Late blight is a fast-spreading, destructive disease of tomatoes and potatoes caused by the water mold Phytophthora infestans, the same pathogen behind the Irish potato famine. It causes large, greasy-looking, gray-green to dark brown patches on leaves, often with a pale green edge and white fuzzy growth on the undersides in humid weather; dark brown to black lesions on stems; firm, greasy brown blotches on green tomatoes; and a reddish-brown rot in potato tubers. In cool, wet weather it can kill entire plants within a week and spread for miles on the wind. There is no cure for infected plants: remove and bag them immediately, do not compost them, protect healthy plants with a fungicide labeled for late blight, plant resistant tomato varieties, and use certified disease-free seed potatoes.
Late blight warning signs
| Plant part | What you see |
|---|---|
| Leaves | Large, irregular, pale green to brown-black patches that look water-soaked or greasy; often start at leaf tips and edges |
| Leaf undersides | White, fuzzy growth around the edges of lesions in humid conditions or early morning |
| Stems and petioles | Dark brown to black, greasy lesions that can girdle stems |
| Tomato fruit | Firm, greasy, brown to olive-colored blotches, usually on the upper half; fruit stays firm at first |
| Potato tubers | Shallow, reddish-brown, dry, granular rot under the skin, often followed by soft rots |
| Whole plant | Rapid collapse; a distinctive rotting smell in heavily infected patches |
What causes late blight?
Phytophthora infestans is not a true fungus but an oomycete, or water mold, more closely related to algae. The Wikipedia article on Phytophthora infestans explains that it produces vast numbers of airborne spores (sporangia) that can travel many miles on storm systems and infect new plants within hours when leaves are wet. It cannot survive on dead plant debris for long; in most northern areas it overwinters only in living tissue, such as infected potato tubers left in the ground or in cull piles, or arrives on infected transplants. The Penn State Extension guide to tomato and potato late blight in the home garden stresses that home gardeners play a key role in limiting regional outbreaks by recognizing and destroying infected plants quickly.
Why is late blight so dangerous?
Three features make late blight different from other tomato diseases. First, speed: under cool (about 60 to 70°F), wet or humid conditions, the pathogen completes a disease cycle in as little as five days, so a few spots can become a dead plant in a week. Second, spread: spores travel long distances on wind and rain, so one infected garden can seed an entire region, including commercial farms. Third, there is no cure: once leaves, stems and fruit are infected, they cannot be saved. That is why prompt action and community awareness matter so much.
How do you tell late blight from early blight and Septoria?
| Disease | Leaf symptoms | Speed |
|---|---|---|
| Late blight | Large, greasy, gray-green to brown patches; white fuzz beneath; no target rings | Very fast; plants die within days |
| Early blight | Brown spots with concentric rings and yellow halos on lower leaves | Gradual, moves up the plant |
| Septoria leaf spot | Many small round spots with gray centers and dark borders | Moderate; lower leaves first |
See our guides to early blight and Septoria leaf spot. If you suspect late blight, contact your local Cooperative Extension office or plant diagnostic lab for confirmation; many offer free or low-cost testing and track outbreaks.
What should you do if you find late blight?
- Act immediately. On a dry day, pull infected plants, including roots and fruit, and seal them in plastic bags. Leave sealed bags in the sun for a few days to kill the pathogen, then put them in the trash.
- Do not compost infected plants or fruit, and do not leave them on the ground or in a cull pile.
- Check nearby plants daily, including potatoes, tomatoes and volunteer plants; remove any with symptoms.
- Protect healthy plants with a fungicide labeled for late blight on tomatoes or potatoes, applied before infection and repeated every 5 to 7 days during cool, wet weather. Copper and chlorothalonil products are protective options for home gardens; they do not cure infected tissue. See copper fungicide.
- Report it to your local Extension office, especially early in the season, so others can be warned.
- Harvest healthy fruit from unaffected plants promptly; green tomatoes without lesions can ripen indoors, but check them daily and discard any that develop spots.
How do you prevent late blight?
- Plant resistant tomatoes. Varieties such as Mountain Magic, Defiant PHR, Iron Lady, Jasper, Mountain Merit, Plum Regal, Matt’s Wild Cherry and Lizzano have good late blight resistance; look for “LB” in catalog listings.
- Use certified seed potatoes, never grocery store potatoes or saved tubers from a garden that had late blight.
- Destroy volunteer potatoes and tomatoes that sprout from last year’s crop, which can carry the pathogen.
- Buy healthy transplants from reputable sources and inspect them before planting.
- Keep leaves dry with drip irrigation and morning watering; use the garden watering calculator.
- Space and prune for air flow; use the plant spacing calculator.
- Hill potatoes well so tubers are covered by several inches of soil, which protects them from spores washing down from infected leaves.
- Watch disease alerts: in North America, outbreaks are often reported by Extension services; when late blight is reported in your region, start protective sprays on susceptible plants.
Community and label note: Late blight spreads between gardens and farms on the wind, so destroying infected plants promptly protects your neighbors and local growers. Report suspected cases to your Cooperative Extension office. Use only fungicides labeled for late blight on your crop, follow the label and pre-harvest interval, and wear the protective equipment the label requires.
