By Lukie Pieterse, Editor and Publisher, Potato News Today

EuroBlight’s 2024 season data is still ricocheting through agronomy circles for one reason: the population of Phytophthora infestans keeps shifting under our feet—and stewardship decisions are already changing as a result.

Introduction

Late blight remains the most economically destructive disease in global potato production—and in Europe, 2024 proved once again that Phytophthora infestans is an adversary in constant evolution.

Fresh data from EuroBlight’s continent-wide monitoring network reveal major shifts in the genetic make-up of blight populations, with aggressive clones like EU_36_A2 gaining dominance, resistant strains such as EU_46_A1 expanding their footprint, and a newly named threat, EU_47_A1, showing the ability to break through some cultivar resistances.

While the decline of the much-feared EU_43_A1 offers a rare success story in fungicide stewardship, the overall picture is one of a disease that continues to adapt to both chemical and genetic control measures.

For farmers, agronomists, and breeders, these findings are more than just research updates—they are a strategic roadmap for what must change in blight control programmes heading into 2025.

Key takeaways at a glance

• EU_36_A2 surged to become the most frequently sampled genotype in Europe in 2024, accounting for roughly half of submissions—and showing signs it can erode some cultivar resistance under selection pressure.
• EU_43_A1—the headline-grabbing clone linked with reduced sensitivity to both CAA fungicides (e.g., mandipropamid) and OSBPI (oxathiapiprolin)—declined in frequency overall in 2024, but expanded its geographic range to more countries.
• EU_46_A1 ticked up slightly and spread to more countries; field reports indicate resistance to oxathiapiprolin but relative sensitivity to mandipropamid.
• EU_37_A2 (fluazinam-resistant) fell below 1%—a practical win tied to changes in how growers deploy chemistries.
• A new clone, EU_47_A1, emerged with novel virulence on certain blight-resistant cultivars, underlining that host resistance must be diversified and actively managed.

What the EuroBlight 2024 dataset actually shows

EuroBlight’s final release for the 2024 season (published 28 May 2025) aggregates nearly 3,000 genotyped samples from 27 countries—its largest annual dataset to date. Pressure was higher than average in many regions, and sampling intensity rose amid resistance concerns. The headline shifts:

• EU_36_A2: rose from ~37% (2023) to ~53% (2024); now found in 27 countries. Researchers note large lesions, heavy sporulation, and worrying evidence that mutations within this lineage can overcome some cultivar resistance and, at low frequency, exhibit oxathiapiprolin resistance. Translation for the field: don’t assume “old” clones are benign—selection pressure reshapes them.
• EU_43_A1: dropped from ~23% (2023) to ~9% (2024) after many growers intensified programmes and strictly mixed/alternated modes of action; yet it spread to additional countries and increased in parts of France. The UK still hadn’t sampled it in crops by season’s end, even as Ireland confirmed it in 2023 research plots.
• EU_46_A1: edged up from ~3.5% to ~4% overall, with the highest frequency reported in Germany; importantly, labs report oxathiapiprolin resistance but mandipropamid sensitivity in most isolates tested to date.
• EU_37_A2: continued a steady multi-year decline to <1%, paralleling reduced reliance on fluazinam and fewer back-to-back applications—a stewardship success story.
• EU_47_A1: named in 2024 after repeated finds since 2021; concentrated in the Netherlands but also in Belgium and Switzerland. It appears to have spread because of virulence traits on specific resistance groups—an early warning that host resistance can be outflanked.

Why the dip in EU_43_A1 matters: EuroBlight explicitly links its fall to more intensive programmes and sharper mixing/alternation of actives in 2024. This is a rare, data-backed example of coordinated on-farm stewardship turning the dial in-season—useful proof that resistance management is not just theory.

The control brief for 2025: practical, not theoretical

The job now is to hard-wire what worked in 2024 and close gaps exposed by EU_36 and EU_47.

1. Lock in fungicide stewardship that tracks the genotypes

• Follow FRAC working-group guidance for CAA and OSBPI groups: mix with effective partners, alternate modes of action, avoid consecutive solo uses, and cap the number of applications per season. These are not “nice-to-haves”—they are now population-level levers.
• UK FRAG (April 2024; updated Feb 2025) reiterates limits, cautions on known insensitivities (phenylamides; fluazinam in some regions), and documents UK detections of OSBPI-resistant isolates and late-season 46_A1 finds in 2024. Use these national guidelines as your operational baseline if farming in Britain or aligning with GB practice.
• EuroBlight’s multi-year trials and workshop papers show oxathiapiprolin is highly vulnerable if misused; alternating rather than “block” applications reduces resistance risk. If an OSBPI is used, anchor it in a robust alternation strategy.
• Peer-reviewed work continues to flag resistance risk for both oxathiapiprolin and mandipropamid; treat these as finite assets.

