Late blight on potatoes

Phytophthora infestans on potatoes is a special case in plant pathology: it is the disease that re-shaped modern agriculture. The same pathogen that caused the Irish Famine remains a serious disease today, requiring active management in any season with cool, wet summers. What makes potato late.

—- title: "Late blight on potatoes" slug: late-blight-on-potatoes hub: problems category: "Disease-by-host" description: "Late blight can destroy a potato planting in days and spread through tubers to next year's crop. Identify the greasy lesions, act immediately, and manage infected tubers to prevent recurrence." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Phytophthora infestans on potatoes is a special case in plant pathology: it is the disease that re-shaped modern agriculture. The same pathogen that caused the Irish Famine remains a serious disease today, requiring active management in any season with cool, wet summers. What makes potato late blight different from late blight on tomatoes is the tuber dimension — infected tubers in the ground or in storage serve as overwintering inoculum for the following season, creating a year-to-year carry-over problem that does not exist with tomatoes in most northern gardens.

I don't grow potatoes at my Long Island property, so this guide draws on Penn State Extension, Cornell, and USDA late blight research.

The pathogen

Per Penn State Extension, Phytophthora infestans is an oomycete that overwinters primarily in infected potato tubers. In the northern US, the pathogen does not survive in soil absent host tissue; infected cull piles, unharvested potatoes, and infected seed pieces are the primary inoculum sources for each season's epidemic.

When infected tubers sprout, the emerging shoot carries the infection and begins producing sporangia. Sporangia are wind-dispersed, travel long distances, and infect new plants through stomates when conditions are favorable.

Per Cornell Cooperative Extension, the USAblight.org network (usablight.org) tracks confirmed late blight reports and provides regional forecasts.

Identification

Foliage symptoms

Per Penn State Extension, late blight on potato produces:

1. Water-soaked, irregular lesions — typically beginning on leaf margins or tips, expanding rapidly into irregular, oily or greasy-appearing brown patches with no defined border
2. White sporulation on leaf undersides — in humid conditions (overnight, in fog, after rain), the undersides of lesions develop white to pale gray cottony sporulation
3. Stem lesions — dark, water-soaked to brown lesions on stems that can girdle the plant; stem infection distinguishes late blight from drought and other foliar problems
4. Rapid progression — in optimal conditions (60–70°F, 15–21°C, with prolonged leaf wetness), the disease can collapse an entire planting within 3–7 days

Tuber symptoms

Per Cornell Cooperative Extension, late blight on potato tubers produces:

• Brown to purplish-brown, slightly sunken lesions on the tuber surface
• When cut open, infected tubers show a granular, copper-to-reddish-brown rot that extends from the skin surface inward; the rot is dry and firm initially but often transitions to a soft, foul-smelling wet rot when secondary bacteria enter
• Infected tubers should never be stored — they will rot and spread the pathogen to adjacent tubers in storage

Distinguishing from early blight and other potato diseases

Per Penn State Extension, early blight lesions are smaller, have defined target-spot rings, and progress over weeks. Common scab produces superficial cork-like patches on tubers without internal rot. Fusarium dry rot on tubers is dry and powdery internally without the granular copper-brown of late blight. Late blight's speed of progression and the white sporulation on leaf undersides are the most reliable distinguishing signs.

Disease cycle and tuber carry-over

Per Penn State Extension:

1. Infected seed pieces or cull piles release sporangia from infected sprout tissue in spring
2. Sporangia are wind-dispersed to potato foliage; infection occurs when temperatures are 60–70°F (15–21°C) and leaves are wet for 10+ hours
3. Infected foliage produces massive secondary sporangia loads within 4–5 days
4. Sporangia from leaves are washed into soil by rain or irrigation, infecting tubers developing in the root zone
5. Infected tubers go to storage and serve as the following season's primary inoculum

This tuber-to-tuber cycle means late blight can reappear in the same garden each year from infected leftover tubers, even without external inoculum introduction.

Management

Preventive fungicides

Per Penn State Extension, preventive fungicide applications must be established before the disease arrives. Begin when the regional forecast (see USAblight.org) indicates high-risk conditions, or when neighboring reports of late blight are confirmed. Apply every 5–7 days during cool, wet periods:

• Chlorothalonil — effective protectant for foliage
• Copper-based fungicides — organic option; apply every 5–7 days during wet periods
• Mancozeb — effective protectant; observe pre-harvest intervals
• Cymoxanil combinations — some curative activity within 24–48 hours of infection
• Phosphonates — systemic oomycide

Per Cornell Cooperative Extension, during high-risk periods (cool, wet weather), shorten spray intervals to every 5 days.

Use certified seed potatoes

Per Penn State Extension, certified disease-free seed potatoes eliminate the primary mechanism by which infected tubers introduce the disease to a new planting. Never plant saved tubers from a field or garden where late blight appeared.

Hilling

Per Cornell Cooperative Extension, hilling soil around potato stems creates a physical barrier between infected foliage and developing tubers. Per Penn State Extension, a minimum of 3 inches of soil over developing tubers reduces tuber infection when sporangia are washed down from infected foliage.

Vine killing before harvest

Per Penn State Extension, if late blight appears on foliage, killing the vines mechanically or with a desiccant before harvest stops sporangia production and dramatically reduces tuber infection. Allow 2 weeks after vine death before harvesting to allow tuber skins to mature and any surface-deposited sporangia to lose viability.

Harvest and storage management

Per Cornell Cooperative Extension, inspect tubers at harvest. Cull and discard any with skin lesions rather than including them in storage. Infected tubers in storage rot rapidly and spread infection to adjacent healthy tubers. Store only firm, clean tubers in a cool (38–40°F, 3–4°C), well-ventilated location.

Remove infected plants

Per Penn State Extension, a plant showing late blight symptoms should be removed promptly, bagged without shaking, and disposed of in the trash. Leaving infected plants in place converts them into regional spore factories for the next 1–3 weeks.

Common problems table

Frequently Asked Questions

Can I save potatoes from a plant with late blight?

Per Penn State Extension, tubers from a plant with foliar late blight may still be usable if harvested promptly after vine killing, inspected carefully, and consumed quickly rather than stored. Do not save any of these tubers as seed for next year.

Does late blight persist in my garden soil from year to year?

Per Cornell Cooperative Extension, in the northern US, P. infestans does not persist in soil absent a host. The disease returns each year via infected seed tubers, cull piles, or wind-blown sporangia from distant sources. Cleaning up all crop debris and cull tubers breaks the local cycle.

Is late blight on potatoes the same as late blight on tomatoes?

Per Penn State Extension, yes — the same species, Phytophthora infestans, infects both crops. Some strains (genotypes) may be more aggressive on one host than the other, but the biology and management are essentially the same. See Late blight on tomatoes for the tomato-specific discussion.

Should I report late blight to anyone?

Per Cornell Cooperative Extension, reporting late blight to the USAblight.org network (managed by NC State and Cornell) is genuinely useful — the mapping helps commercial growers and home gardeners in your region prepare. Send a photo or plant sample to your local Extension office for confirmation before reporting.

—-

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 — Late Blight of Tomato and Potato
2. Cornell Cooperative Extension — Potato Late Blight Management
3. USAblight.org — Late Blight Tracking Network