Late blight can spread quickly, leading to huge yield losses in potato crops.
Late Blight in potatoes
What is Potato Late Blight?
Phytophthora infestans, more commonly known as Potato Late Blight (PLB) is one of the oldest recorded plant diseases in human history. It is generally spread by bridging between seasons on tubers as volunteers, unharvested crop, cull piles or home-saved/commercial seed. It may also be soilborne in sexually-derived resting bodies called oospores, though in practice, the pathogen population faced at the outset of one season is composed of the strains that ended the previous one.
During early infection, PLB is made visible by brown, moist lesions on the green leaf tissue, often outlined with a white mildew layer on the underside of the leaf, characteristic of Oomycetous diseases.
This white layer is predominantly made up of sporangia; asexual spores that are fundamental to the primary stage of infection and indirectly to secondary infections.
At this point, PLB can spread systemically within the plant and via the production of prodigious numbers of spores. These are carried by air currents or splashed onto neighbouring plants, germinating to infect tissue directly or via the release of mobile zoospores that spread over the leaf surface, depending on temperature. Indeed it is the production of motile zoospores during the lower temperature period in the latter part season which may be washed into the soil and can enter tubers via lenticels or weak spots such as wounds.
Here, PLB sporangia can overwinter and survive until the next season, or cause significant problems in storage.
Both Zorvec™ active and Cymoxanil (Option®) have curative activity, providing strong antisporulant activity as a result. Additionally, by targeting the pathogen early in the epidemic, it will prevent the build up of spore numbers and more effectively, prevent PLB sporangiaphores being produced at the leaf surface.
The conditions favouring P. infestans are those used in the Decision Support System (DSS) behind the ‘Hutton Criteria’. A Hutton Criteria occurs when two consecutive days have both a minimum temperature of 10°C and at least six hours of relative humidity (RH) at or above 90%. This is calculated by looking at the relative humidity at or greater than 90% and the minimum temperature, on each of two days.
- If RH > 90% occurs for 5 hours on either day, or the minimum temperature is 9°C on either day then a near miss is illustrated on the map, as the full criteria isn't met for Hutton.
Source: https://blightspy.huttonltd.com/#/
It is worth noting that the weather data is collected from standard meteorological sites across the potato growing regions of the UK and while an invaluable guide, this will not reflect the exact conditions which arise within the canopy of a specific area of a field crop.
What causes Late Blight in potatoes?
Late blight infection can originate from seed, non-marketable tubers left in the field, or volunteer plants. Prioritising cultural control by managing potential inoculum sources is crucial for effective late blight management.
During the growing season, the risk of infection is highest when temperatures consistently range from 8-15°C paired with precipitation, maintaining high humidity levels above 80% relative humidity (RH).
When spores land on a leaf, they will either germinate directly or produce motile zoospores (depending on temperature). These zoospores spread along moist leaves before anchoring and producing a germ tube, entering through lesions or stomata to continue the infection cycle. Such conditions are typically seen from May to September.
In the early stages of infection, late blight is characterised by brown, moist lesions on green leaf tissue, often accompanied by a white mildew-like margin layer common to Oomycete diseases. This layer consists largely of sporangiaphores and their accompanying sporangia; asexual spores essential to both primary and secondary infection stages.
Late blight remains active throughout the growing season, making plants vulnerable to infection from emergence. Maintaining good crop hygiene and following a robust blight spray programme is vital for protecting this economically important crop.
How to spot Late Blight?
- The disease develops from infected seed or in some instances, direct germination from the resting oospores in the soil (more common in cooler regions).
- The fungal spores and developing lesions are carried from the tuber and will show initially as stem lesions, producing spores at the base of the developing canopy, which can also spread to nearby plants.
This is why early fungicide applications in a blight programme should contain a curative active ingredient to ensure no hard-to-reach lesions survive deep in the canopy to act as a source of infection throughout the season. Examples include cymoxanil (Option), propamocarb, or potassium phosphates.
Early season infection
Late season infection
Close up lesions
Why is Late Blight a problem?
- The pathogen can cycle very quickly once infection sets in, and the symptoms can appear in as few as 4-5 days under the right conditions. With PLB sporangia being capable of overwintering, if infected tubers are stored, the disease can further develop there, leading to store breakdown and significant losses.
- The main method of fungal dispersal is through airborne sporangia from already infected crops, volunteer potatoes and cull piles.
- In recent years, PLB has become more aggressive and harder to control through the evolution of strains which produce ever greater spore numbers and cycle faster, as well as evolving mutations which confer resistance to some fungicide modes of action. For this reason, alternating modes of action, maintaining spray intervals, and mixing active ingredients at each application are now strongly advised.
See Corteva's Best Practice Advice for more information.
Late Blight life cycle
When both mating types (fungal genders) are in proximity, the sexual life cycle begins, producing oospores. This stage will only progress if the RH is 80% and temperatures exceed 9°C.
What is most important about these oospores is that they can survive for multiple growing seasons, making it difficult to combat this inoculum once it has formed.
All the more reason to target PLB at its primary stage to prevent development into the sexual form. Additionally, it is important to minimise leaf litter and tuber residues on crops, as these provide reproduction sites for both the sexual and asexual life cycles of PLB to overwinter.
This will reduce the inoculum for the following year and lower the resistance potential of the population.
Corteva solutions for controlling potato Late Blight
Zorvec™ active
Zorvec™ active is a member of a novel class of fungicides that control diseases caused by oomycetes, acting at a unique target site. It also works within plants to protect new growth.
Zorvec Entecta™ Fungicide
Zorvec Entecta™ is proven to be the most effective control for Late Blight, ensuring a clean start to your blight programme—even under the most challenging conditions.
Option® Fungicide
Option® is a fungicide for potato disease protection. It contains cymoxanil, which is locally systemic and offers both preventative and curative control.
Latest news
Zorvec™ active application advice for 2025
Zorvec remains a highly effective product for the control of Late Blight. Corteva strongly advises following the 2025 application guidelines when recommending and applying Zorvec products.
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