What new potato blight strain means for fungcide strategies in 2025

It was at the tail end of last season that monitoring by Fight Against Blight (FAB) confirmed the EU46 strain had been found in Wales and eastern Scotland, marking the first appearance of this genotype in the UK.

EU46 is already present in Germany, Denmark and the Netherlands and is known to have resistance to oxathiapiprolin (OXTP) chemistry, as well as isolates with reduced sensitivity to mandipropamid and metalaxyl-M.

See also: Potato blight: What UK farmers can learn from Europe

How it reached the UK is unclear, although wind-blown spores are one possible explanation, according to Dr David Cooke, a plant pathologist at the James Hutton Institute.

Changing threats

The discovery is yet another illustration of the constantly changing blight threats that growers face; threats that are increasingly hard to tackle as fungicide options become more limited, due to either resistance shifts or tightening legislation.

“The fact we picked up two cases of EU46 late in the season means we can’t be sure it isn’t elsewhere that we just haven’t sampled, as sampling intensity had dropped dramatically by then,” says David.

“It emphasises the importance of mixing [fungicide] actives and modes of action throughout the programme to help avoid the disasters that have befallen other parts of Europe, where they have relied on blocks of single actives to fight infections and have selected for resistance.”

Although the more varied approach taken by UK growers has helped avoid many of the serious issues on the continent, the arrival of EU46 shows there is no room for complacency, he says.

East Anglian Association of Independent Potato Consultants (AIPC) agronomist Martyn Cox says: “We know blight strains can change and adapt every season, so we simply don’t know what’s going to happen.

He says it is a case of “start every year knowing that whatever you’ve seen before may not happen again”.

Selection pressure

Currently, FAB testing shows that EU36 continues to dominate late blight populations in England, particularly eastern counties. However, in Scotland it is EU6, although 2024 did also see a re-emergence of EU41.

Testing of GB isolates last year showed no presence of DNA markers linked to resistance in 33 EU36 and EU41 samples, but David says there is EU36 resistance to certain actives (OXTP or mandipropamid) elsewhere in Europe, so the risks are very real.

“Solo exposure of any active, even at low doses, may result in a selection of some genotypes with reduced sensitivity, so should be avoided at all times,” says Martyn.

He also warns that EU36 is a very prevalent genotype that is aggressive and can evolve to challenge varietal resistance.  

He cites the example of a cultivar that has been remarkably resilient to UK strains of potato blight for several years.

Last year in one of his Norfolk crops, grown organically, the variety succumbed to the only known isolate of EU36 that can overcome its resistance. 

The outbreak was late in the season, the haulm was quickly burnt down, and the crop still yielded well, but it is a warning that any variety – or fungicide – can succumb when exposed to a large enough population of the late blight pathogen. 

“I’ve never seen anything like it. It was unbelievably aggressive,” he says.

“Varietal resistance is still an important tool in tackling blight, but it’s got to be stable and reliable. If we are not vigilant, nature will always find a way.”

Mix and match

What does all this mean for blight fungicide programmes? As ever, the key advice is to start early, maintain tight intervals and, crucially, mix and alternate a range of active ingredients with different modes of action throughout the season, tailoring choices to specific situations and blight risk.

Mixing and matching in this way helps to reduce the chances of fuelling resistance build-up, and spreads risk against the different strains of blight that may be present.

When considering which products to use, the advice is to understand the active ingredient and where it is best used in the programme, asking questions such as: what is its main mode of activity (systemic or translaminar protectant, for example)?

Which Fungicide Resistance Action Committee (Frac) group is it from? Is it effective on tuber blight, or just foliar blight? What is the most suitable mix partner? What conditions does it work best in? How and when should it be applied?

Products such as Ranman Top (cyazofamid), for example, can sometimes get pigeonholed as an end-of-season option, applied around desiccation for its tuber blight activity.

However, while this is an important timing, it should also be considered earlier in programmes too, says Certis Belchim technical manager James Cheesman.

“Ideally, you need coverage from zoospores from as soon as tubers start to initiate,” he says.

“If the ridge cracks and rain washes spores down to the tubers, no fungicide is ever going to get down there to sort the problem out, so it’s important to cover all bases with strategic use of tuber blight-active products throughout the programme.

“Six applications of Ranman Top are available; you don’t necessarily need to use all of those, but it is worth looking at applications towards the start (second or third blight spray), middle and end of the season.

“There’s no right or wrong place to use it, so long as you’re spreading it through the programme to get the benefit of the foliar and tuber blight activity.”

