The latest findings from Fight Against Blight (FAB) highlight the importance of choosing potato varieties with built-in resistance.
FAB is the industry-sponsored campaign that tracks the emergence and spread of the Phytophthora infestans (late blight pathogen) clones.
2025 low pressure
Dr David Cooke of the James Hutton Institute leads the monitoring programme. He notes that the 2025 potato growing season began quietly for the FAB scouts – a nationwide network of agronomists, growers and industry representatives who submit field samples from suspected outbreaks.
“The exceptionally dry weather in April and May suppressed sources of primary infection. Remarkably, some potato-growing areas in the east of England didn’t see conditions suitable for blight until mid-July, and by then, the early sources of infection had already been held in check,” he explains.
The first outbreak was in Ceredigion in mid-May, followed in mid- to late-June with outbreaks in Cornwall, Pembroke, Perth and Kinross, and Aberdeenshire.
Across the UK, 81 outbreaks were sampled by JHI scientists in 2025 and more than half of these came from volunteer potato plants – not in commercial crops – that continued to grow in the warm and wet conditions from September onwards.
EU46 prevalence
One strain of the pathogen, EU46, saw a sharp increase amongst the samples tested, making up 23% of all of the 300 FAB samples. However, it appeared in only seven locations, of which just two were conventional commercial potato crops, while the others were trial sites and organic crops, explains David.
These outbreaks caused by EU46 began in Wales and later spread to the Midlands and Derbyshire, possibly helped by Storm Amy. The fact that EU46 has become established in Wales suggests it may return in 2026, believes David.
“EU46 has a known resistance to oxathiapiprolin, a key fungicide ingredient. All the FAB strains tested in 2025 were found to have the mutation that confers that resistance.
“However, unlike on the continent in 2023 and 2024, management failures were not reported which implies that growers are following industry and Fungicide Resistance Action Committee (FRAC) guidelines correctly when implementing their blight control programmes.
“The absence of EU43, in which resistance to mandipropamid has been widely reported, was good news for the British industry, although the prevalence of the EU41 clone at multiple sites in Scotland will have to be closely monitored given last year’s reports of it having reduced sensitivity to low doses of cyazofamid.”
Best practice
Despite the doses in question being well below field rate, it does flag the potential for population shifts through selection, adds David. “Bearing in mind the effective dose in the crop progressively reduces in the intervals between applications, growers must avoid consecutive applications of any fungicide product, particularly those with only a single active ingredient.”
Other key findings from 2025 were:
- EU36 was again the most common strain, making up more than 40% of samples. It’s known for breaking down resistance in potato varieties and has shown signs of fungicide resistance in parts of Europe. In the past this clone was more prevalent in the East, but has now spread more widely across the UK than in previous years.
- EU37, which is resistant to fluazinam, has almost disappeared, being found on just two sites in 2025, an encouraging sign that careful fungicide management is working.
- EU13, resistant to metalaxyl, caused only two outbreaks, both on volunteer plants.
Although this year’s campaign received far fewer samples of suspected late blight than usual, David warns against complacency. “It remains crucial that growers continue to alternate the actives in their spray programmes to reduce the threat of resistance developing. There’s no place for block spraying.
“Cultivar resistance has become increasingly important to complement fungicides, and care should still be taken to maintain long rotations to reduce the numbers of long-lived sexual oospores in the soil, plus managing other key sources of primary inoculum such as cull piles and potato volunteers.”
Fungicide efficacy
James Lynott, research assistant at JHI, tested several fungicides for their efficacy against current strains and found the following:
- Oxathiapiprolin: EU36 and EU41 remain sensitive; EU46 is resistant
- Mandipropamid: all UK isolates (the cultures grown from samples of late blight) remain sensitive, unlike some resistant strains found in Europe
- Amisulbrom, propamocarb, and fluopicolide: all UK isolates tested remain sensitive
James also expanded DNA-based fungicide resistance testing during the season, reporting issues immediately to give growers real-time guidance to optimise IPM of late blight in line with the new UK Pesticides National Action Plan for 2025.
“Care must be taken in 2026 to ensure that industry continues to follow FRAC guidelines on mixing and alternating active ingredients, in particular considering the loss of multi-site mancozeb products,” says David.
Future actions
He emphasises three key actions for 2026:
- Rotate fungicides and avoid repeated use of the same active ingredient, especially with the loss of mancozeb from the market
- Choose more blight-resistant potato varieties to reduce reliance on chemicals