2. Use decision support—don’t fly blind

• The Hutton Criteria remains the benchmark for forecasting infection periods (two consecutive days ≥10 °C and ≥6 h RH ≥ 90%). Free tools such as BlightSpy and commercial services like BlightCast operationalize this for timely sprays and interval tightening.
• In a season with above-average pressure—as EuroBlight reported for 2024—interval discipline is not optional. Where conditions allow, shorten intervals, maintain dose integrity, and protect the canopy before risk spikes, not after.

3. Cut inoculum—local sources are driving outbreaks

• 2024 metadata show primary inoculum is largely local—volunteers, dumps, infected seed, and oospore reservoirs in fields. Tighten seed health, destroy cull piles, sweep regrowth in breaks, and consider longer rotations and field hygiene to starve the pathogen.

4. Re-think host resistance deployment

• The emergence of EU_47_A1 on specific resistance groups is a blunt reminder: do not lean on single resistance sources. Deploy a diverse portfolio of cultivars across acreage, rotate resistance profiles across seasons, and expect virulence shifts where selection pressure is high.

5. Country-level surveillance still matters

• Programmes such as the UK Fight Against Blight (FAB) continue to secure funding and deliver within-season genotype intelligence. Plug your agronomy into those alerts; they’re often the first to flag local appearances of problem clones.

What this means for growers, processors and breeders

• Growers: Stewardship works—but only when it’s systematic. If you used tighter alternation/mixing in 2024, keep doing it. If not, start now. Align spray plans with forecast tools and avoid back-to-back single-site uses.
• Processors: Expect variability in foliar protection efficacy if programmes slip and if EU_36 continues to adapt. Contract guidance should stress programme integrity and seed/cull hygiene.
• Breeders: The EU_47 signal is unambiguous—stack and diversify resistance, and phenotype against the current clonal spectrum, not last decade’s.

Conclusion

The EuroBlight 2024 findings confirm what many agronomists already suspected in the field: Phytophthora infestans is not standing still, and neither can we. The rise of EU_36_A2, the spread of EU_46_A1, and the emergence of EU_47_A1 underscore that blight management is now a moving target, shaped by the interplay of fungicide stewardship, host resistance, and local inoculum control.

The drop in EU_43_A1 is proof that coordinated, disciplined strategies work—yet the pathogen’s adaptability ensures the next challenge is already in motion. For growers, the message is clear: integrate resistance management into every decision, from seed sourcing to spray scheduling, and stay plugged into surveillance networks.

For processors and breeders, the call is to align with these realities, ensuring cultivar development and supply contracts reinforce rather than undermine resistance stewardship. In the battle against late blight, complacency is a luxury we can’t afford; vigilance, adaptability, and science-led action will remain our most effective defences.

References

1. EuroBlight. EuroBlight final data release, 2024 (28 May 2025). PDF: https://agro.au.dk/fileadmin/euroblight/Monitoring_2013_project/EuroBlight_final_data_release_V3.pdf; Aarhus University Agroecology
2. EuroBlight news post summarising the 2024 results, including notes on EU_45, EU_46, EU_47: https://agro.au.dk/forskning/internationale-platforme/euroblight/currently/news/nyhed/artikel/euroblight-final-data-release-2024; Aarhus University Agroecology
3. Teagasc. Judicious use of chemistries now critical to control late blight (EU_43_A1 detection in Ireland; resistance context): https://www.teagasc.ie/news–events/news/chem-blight-control/
4. FRAC Working Group pages – OSBPI (oxathiapiprolin/fluoxapiprolin) and CAA (mandipropamid, dimethomorph, etc.): https://www.frac.info/frac-teams/working-groups/osbpi-fungicides
5. FRAG-UK. Fungicide resistance management in potato late blight (April 2024; updated Feb 2025 summary): https://projectblue.blob.core.windows.net/media/Default/Imported%20Publication%20Docs/AHDB%20Cereals%20%26%20Oilseeds/Disease/FRAG/FRAG-UK%20potato%20late%20blight%20guidelines%20%28April%202024%29.pdf and https://media.potatopro.com/frag-uk-potato-late-blight%20-guidelines-feb-2025.pdf; Project Blue
6. EuroBlight Workshop materials: Oxathiapiprolin resistance management trials (May 2024): https://agro.au.dk/fileadmin/euroblight/Workshops/Netherlands/Presentations/Session_1/8_EuroBlight_OXTP_management.pdf; Aarhus University Agroecology
7. Hutton Criteria background (AHDB/GB warning system): https://potatoes.ahdb.org.uk/development-and-implementation-of-a-new-national-warning-system-for-potato-late-blight-in-great-britain-hutton-criteria and Hutton’s BlightSpy tool: https://blightspy.huttonltd.com/

Cover image: Credit Teagasc