LOOK at any of the Fight Against Blight (FAB) or Euroblight data and it is clear to see that the threat from late blight is constantly changing as the blight pathogen evolves and adapts to the way it is managed.
EU36 continues to dominate populations in England, particularly eastern counties, whereas in Scotland, it is EU6, although 2024 did also see a re-emergence of EU41, a genotype that has demonstrated insensitivity to active substance metalaxyl-M.
Last season also saw the UK’s first confirmed cases of the EU46 genotype in Wales and Eastern Scotland. EU46 is already present in Germany, Denmark and the Netherlands and is known to have resistance to oxysterol binding protein inhibitor (OSBPI) oxathiapiprolin, as well as isolates with reduced sensitivity to mandipropamid and metalaxyl-M.
The historical reliance on blocks of chemistry in these countries may well have led to this resistance issue, and while UK growers have long taken a far more varied approach to fungicide use, the discovery of EU46 shows there is no room for complacency.
Alongside these constantly-shifting threats, the control options available are becoming increasingly limited as actives, such as mancozeb – available for the last time this season – are lost to regulation, and others succumb to resistance, putting greater pressure on those that remain.
Agronomists and manufacturers of disease control products are advising growers that they will need to rely more heavily than ever on an integrated approach to manage late blight this season.
Certis Belchim’s Global Crop Manager for potatoes and beet, Ed Bingham, who also farms in Norfolk, said: “As an industry, we need to take a more integrated approach to tackling blight. It’s not just about what sprays we use, it’s about the nutrition of the crop, using resistant varieties, controlling volunteers across the rotation, and staying one step ahead of whatever’s coming along next.”
The independent work by initiatives such as FAB is invaluable for understanding the disease, the genotypes present, and the control strategies required, he adds.
Sipcam UK’s Technical Manager Stewart Woodhead said there’s more to consider than which chemistry is under pressure from various blight strains and using the fungicide programme to combat resistance build-up should be a key focus.
He said: “A combination of understanding blight strain populations, adjusting chemistry to best support resistance management and choosing cultivars to compliment the programme are just some of the considerations that will be needed going forwards.”
VCS Potatoes Ltd Agronomist Graham Tomalin recognises late blight is becoming more aggressive, with the arrival of EU36 a few years ago marking a step-change from the likes of Blue 13 and Pink 6.
“What we see in the field correlates with research that shows lesion growth and sporulation capacity is greater than the strains we were previously working with. If the right conditions exist, blight is now much quicker to spread and infect new plants.”
Indeed, research shows EU36 can produce more than 800,000 spores per cm2 of lesion per week in lab conditions, says Certis Belchim’s Technical Specialist, Cristina Ruiz Alonso. That could be equivalent to 1.5 billion spores per ha at 1% disease severity.
That spore production over seven days is almost 20% greater than 43_A1, for example, highlighting the aggressiveness of 36_A2, she says.
“It’s important to understand the behaviour of different genotypes and how aggressive they are, as this helps us manage them in the field.”
Graham agrees on the importance of understanding the blight populations in a specific area, but says this testing only gives a snapshot of whatever samples are collected, so other genotypes may be present, but undetected.
How then do growers guard against these known, and unknown threats when planning fungicide programmes?
For Graham, the key is to start early and aim to remain in a protectant situation, utilising the most suitable active ingredients for specific crop stages and risk situations throughout the season.
Alternating chemistry and modes of action is vital, and in most situations, he says growers should use two different forms of chemistry in every application to avoid placing too much pressure on any single active, either by using a co-formulation, or mixing suitable partners.
Blight protection begins from the outset, as risk will be there as soon as crops emerge if conditions are conducive.
“In my experience, those crops that have not been properly protected at the start will be the first to be at risk of blight because they haven’t built up a level of protection,” Ed said. “Whatever you use, you’ve got to have a good programme, especially on long-term storage crops were tuber blight is more of a concern. There’s no sense trying to cut corners.”
Graham says growers cannot be too prescriptive though.
“There are certain products you’d use at certain times, such as those with tuber blight activity, or good systemic activity during rapid canopy expansion, but nothing is fixed in stone. Any blight programme has to react to whatever infection pressure and risk develops through the season.”
Growers are advised to maintain tight intervals to avoid getting into curative situations, and by mixing and matching actives, they can reduce the chances of fuelling resistance build-up, while spreading risk against the different strains of blight that may be present.
When considering which products to use, the advice is to understand the active ingredients available and where they are best used in the programme.
Consider aspects such as the main mode of activity (systemic or translaminar protectant, for example), FRAC classification, effectiveness on tuber blight and/or foliar blight, suitable mix partners, optimum application conditions, and how and when should it be applied.
Products such as Ranman Top (cyazofamid), for example, can sometimes get “pigeon-holed” 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 from tuber initiation onwards to protect developing tubers.
