Syngenta blight advice for the 2025 season

After two successive seasons of high and prolonged blight pressure, more aggressive blight strains and the risk of resistance developing should be top of mind for potato growers and agronomists this summer, warns Syngenta Technical Manager, Andy Cunningham.

Speaking at Syngenta Potato Power meetings this spring, Andy highlighted that last year was especially challenging when high risk and early infections started in May, compared to July in 2023. Where an early foci of infection establishes in crops, any further periods of wet weather, or extended leaf wetness from irrigation, could trigger further flare ups. 

″That has been further compounded by the plethora of blight strains that now make up populations in the UK, some of which create a real challenge for putting together highly effective blight control programmes,″ he reports.

″Currently the population in England is dominated by the EU_36 strain, which is not overly problematic in the sense that it has not got any resistance issues, however it is quite aggressive and reinforces the priority to start spray programmes early, and maintain them through to full senescence.″ The EU_6 strain tends to be the most prevalent in Scotland.

Alert to the threats of new blight strains, Andy outlined that EU_43 that appears to have some resistance to CAA fungicides, for example, had become a serious issue in Denmark and the Netherlands, primarily due to the limited number of fungicide options they had available, and the reliance on blocking repeated CAA treatments. In practice, that selected for the resistant strains and allowed the populations to build rapidly in 2021 and 2022.

When in 2023 growers in Denmark changed their practices to alternating fungicide modes of action in the programme and, where possible, to use a fungicide mixing strategy, the population of EU_43 declined rapidly. However, in Holland where the single active blocking protocol continued, the population of problematic EU_43 continued to dominate.

″With the catastrophic effects seen in Holland, in 2024 both countries adopted what we see as good practice, to alternate and mix fungicide actives, and the dominance of EU_43 has effectively been brought under control. 

″This was achieved even when they were extensively using CAA chemistry, including Revus and mandipropamid products in the programme. It’s a very positive message that the UK approach of alternation and mixing really can be effective against new strains of blight, even which show some levels of resistance.″

A further issue with not maintaining effective control of blight is the inherently increased risk of more difficult strains being able to evolve. The explosion of EU_43 in Holland, for example, was the precursor of its evolution into EU_46 – which has now been identified in the UK and, with its genetic make-up, poses a greater threat of resistance issues to the highly valuable OXTP active.   Andy points out towards the end of the season last year, monitoring did identify two samples of EU_46 in Wales and Scotland, which had probably arrived by wind blow.

″Growers and agronomists are going to have to find a way to retain the use of the potency of OXTP within programmes, but mitigating the implications of the resistant strain developing. 

″What does help is that we know that the resistance to CAA mandipropamid is a recessive gene, so if that is diluted in anyway with new gene evolution, then it will remain susceptible to the fungicide group, which is what we see with EU_46 that is sensitive to Revus.″

However, the genetic make-up responsible for EU_46 resistance to OXTP (oxathiapiprolin) is a dominant gene, which means any further evolution of the blight strain will inevitably be carrying that mutation for resistance as well. 

Syngenta Potato Power research in the UK and across Europe has been focused on developing strategies proven to work on managing blight strains with reduced sensitivity to CAA mandipropamid products, along with minimising the risk of other strains developing resistance. 

″Mandipropamid, in Revus, is one of the most potent actives on the toolbox, outside of OXTP. Being able to utilise its strength is extremely important for robust blight programmes,″ he says.

Trials in Denmark, in the presence of both EU_43 and EU_46 demonstrated the importance of using Revus in mix with other actives to continue to retain very high levels of foliar control - even where the same mix was used right through the season in the trial scenario. In combination with amisulbrom (Evagio Forte), for example, infection was right down at around 10%, compared to over 66% in untreated and 36% with Revus alone. 

When Revus was used with propamocarb, foliar blight was assessed at just 1% and with fluazinam at 0.6%. Mixes with cyazofamid but also azoxystrobin (Amistar), which would be applied to target Alternaria, also proved to strengthen mandipropamid against the mixed blight population.

″Fluazinam, for example, is a really good partner for Revus, where individually the trials show some gaps, but the synergy works exceptionally well together to effectively eliminate the problems.″ Plots where different combinations were used through the season, including four Revus mixes in the programme, completely eliminated any infection.

In the UK at Euroblight trials, Andy reported combinations of Revus and fluazinam also performed exceptionally well, with significantly improved control over straight fluazinam.

The research using programmes and combinations showed that using a full complement of six mandipropamid applications strictly alternated in a 12- spray programme, to meet FRAC guidelines, achieved the best levels of blight control. A programme with no mandipropamid did achieve 75% control compared to untreated, but with a 22% improvement over that from including two mandipropamid treatments, and a further 29% enhancement with the six mandipropamid treatment programme. 

″The visual effects of blight suppression and the greenness of the crop protected with more mandipropamid in the programme was clear to see,″ he adds.

The work did also show that while adding cymoxanil to a mix could give useful 36-hour kick-back activity where blight sprays may have been delayed by weather, for example, its protectant ability is not its strength. That means if there was any risk of blight development, cymoxanil would also need another fungicide partner when in mix with Revus, he advises.       

As an early look see at future strategies, Andy highlighted that phosphonate products in development may offer a partner solution that looks as good as fluazinam for performance and to mitigate against resistance – adding another option in the event of mancozeb loss. 

Andy also emphasises the importance of application, to get blight products where they are required in the canopy and to stay on the leaf. The angled Syngenta 3D ninety nozzle has been a step forward in moving spray through the leaf canopy at every stage of the crop growth. 

Ongoing work has also shown that including a drift retardant can significantly increase performance with Revus and even new products, such as a mandipropamid + propamocarb development product that has shown exceptional results and would offer an extremely valuable option for a flexible early season systemic treatment, with built-in resistance management, he suggests. 

Syngenta advice for 2025 season blight control

• Always apply Revus in mixture with a fungicide with a different mode of action.

• Apply Revus, or other CAA fungicide, in strict alternation with a fungicide with a different mode of action. 

• Preferably apply Revus or other CAA fungicides as preventative treatments.

• Use up to six applications of mandipropamid, or other CAA fungicide, making up no more than 50% of the intended number of sprays. 

• Good mix protectant partners for Revus include: fluazinam; propamocarb; amisulbrom; cyazofamid and mancozeb.