Tomato blight is caused by two different microorganisms that thrive in warm, humid conditions: a fungus that causes early blight and a water mold that causes late blight. Both attack leaves, stems, and fruit, but they behave differently, look different on the plant, and require slightly different approaches to manage. Understanding which one you’re dealing with, and what conditions invite it, is the key to keeping your tomato plants healthy.
Two Diseases, Two Different Pathogens
The word “blight” gets used as a catch-all, but early blight and late blight are caused by entirely separate organisms. Early blight comes from a group of fungi in the genus Alternaria, primarily Alternaria solani and Alternaria alternata. These are true fungi that produce dark spores on infected tissue and spread readily through wind and rain splash.
Late blight is caused by Phytophthora infestans, which is technically not a fungus at all. It’s an oomycete, sometimes called a water mold, more closely related to algae than to the organism behind early blight. This is the same pathogen that triggered the Irish Potato Famine in the 1840s, and it remains one of the most destructive plant diseases worldwide. It attacks both tomatoes and potatoes aggressively and can destroy a healthy plant within days under the right conditions.
How Early Blight Infects Your Plants
Early blight typically shows up on the oldest, lowest leaves first and works its way up the plant. The telltale sign is dark brown spots with concentric rings, sometimes described as a bull’s-eye or target pattern. The tissue around the spots often turns yellow before the leaf dies and drops off. Stems can develop dark, sunken lesions, and fruit may show leathery, dark patches near the stem end.
The fungus gets into the plant by penetrating leaf tissue directly, entering through natural pores on the leaf surface, or slipping through wounds from pruning, insect damage, or wind abrasion. Once inside, it produces enzymes and toxic compounds that kill plant cells ahead of the infection, essentially clearing a path for itself to spread deeper into the tissue. Plants that are stressed from poor nutrition, heavy fruit load, or drought are especially vulnerable.
How Late Blight Looks Different
Late blight moves faster and looks different. Instead of neat concentric rings, you’ll see large, irregular, water-soaked patches on leaves that quickly turn dark brown or black. In humid conditions, a fuzzy white growth often appears on the undersides of affected leaves. Stems develop dark lesions, and infected fruit turns brown and mushy with a rough, firm texture on the surface. The speed is what distinguishes late blight most clearly: a plant can go from a few spots to complete collapse in under a week during cool, wet weather.
The Weather Conditions That Trigger Blight
Both forms of blight need moisture on the leaves to get started, but they prefer slightly different temperatures. Early blight spores germinate across a wide range, from about 59°F to 104°F (15°C to 40°C), but the sweet spot is 77°F to 86°F (25°C to 30°C). At 86°F with high humidity, spore germination reaches roughly 75% within 24 hours. Disease severity peaks at those same temperatures, with studies recording over 80% severity at 86°F under controlled conditions. Early blight is a warm-weather disease.
Late blight, by contrast, favors cooler, wetter conditions. It’s most aggressive when nighttime temperatures drop into the 50s and 60s (°F) and daytime highs stay below 80°F, especially with fog, drizzle, or extended periods of overcast skies. Prolonged leaf wetness is critical for both diseases. Leaves need to stay wet for several hours (often overnight or longer) for spores to successfully germinate and penetrate the plant.
This is why blight outbreaks tend to follow predictable weather patterns. A stretch of warm, rainy days in midsummer sets the stage for early blight. A cool, damp spell, common in late summer or early fall in many regions, is prime time for late blight.
Where Blight Comes From Each Season
The early blight fungus overwinters in infected plant debris left in or on the soil, where it can survive for at least one year and possibly several. It can also be carried on seed. When conditions warm up in spring, the fungus produces new spores that land on young plants through wind and rain splash. This is why gardens that grew tomatoes the previous year often see blight appear earlier and more severely.
Late blight doesn’t survive as easily in cold-winter climates because the pathogen needs living tissue. Its main reservoirs between seasons are potatoes saved for seed, tubers that survive unfrozen in the soil over winter, and volunteer potato or tomato plants growing in compost piles or fields. In milder climates, infected crops can produce wind-dispersed spores that travel long distances. Infected transplants purchased from greenhouses have also been a significant source of late blight outbreaks in recent years.
Both pathogens also survive on related plants. Nightshade weeds like hairy nightshade, bittersweet nightshade, and jimson weed serve as hosts. For late blight specifically, even ornamental plants like petunias and calibrachoa can harbor the pathogen, a detail many gardeners don’t expect.
Cultural Practices That Reduce Blight Risk
The most effective prevention starts with how you manage your garden, not what you spray on it. Crop rotation is essential. Wait at least two years before planting tomatoes (or any related crop like peppers, potatoes, or eggplant) in the same spot. This gives the early blight fungus less debris to feed on and reduces the spore load in the soil.
At the end of each season, remove or bury infected plant material rather than leaving it on the surface. Don’t compost visibly diseased plants unless your compost reliably reaches high temperatures.
During the growing season, three practices make the biggest difference:
- Water at the base, not overhead. Drip irrigation or soaker hoses keep leaves dry. Overhead sprinklers splash spores from the soil onto lower leaves and create the prolonged leaf wetness that both pathogens need.
- Stake or cage plants and space them generously. Good airflow between plants helps leaves dry quickly after rain or morning dew. Crowded, sprawling plants create a humid microclimate that favors infection.
- Mulch around the base. A layer of straw or other mulch acts as a barrier between the soil (where spores overwinter) and the lowest leaves, reducing splash-up during rain.
Pruning lower branches so that no foliage touches the ground also helps, since early blight almost always starts on the leaves closest to the soil.
Varieties With Built-In Resistance
Choosing the right variety won’t make your plants immune, but it can dramatically slow down infection and reduce losses. Several tomato varieties have been bred with resistance to one or both types of blight.
For late blight resistance, Cornell University’s list of disease-resistant varieties includes Mt. Merit F1 (which also resists early blight), Juliet, Roma, Old Brooks, Cherry Bomb F1, and Old Fashioned Goliath F1. Mt. Merit F1 stands out as one of the few varieties with documented resistance to both early and late blight.
For early blight specifically, options are more limited. Honey Bunch F1 and Mt. Fresh Plus F1 both carry early blight resistance. Mt. Fresh Plus is a larger slicing tomato that also resists several other common diseases.
Keep in mind that “resistance” means the plant tolerates infection better and develops symptoms more slowly. It does not mean the plant is bulletproof. Resistant varieties still benefit from good cultural practices, especially in years with heavy disease pressure.
Fungicide Options for Active Infections
When cultural practices aren’t enough, fungicides can help protect healthy tissue from new infection. Copper-based sprays are the most widely available option for home gardeners and are approved for organic use. They work as protectants, meaning they need to be on the leaf surface before spores arrive. Once blight has visibly infected a leaf, no spray will cure that tissue.
Timing matters more than the product itself. Forecasting tools like TOM-CAST (for early blight) and the Cornell Late Blight Decision Support System use local weather data to predict when conditions favor infection, helping growers time applications more precisely rather than spraying on a fixed schedule. Some regional extension services publish blight alerts based on these models during the growing season.
For most home gardeners, the practical approach is to begin protective sprays when weather conditions turn favorable for blight (warm and humid for early blight, cool and wet for late blight) and reapply after rain washes the product off. Removing and disposing of infected leaves as soon as you spot them slows the spread while you protect the remaining healthy foliage.