James Cheesman © James Cheesman

Suitable partners

As with any single active product, Ranman Top should always be used with a suitable mix partner, tailored to individual situations and risk, adds James.

David notes that FAB lab testing of cyazofamid and several other active ingredients against isolates from the main GB genotypes did find slight differences in sensitivity amongst clones, with EU41 showing reduced sensitivity.

However, he stresses this does not mean there is resistance.

“These are only representative isolates. It doesn’t imply that the whole of that clone responds in the same way,” he says.

“There is a very similar situation with EU46 and mandipropamid, where we have seen some differences in the sensitivity of EU46 to mandipropamid compared to other clonal lineages.”

He also points out that the concentrations of actives used in the testing are far below field rates.

“Nevertheless, it is something to bear in mind. Given the solo inclusion of cyazofamid in Ranman Top, growers need to mix actives and alternate modes of action.

“Again, it hammers home the message that solo exposure, even at low doses, may result in selection within the population.

“During the period between sprays, concentrations on plants may drop to the point where selection can occur, with some genotypes being more active in a crop than others.”

Build protection

Maintaining tight spray intervals is crucial to late blight control, and is more effective than relying on big mixes, stresses Martyn.

“We know from work with desiccant products that sprays often aren’t hitting as much of the leaf as growers think they are – sometimes only 30% of the canopy – and there’s no reason why blight sprays would be any different.

“We, therefore, need to build that protection up as best we can with regular applications.”

Using the correct water volume, nozzle choice (and therefore droplet size), pressure, boom height and forward speed all help improve coverage and efficacy.

“Speed is not of the essence, it is often part of the problem,” he adds.

Also pay attention to any potential areas where application might get missed, such as when backing into corners, or manoeuvring around obstacles, he notes.

© GNP

Beyond mancozeb

This season will be the last for using mancozeb, and while growers will want to make the most of this important multisite fungicide one last time. It is also the final opportunity to compare its performance to potential alternatives for next year.

“There’s no real replacement,” says Certis Belchim’s James Cheesman. “When you lose an active ingredient, the default position for some is to use it until the final cut-off, then work out what to do next.

“But actually, why not look at the alternatives on a field or part-field to see how they compare to mancozeb?”

Since 2020, Certis Belchim has been evaluating mancozeb alternatives from its portfolio at the Eurofins blight trials in Derbyshire.

“We’re not necessarily looking to completely replace mancozeb, but maybe complement the blight programme once it’s finally gone,” James says.

The trials, which are deliberately managed to be put under the highest possible blight pressure, compared an untreated control against plots that received eight applications of either Proflux (cymoxanil + mancozeb), straight cymoxanil, or cymoxanil plus Shirlan (fluazinam).

“It’s not a strategy we’d ever recommend in the field, but the trials did show that Shirlan wasn’t letting us down, with just a 10-15% difference in the visual assessment of foliar blight in the cymoxanil plus Shirlan compared with cymoxanil + mancozebm” he says.

“A simple Shirlan plus Ranman Top mix would give you a good, robust, blight application based on multiple active ingredients, that covers pretty much every base and genotype that we face.”

Martyn expects fluazinam will be carefully reintroduced into more programmes, but given the history with resistance in EU37, it cannot be used alone. “Even if there’s a low incidence of EU37, we know it will come back if we’re not careful.”

Holistic view

Mixing and matching chemistry and different modes of action is only part of the story when it comes to controlling blight, he says.

“My concern is that with every resistance change and every removal of product through legislation, like mancozeb, we simply pile more pressure on the ones that we’ve got left.

“Growers have to also look at the bigger picture.”

This includes several key areas, including:

• Monitoring risk closely In-field weather stations can be a “game changer”. Identify higher-risk areas where blight may start (areas in morning shade, for example)
• Control volunteers across the rotation, including in cover crops “Although only a small percentage of blighted tubers will produce a blighted plant, you only need one,” says Martyn. He also says to watch out where soil is moved outside of the sprayed area when de-stoning, which could result in untreated volunteers
• Watch field edges Less determinate varieties (Markies, Royal or Brooke, for example) can grow beyond the edge of the field and sprayer reach, often underneath other vegetation, so consider leaving a clear strip around the edge of fields to reduce such risks
• Manage indeterminate varieties carefully Avoid lengthy desiccation periods and greater tuber blight risk (tailor nitrogen applications and consider maleic hydrazide to stop secondary growth).