“In the Netherlands two years ago, we were finding tuber blight in August, well before desiccation,” said Ed. He says Ranman Top has proven to be a very effective option over the 25 years it has been on the market, with no known resistance, suitability for a range of timings, and excellent rain fastness.
“With mancozeb going and question marks over other active ingredients, I think it could be the backbone of many blight sprays going forward. But as a single active ingredient product, it should always be used in mix with an effective partner.”
One potential mix partner is Shirlan (fluazinam), an active ingredient that is being carefully reintroduced into more programmes.
Although there is a history of fluazinam resistance in genotype EU37, a change in use of the active several years ago led to a sharp decline in resistant populations, and fluazinam is still effective against most other genotypes, including EU43 and EU46, but should not be used alone.
“I’ve been using fluazinam in combination with other actives for the past couple of years as it is a useful mixing partner at various points throughout the season,” Graham said. “Target alternation with other actives and aim to avoid back-to-back applications though. I would, however, change this approach if EU37 was identified by FAB monitoring in the local area.”
For early canopy development, Stewart said Lieto from Sipcam UK is also a great option, containing 330g/kg Cymoxanil (Group 27) and 330g/kg Zoxamide (Group 40), providing two alternate modes of action to the Quinone inside Inhibitors (QiI) e.g. Cyazofamid and Carboxylic Acid Amide (CAA). It effectively combines two Mandipropamid fungicides in one product.
“With up to six applications permissible per crop, it should be considered as a preventative application at first risk of blight infection. With flexible application rates, consider using Lieto as part of the blight programme in combination with actives from differing fungicide groups, as part of an overall resistance strategy,” he said.
“These include cyanoacetamide oxime, valifenalate, fluopicolide, propamocarb and benthiavalicarb. It is also important not to forget fluazinam, cymoxanil and oxathiaptprolin to ensure a good success rate vs resistance build up, limiting the impact from the increasing resistant strains.”
Although very rare, seed-borne blight infection can be a potential risk if lower quality stocks have been bought in, so growers must be vigilant.
Seed infection is often characterised by multiple lesions on isolated plants soon after emergence, while those around them are unaffected, Graham said.
“It’s quite rare to find blight in the seed, but if you did, a robust fungicide programme would be needed right from the start.”
Early use of Ranman Top would be an effective option for managing any possible seed-borne infection, says Ms Ruiz Alonso.
Again, it should be used at full rate, with a suitable mix partner, such as mancozeb or cymoxanil, the latter offering more curative action if required, she notes.
Some growers may also choose to visit seed suppliers in-season to check the quality and health of crops, ensuring they will get the best possible seed, she adds.
Unlike the UK, growers in many continental countries have historically taken a different approach to blight control, often involving “blocks” of key products, not always used in mix, and sometimes at reduced rates if disease pressure is lower.
Inevitably this has put a lot of pressure on fungicides and selected for resistant genotypes, such as EU43, which has CAA resistance, and was first found in Denmark. It increased rapidly to account for 75% of the population in 2022, according to Euroblight monitoring data (www.euroblight.net).
However, Ed says a change in strategy saw EU43 fall to 15% in 2024, with growers moving away from blocks of actives, alternating chemistry, and introducing fluazinam into many mixes.
EU46 has also stayed at a relatively low level with this approach, although Denmark is unique in also having a large “miscellaneous” category of blight genotypes which have not yet been defined.
The Netherlands too has traditionally relied on blocks of two, three, sometimes four applications of the same product, which has again created a “hotbed for resistance”, he says.
EU43 was identified in the Netherlands in 2022 and exploded in the bad blight year of 2023. At the same time, a new strain with resistance to CAA and OSBPI chemistry also appeared.
“Last season was the first for real change though, with far greater focus on mixing and alternating available chemistry, more similar to the UK, in an attempt to combat late blight evolution without mancozeb.
“There are signs of success, as EU43 declined considerably in 2024, although EU46 remained at the same level. EU36 increased and although it does not have any resistance, it is a very aggressive form of late blight.”
“The key thing to remember is that pathogen evolution will always be one step ahead, but even without mancozeb, we can still have confidence in effective management.
“We’ve got the right building blocks in terms of chemistry available; growers need to keep alternating and mixing as much as possible and using active ingredients at the full effective rates.”
David Cooke from the James Hutton Institute notes that FAB lab testing of cyazofamid and several other active ingredients against isolates from the main GB genotypes last year did find slight differences in sensitivity among 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. 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 mode 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.”
UK potato growers may soon have another option for blight control, as Certis Belchim hopes to bring its new cyazofamid and valifenalate-based fungicide to the UK market.
The product is already available in three other European countries and is currently going through the UK regulatory approvals system.
“It’s a mix of two active ingredients from different classifications, mixed at the full rates of each in one product, which is very unique,” says Ed.
“We are seeing a benefit from mixing those two active ingredients together at full rate, especially on foliar blight.”
See the May issue of British Potato Review for advice on Choice of Cultivar and Cultural Control
