Blight on Potatoes: Early Blight, Late Blight, and How to Tell Them Apart

Two diseases named "blight" affect potatoes, and they are not the same organism, do not cause the same damage pattern, and do not require the same management. Early blight is a fungal disease that is common and manageable. Late blight is a different organism -- an oomycete (water mold) -- that is.

—- title: "Blight on Potatoes: Early Blight, Late Blight, and How to Tell Them Apart" slug: blight-on-potatoes hub: problems category: "Problem-by-host" description: "Early blight and late blight on potatoes look similar but are caused by different organisms and require different management. Here's how to identify and manage both." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 9 scientific: "Solanum tuberosum" —-

Two diseases named "blight" affect potatoes, and they are not the same organism, do not cause the same damage pattern, and do not require the same management. Early blight is a fungal disease that is common and manageable. Late blight is a different organism — an oomycete (water mold) — that is the pathogen responsible for the Irish Potato Famine of 1845–52 and can destroy an entire potato planting in 5–10 days under favorable conditions.

Treating late blight as if it were early blight is a serious mistake. The fungicides effective against one are not necessarily effective against the other.

Early Blight

Pathogen

Early blight is caused by Alternaria solani, a fungus that is widespread in soils where potatoes and tomatoes have been grown. Per Penn State Extension, it overwinters in infected plant debris and soil; spores are dispersed by wind and rain splash.

Symptoms

Per Cornell Cooperative Extension:

• Dark brown spots on lower, older leaves first
• Spots are 1/4 to 1/2 inch, angular to circular
• Target pattern (concentric rings): Spots develop characteristic rings-within-rings that look like a target or bulls-eye — this is the most reliable visual identifier for early blight
• Yellow halo around the dark spot
• In severe cases: defoliation progressing upward through the canopy
• Tubers sometimes develop slightly sunken, dark lesions on the skin, but tuber infection is generally minor

Conditions

Per NC State Extension, A. solani is favored by:

• Warm temperatures (75–86°F optimal)
• Alternating wet and dry conditions
• Older plant tissue, especially leaves below the flowering zone
• Plant stress (drought, low nitrogen)

Early blight is a "mature plant" disease — it rarely causes significant damage on young, vigorously growing plants. It becomes most problematic after flowering when plants begin to senesce.

Late Blight

Pathogen

Late blight is caused by Phytophthora infestans, an oomycete (water mold; not a true fungus). Per Cornell Cooperative Extension, this distinction is critical: oomycetes have different cell wall biochemistry than true fungi, which means many fungicides effective against fungi have poor activity against P. infestans.

P. infestans spreads rapidly by sporangia (spore structures) that are carried by wind and germinate in films of water on leaf surfaces. A single infected lesion can produce 100,000+ sporangia overnight, which can infect thousands of new plants the following day.

Symptoms

Per Penn State Extension:

• Pale green to grayish water-soaked lesions on leaves and stems — not dark brown target patterns
• Lesions expand rapidly, often with an irregular, oily-looking margin
• White cottony sporulation on the underside of leaves in humid conditions — this is the most diagnostic feature; if you see white fuzzy growth on the underside of expanding lesions, you have late blight
• Entire leaves collapse and turn brown within days
• Stem lesions (dark brown to black, often girdling the stem)
• Tuber infection: Firm to soft, reddish-brown discoloration that extends into the tuber flesh; affected tubers smell foul and rot

The rapidity of spread separates late blight from early blight. Early blight progresses over weeks; late blight can collapse a planting in 5–10 days under cool (60–70°F), wet conditions.

Conditions

Per Clemson HGIC, P. infestans is favored by:

• Cool temperatures (60–70°F optimal; activity stops above 77°F)
• Extended leaf wetness (rain, fog, heavy dew)
• High humidity (above 90% for 10+ hours)
• Presence of infected plant material (often spread by infected seed potato pieces or from neighboring tomato plantings)

Late blight is most severe in cool, wet springs and early summers. In hot, dry summers it rarely develops. In the Northeast, outbreak risk is highest in June–July.

Differential Diagnosis

Management: Early Blight

Per Penn State Extension:

• Copper fungicides: Applied at 7–10 day intervals; most effective preventively
• Chlorothalonil (Daconil): Effective conventional option at 7-day intervals (not organic-approved)
• Cultural controls: Adequate nitrogen fertility reduces senescence-induced susceptibility; remove infected lower leaves; mulch to reduce soil splash
• Crop rotation: 2–3 year rotation out of solanaceous crops (potatoes, tomatoes, peppers, eggplant)
• Resistant varieties: Some potato varieties show reduced early blight susceptibility; check variety descriptions

Management: Late Blight

Per Cornell Cooperative Extension, late blight requires a different response:

Fungicides with oomycide activity (not copper alone):

• Mefenoxam (Ridomil Gold) — highly effective; phenylamide class; resistance possible, rotate with other classes
• Cymoxanil (Curzate) — systemic; use in combination with protectant
• Mandipropamid (Revus) — effective protective and systemic activity
• Chlorothalonil alone — has some protectant activity against late blight but insufficient for active outbreaks

Per Penn State Extension, for home garden use:

• When late blight is confirmed in the region (check local Extension disease alerts), apply a protective oomycide-class fungicide before symptoms appear
• Once symptoms are present: apply mefenoxam or cymoxanil-containing product immediately; remove and destroy infected plant parts (do not compost)
• If tubers are already infected: harvest immediately even if immature; infected tubers continue to rot in storage

Destroy plant material immediately when late blight is confirmed. Bury or bag infected plants; do not compost.

Certified Seed Potatoes

Per Penn State Extension, using certified disease-free seed potatoes is one of the most important preventive practices. P. infestans can overwinter in infected tubers; planting infected seed pieces introduces the pathogen into a previously clean bed. Grocery store potatoes are not a reliable seed source for this reason — they are not inspected for disease.

Common Problems

Frequently Asked Questions

How do I know if late blight is in my area?

Per Cornell Cooperative Extension, the US Blight information network (USABlight.org) and your state's Cooperative Extension publish late blight alerts when the disease is confirmed in a region. Check these sources in June and July before and during cool, wet periods. Extension hotlines often have recorded regional alerts during active disease seasons.

Can late blight spread from my potatoes to my tomatoes?

Per Penn State Extension, yes. P. infestans infects both Solanum tuberosum (potato) and Solanum lycopersicum (tomato) and moves freely between them. An infected tomato plant is a source of inoculum for potatoes and vice versa. Manage both crops together; do not grow them adjacent in years when late blight pressure is high. See also: Blight on Tomatoes: Early vs. Late.

If I find late blight, should I pull all my plants?

Per Cornell Cooperative Extension, if late blight is present on a small portion of the planting and tubers are not yet infected, aggressive fungicide application and removal of infected plants may save the remaining crop. If late blight is widespread and tubers show any sign of infection, harvesting immediately and destroying all foliage is the recommended response. Leaving infected plants in the ground allows tubers to continue rotting and spreads the pathogen to neighboring solanums.

Can I save infected tubers?

Per Penn State Extension, tubers with visible late blight lesions (reddish-brown discoloration in the flesh) cannot be saved. They will continue to rot and will infect healthy tubers in storage. Sort all harvested tubers carefully; discard any with soft spots, discoloration, or foul odor.

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Recommended gear: Best tomato varieties for the home garden — determinate vs indeterminate — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Sources

1. Penn State Extension — Early Blight of Potato and Tomato
2. Penn State Extension — Late Blight
3. Cornell Cooperative Extension — Potato Disease Management
4. NC State Extension — Vegetable Gardening Handbook
5. Clemson HGIC — Potato Diseases