate in as little as three to five days under ideal conditions, which is why an entire field can go from healthy to devastated in under two weeks.
The Irish Potato Famine
Late blight’s destructive power is not theoretical. In 1845, Phytophthora infestans arrived in Ireland and infected an estimated one-third to one-half of the country’s potato acreage. The following year was worse: three-quarters of the harvest was lost. Ireland’s population depended heavily on potatoes as a staple food, and the consequences were catastrophic. Between 1845 and 1850, roughly 2.5 to 3 million people, nearly one-third of the population, disappeared from the island through death and emigration. The worst year, 1847, known as “Black ’47,” saw at least 400,000 deaths. The potato harvest didn’t meaningfully recover until the fall of 1848.
Early Blight
Early blight is a separate disease caused by the fungus Alternaria solani, and it also targets potatoes and tomatoes. Its signature symptom is distinctive enough to identify at a glance: dark brown lesions with concentric rings, like a target or bullseye, often surrounded by a yellow halo. These lesions always start on the oldest, lowest leaves and work their way upward.
Early blight favors warmer conditions than late blight. Spore germination peaks at temperatures between 77°F and 86°F with relative humidity above 90%. At 100% humidity and 86°F, disease severity on tomato plants can reach over 80%. The disease drops off sharply at extreme heat (above 104°F) or when humidity falls below about 40%.
Fire Blight
Fire blight is the major bacterial form of blight, caused by Erwinia amylovora. It attacks plants in the rose family, and its host list reads like a catalog of common fruit and ornamental trees: apple, pear, flowering crabapple, ornamental pear, hawthorn, mountain ash, quince, cotoneaster, and firethorn.
The name comes from its appearance. Infected branches look scorched, as if someone held a torch to them. Blossoms wilt and turn brown, then the infection moves down into the branch, blackening bark and causing the tips of young shoots to curl into a characteristic “shepherd’s crook” shape. Fire blight spreads during warm, wet spring weather, often carried from flower to flower by pollinating insects.
What Triggers Blight Outbreaks
Every type of blight shares one basic requirement: moisture. Fungal and bacterial spores need water on plant surfaces to germinate and penetrate tissue. Extended periods of leaf wetness, whether from rain, heavy dew, or overhead irrigation, create the conditions blight needs to take hold. High humidity (above 90%) keeps that moisture in place long enough for infection to establish.
Temperature determines which type of blight you’re likely to encounter. Late blight favors cooler conditions in the 50°F to 72°F range. Early blight thrives in warmer weather, peaking around 77°F to 86°F. Both are most aggressive when warm or cool temperatures combine with persistent wetness, which is why rainy summers are notorious blight seasons.
Overcrowded plantings compound the problem. Dense foliage traps moisture, blocks airflow, and creates the humid microclimate blight pathogens prefer. Stressed plants, whether from poor nutrition, compacted soil, or drought, are also more susceptible.
Organic and Chemical Controls
Copper-based fungicides are the backbone of blight treatment for both conventional and organic growers. Copper kills fungi and bacteria on contact, making it effective against fungal blights like early and late blight as well as bacterial diseases like fire blight. Several formulations exist, but one of the oldest and most proven is Bordeaux mixture, a combination of copper sulfate and lime that has been used successfully for more than 150 years. The lime acts as a safener, neutralizing the acidity of the copper sulfate so it’s less likely to burn plant tissue.
Timing matters more than product choice. Copper fungicides are preventive, not curative. They protect healthy tissue from infection but can’t reverse damage that’s already occurred. You need to apply them before symptoms appear or at the very first sign of disease, then reapply after rain washes the coating off. Avoid applying Bordeaux mixture when temperatures exceed 85°F or right before rain, as both situations increase the risk of leaf burn.
Sulfur is another organic option, though it’s more effective against powdery mildew and rusts than against blight specifically. One critical rule: never combine sulfur with oil sprays or apply them within a month of each other. The combination is toxic to plants. Sulfur also shouldn’t be used when temperatures are expected to exceed 80°F.
Prevention Through Cultural Practices
The most effective blight prevention starts before you ever reach for a spray bottle. Crop rotation with unrelated plants is probably the single most impactful practice. Blight pathogens survive in soil and plant debris between seasons. If you grow tomatoes in the same spot year after year, you’re giving the pathogen a ready supply of hosts. Rotating to an unrelated crop, something outside the nightshade family for tomato or potato blight, breaks the cycle.
Spacing plants generously improves air circulation and helps foliage dry quickly after rain or dew. Water at the base of the plant rather than overhead to keep leaves dry. Morning watering is preferable to evening watering because leaves have the full day to dry before nighttime humidity sets in. Removing and destroying (not composting) infected plant material prevents spores from overwintering in your garden.
Raised beds help prevent certain soil-borne blight diseases by improving drainage and keeping plant crowns above soggy ground. Mulching around the base of plants reduces soil splash, which is one of the main ways fungal spores travel from contaminated soil onto lower leaves.
Blight-Resistant Varieties
Choosing resistant varieties is one of the simplest ways to sidestep blight entirely. Plant breeders have developed tomato and potato cultivars with strong resistance to both early and late blight. For tomatoes, the variety “Legend,” bred at Oregon State University, is a determinate (bushy) plant that produces large red fruit and carries resistance to late blight. It’s compact enough for container growing, making it a practical choice for home gardeners in blight-prone climates.
When shopping for transplants or seeds, look for disease resistance codes on the label. “LB” indicates late blight resistance, and “EB” indicates early blight resistance. Many modern hybrid tomatoes carry resistance to multiple diseases. For fruit trees susceptible to fire blight, nurseries can recommend cultivars with higher natural resistance, which varies by apple and pear variety.