Late blight is the most destructive disease tomatoes face, capable of killing an entire plant within days under the right conditions. Caused by a water mold organism called Phytophthora infestans (the same pathogen behind the Irish potato famine), it thrives in cool, moist weather and spreads with alarming speed. If you’re seeing dark, water-soaked patches on your tomato leaves that seem to be growing by the hour, late blight is likely the culprit.
How to Identify Late Blight
Late blight lesions look different from other tomato diseases, and learning the visual cues will save you time and guesswork. The first sign is irregular, water-soaked, greenish-gray patches on the leaves that look greasy or oily. These patches expand rapidly into large brown-gray dead areas that can consume most of a leaf within days. Unlike other leaf diseases, late blight can appear anywhere on the plant and spreads in all directions at once rather than working its way up from the bottom.
The single most reliable diagnostic feature: flip an affected leaf over and look at the underside. Under humid conditions, you’ll see white cottony fuzz along the edges of the lesions. That fuzz is the organism actively producing spores. White mold on the leaf underside means late blight. No other common tomato disease produces it.
Stems develop dark brown or black lesions that can girdle the entire stem, cutting off water and nutrients to everything above. Fruit develops large, brown, leathery patches that often have an oily sheen. Affected tomatoes rot quickly and are not salvageable.
Late Blight vs. Early Blight
These two diseases are easy to confuse by name, but they look quite different in person. Early blight produces small, circular spots (about a quarter to half an inch wide) with concentric brown rings that create a target or bullseye pattern. It starts on the oldest, lowest leaves and moves upward over several weeks. Late blight skips the bullseye pattern entirely, producing large, irregular, water-soaked patches with no ring structure.
Speed is another giveaway. Early blight is a slow burn, taking weeks to defoliate a plant. Late blight can destroy a plant in under a week during favorable weather. And again, flip the leaf: early blight shows brown spots on the underside but no white fuzz. If you see cottony white growth, it’s late blight.
Weather Conditions That Trigger Outbreaks
Late blight is a disease of cool, wet weather. The organism produces spores most aggressively at 90 to 100% relative humidity with temperatures between 60 and 80°F. The pathogen’s growth peaks around 73°F. Spores germinate best between 64 and 75°F, and the swimming spores that actually infect plant tissue are released at even cooler temperatures, between 46 and 64°F.
This means late blight is most dangerous during stretches of cool, rainy, or foggy weather, particularly in late summer and early fall. Hot, dry conditions slow or stop the disease. If your area is experiencing a wet spell with overnight lows in the 50s or 60s, conditions are ideal for an outbreak. Gardeners in the Northeast, Pacific Northwest, and upper Midwest face the highest risk, though late blight occurs in every region.
How It Spreads
Phytophthora infestans does not survive in soil on its own. It only persists in living plant tissue. In northern climates, the pathogen overwinters in infected potato tubers that don’t freeze, whether left in the ground, sitting in compost piles, or stored in root cellars. Infected tubers sprout in spring, and the new growth produces spores that can travel on the wind for miles.
Other plants in the nightshade family are also susceptible, including petunias and wild nightshade weeds. Volunteer potato plants and these related species can serve as early-season reservoirs. Once spores land on a tomato leaf in moist conditions, infection can begin within hours. A single infected plant can release millions of spores into the air, which is why late blight outbreaks often affect entire neighborhoods or regions in quick succession.
One piece of good news: the pathogen is not seed-borne. Seeds saved from an infected plant can be used the following season without carrying the disease forward.
What to Do With Infected Plants
Plants showing late blight symptoms cannot be saved. Spraying fungicide on an already infected plant in a home garden offers very little protection. The priority is removing diseased plants immediately to limit spore production and protect nearby gardens.
Pull affected plants out entirely, roots and all, and place them in plastic bags. Leave the bags in the sun for a few days to ensure the organism is completely killed, then put them out with the trash. Do not compost infected plants or fruit. The temperatures in a home compost pile are not reliable enough to kill Phytophthora infestans, and you risk reintroducing it later.
The following spring, watch for and remove any volunteer tomato or potato plants, as well as nightshade weeds. All of these can harbor the pathogen and restart the cycle.
Preventive Fungicide Options
Fungicides for late blight are preventive, not curative. They protect healthy tissue from new infection but cannot reverse damage already done. Timing matters more than product choice.
For home gardeners, the two effective active ingredients are copper (available in many organic-approved formulations) and chlorothalonil (sold under the trade name Daconil, among others). Copper is the only option with organic certification that provides decent protection against late blight. Apply either product before symptoms appear, on a 7 to 10 day schedule. If late blight has been reported in your area, tighten that schedule to every 5 to 7 days, and be thorough about covering all leaf surfaces.
Commercial growers have access to a wider range of products and typically rotate between different chemical classes to prevent the pathogen from developing resistance. But for backyard gardens, copper or chlorothalonil applied consistently before infection is the realistic approach.
Resistant Tomato Varieties
Planting resistant varieties is the most effective long-term strategy, especially if you garden in a region where late blight appears regularly. Breeding programs have produced a solid lineup of options in recent years, and many are available from mainstream seed companies.
Determinate (Bush) Types
- Defiant: Red, medium-sized fruit (8 to 12 oz), also resistant to early blight
- Mountain Merit: Red, 8 to 10 oz fruit, resistant to several other diseases and nematodes. An All-America Selection winner.
- Iron Lady: Red, smaller fruit (about 5 oz), with resistance to multiple diseases
- Galahad: Red, 7 to 12 oz fruit, broad disease resistance including nematodes
- Mountain Gem: Red, 6 to 8 oz fruit
Indeterminate (Vining) Types
- Damsel: Pink, 8 to 12 oz fruit, also resistant to verticillium wilt
- Mountain Rouge: Pink, large fruit (12 to 14 oz), an All-America Selection winner
- Abigail: Heirloom-type with pink fruit, 10 to 16 oz, minimal cracking
- Strawberry Fields: Red, heirloom-type, 7 to 10 oz, tolerant of blossom end rot and shoulder cracking
Resistance doesn’t mean immunity. In severe outbreaks with heavy spore pressure, even resistant varieties can show some symptoms. But they buy you time and often produce a viable harvest in conditions that would destroy susceptible heirloom varieties entirely. Pairing resistant cultivars with good airflow (proper spacing and staking) and preventive copper sprays during wet weather gives you the strongest defense a home gardener can build.