Diseases

Late blight of potato (Phytophthora infestans)

Late blight is the most economically damaging potato disease worldwide. It is caused by an oomycete, not a true fungus, and it moves fast in cool, wet weather. Management combines clean seed, field hygiene, forecasting, resistant varieties and a fungicide programme that is shrinking under resistance and regulation.

Last updated: 7 November 2026

At a glance
Cause	Phytophthora infestans, an oomycete (water mould)
Worst weather	Cool, humid or wet, with leaf wetness
Global cost	Over $10 billion a year (peer-reviewed reviews)
Europe cost	About EUR 900 million a year in control and losses
Quarantine pest	No

Cause

P. infestans caused the Irish famine of 1845 to 1852, which killed about a million people and drove another million to emigrate. Until about 1980 a single clonal lineage of one mating type dominated worldwide. New strains reached Europe in the 1970s and brought the second mating type, so the pathogen can now reproduce sexually and form oospores that survive in soil (PMC reviews; EuroBlight).

Annual losses worldwide exceed $10 billion in published reviews, and in Europe about EUR 900 million a year in control costs and crop losses. In India, ICAR-CPRI cites losses of 15 to 25 percent of the crop in affected years, valued at INR 5,000 to 8,000 crore (CPRI Annual Report 2020-21; Indian Phytopathological Society estimates).

Symptoms

Leaves: pale green to dark, water-soaked lesions that turn brown, with a white sporulating fringe on the underside in humid conditions.
Stems: brown to black lesions that can girdle the stem.
Tubers: firm, copper-brown to reddish dry rot under the skin, often followed by soft rot from secondary bacteria in store.

How it spreads

Spores (sporangia) travel on wind and rain from infected seed tubers, volunteer plants, cull piles and, now, oospores in soil. Infected seed is the main way the disease starts in a new crop.

Warmer winters are raising oospore survival and extending the growing season, while 2024 showed that a warm, dry spring can lower pressure early and still leave a damaging late season (EuroBlight, PMC reviews).

Where the risk is highest

Worldwide wherever potatoes meet cool wet weather: north-west Europe, the Andes, the East African highlands (Kenya, Ethiopia, Rwanda, Uganda), the Indian hills and plains in foggy winter spells, and eastern China.

EuroBlight's first 2025 season results, from more than 1,200 genotyped samples, put the aggressive EU_36 genotype at 34 percent of European samples (52 percent in 2024), EU_43 at 12 percent, EU_46 at 7 percent and EU_41 at about 5 percent, with about a third of samples in an 'other' group, higher in the north and east, consistent with soil oospore inoculum (EuroBlight, James Hutton Institute).

Management: cultural

Plant certified seed and destroy cull piles and volunteers, which carry the disease between seasons.
Use a decision support system. About 40 percent of Dutch growers use one, against about 8 percent of UK area. Tools include BlightScout, NEGFRY, SimCast, PhytoPRE and DACOM. Combined with resistant varieties, they can cut fungicide use by up to 30 percent.
In India, ICAR-CPRI's JHULSACAST model, built on NEGFRY and SIMCAST ideas, adds daily weather values into a Disease Severity Value and recommends the first spray when the cumulative value passes about 18 to 20 units. CPRI reports 75 to 85 percent accuracy for first appearance (CPRI Annual Report 2022-23).

Management: resistant varieties

Foliage and tuber resistance are separate traits and are rated separately, usually on a 1 to 9 scale. A variety with good tuber resistance can still lose its haulm early.

Resistant European varieties such as Sarpo Mira and Sarpo Axona are used in organic and low-input systems. Kufri Jyoti, released in India in 1968, carries blight resistance derived from Solanum demissum material.

Resistance-gene stacking uses genes from wild relatives, for example Rpi-amr3 and Rpi-amr1 from Solanum americanum, which showed field resistance (GM Crops & Food, March 2025). Using resistant varieties raises average net income by about EUR 900 per hectare through fewer sprays (PMC reviews).

Management: chemical and regulatory notes

Spray programmes in north-west Europe run to 5 to 15 applications a season at EUR 200 to 400 per hectare. Classes in use: contact products (mancozeb, chlorothalonil, fluazinam), systemics (metalaxyl-M, cymoxanil), CAA fungicides (mandipropamid, dimethomorph), OSBPI fungicides (oxathiapiprolin), phosphonates and QoI products such as azoxystrobin.

Resistance is rising. Metalaxyl resistance is widespread, and new resistance mechanisms to CAA and OSBPI fungicides have been found in the EU_43 and EU_46 genotypes (PMC, Springer Potato Research).

Regulatory notes, EU: chlorothalonil was not renewed (Commission Implementing Regulation (EU) 2019/677) and mancozeb was not renewed (Regulation (EU) 2020/2087), so neither is available to EU growers. Check current national authorisations before relying on any product.

Biological options, still at research stage: Trichoderma strains cut severity by more than 90 percent in some trials, and chitosan, Bacillus strains and potassium phosphite are also being studied (Springer Potato Research, Põldmets et al. 2025).

Seed certification

Late blight is not a quarantine disease, but infected seed tubers are the main route by which it re-enters crops, so field inspections and tuber grading for blight are part of seed production.

Pre-basic seed is grown under strict isolation, and tissue-culture starting material removes carry-over. True potato seed is free of tuber-borne disease, including late blight tuber rot (CIP).

Seed certification schemes set maximum tolerances for disease by class, from Pre-Basic to Certified, and regulated bacteria such as ring rot and brown rot are held at zero in every class (EU Directive 2002/56/EC framework; Scottish SPCS and Dutch NAK schemes as summarised in the Potatopedia corpus).