Late blight on potatoes
On potatoes, late blight causes the same greasy leaf and stem lesions and can kill the foliage of a whole planting in a week or two. Spores washed into the soil infect tubers, causing a reddish-brown, granular rot just under the skin that often turns into a soft, smelly rot in storage. If foliage becomes infected, cut and remove the vines and wait two to three weeks before digging, so the skins set and spores in the soil die off. Harvest in dry weather, discard any tubers with lesions, and store only sound potatoes in a cool, dark place. Never leave infected tubers in the ground or in a compost or cull pile, because they can carry the pathogen to next season.
Can you eat tomatoes or potatoes with late blight?
The UMass Extension fact sheet on late blight on tomatoes and potatoes advises removing and bagging infected plants promptly and explains how the disease spreads between gardens.
Late blight does not make healthy-looking produce unsafe, and it does not infect people. Fruit and tubers with lesions should be discarded, though, because they rot quickly and taste bad. Unblemished tomatoes from infected plants can be eaten, but watch them closely, since symptoms may still develop. Do not save seeds from infected fruit. Potatoes with late blight lesions are often invaded by secondary rots and should be thrown away.
When does late blight occur?
Late blight can appear any time conditions are cool and wet, but in most of North America it shows up from midsummer to fall, often after a stretch of rainy, foggy or cool weather with nights in the 50s and days in the 60s and 70s°F. Hot, dry weather above about 85°F slows or stops it. Because spores blow in from elsewhere, its appearance varies widely from year to year: some summers it is absent, in others it sweeps through entire regions.
Can late blight survive the winter in my garden?
In most northern areas, the late blight pathogen does not survive freezing winters in soil or dead plant debris; it needs living tissue such as infected potato tubers. That is why removing volunteer potatoes and never planting saved or grocery-store potatoes are so important. Tomato seeds and debris are not considered significant sources. In mild climates and in places where both mating types are present, the pathogen can produce resistant spores (oospores) that survive in soil, making rotation and resistant varieties even more important.
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Late blight FAQ
Can tomato plants recover from late blight?
No. Infected plants should be removed and destroyed to protect others.
What does late blight look like on tomatoes?
Large greasy gray-green to brown leaf patches, black stem lesions and firm brown blotches on fruit.
Is late blight in the soil?
In most northern areas it does not survive in soil over winter; it survives in infected potato tubers.
Can I plant tomatoes next year after late blight?
Yes, after removing all infected material and volunteer potatoes; choose resistant varieties.
What spray stops late blight?
Protective fungicides labeled for late blight, such as copper or chlorothalonil, applied before infection; they do not cure infected plants.
Is late blight the same as the Irish potato famine disease?
Yes, the same pathogen, Phytophthora infestans, caused the famine in the 1840s.
How do you choose late blight resistant tomatoes?
Plant breeders at universities such as Cornell and North Carolina State have developed tomatoes carrying resistance genes (named Ph-2 and Ph-3) that slow or stop late blight. In seed catalogs, look for “LB” or “late blight resistant” in the description. Popular choices include Mountain Magic (a red cocktail tomato), Defiant PHR (medium slicer), Iron Lady (resistant to late blight, early blight and Septoria), Mountain Merit and Plum Regal (paste type). Cherry tomatoes such as Matt’s Wild Cherry and Jasper also show good field resistance. Resistance is strong but not absolute, especially against new strains of the pathogen, so combine resistant varieties with sanitation and good spacing.
Should you spray tomatoes preventively for late blight?
In most home gardens, routine spraying is not necessary every year, because late blight does not appear every season. A practical approach is to watch for regional reports from your Extension service or local farm networks. When late blight has been confirmed within your area, or after a long spell of cool, wet weather, begin protective sprays on susceptible tomatoes and potatoes and continue every five to seven days while conditions favor the disease. If you grow resistant varieties, you may not need to spray at all.
Late blight in community gardens
Because spores spread readily between plots, one unattended infected tomato can threaten a whole community garden. If you garden in a shared space, check plants regularly, tell the garden coordinator immediately if you suspect late blight, and help remove infected plants from abandoned plots. Many community gardens now encourage members to grow resistant varieties to reduce the risk for everyone.
Can late blight affect other plants?
The tomato and potato strains of Phytophthora infestans mainly infect those two crops and some related nightshades, such as petunias and hairy nightshade, occasionally. They do not infect peppers, eggplant, cucumbers or squash in any meaningful way. Other Phytophthora species cause blights and root rots on peppers, squash and many ornamentals, but those are different diseases.
Staying alert during cool, wet spells and acting within a day of the first symptoms gives you the best chance of saving the rest of your crop.
Is late blight a reportable disease?
In the United States, home gardeners are not legally required to report late blight, but many Extension services ask for reports so they can warn farmers and gardeners and track which strains are present. In some other countries, potato diseases are monitored more closely. Reporting quickly is a simple way to help protect local growers.
Further reading: Penn State Extension, Tomato and Potato Late Blight in the Home Garden; UMass Extension, Late Blight on Tomatoes and Potatoes; Wikipedia, Phytophthora infestans.
