Potato blight: What UK farmers can learn from Europe
© David Hanlon Dutch Image/Alamy Stock A combination of wet weather, new double fungicide-resistant strains of blight and repeated block applications of fungicides resulted in 2023 being one of the worst late blight seasons in the Netherlands.
This combination saw the disease explode and by July, farmers were unable to manage it for the rest of the season.
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It was a similar situation in Germany, while Danish growers achieved good control and have seen a decline in the prevalence of resistant strains.
Bayer’s strategy lead for vegetable crops Albert Schirring says the one positive for the UK is that it can learn from the contrasting experiences seen on farms in the Netherlands, Denmark and Germany.
Effectively, the Dutch approach last year should be avoided by the UK, and the Danish experience is the better approach, he says.
These lessons have led to changes in the Fungicide Resistance Action Committee (Frac) guidelines for using oxysterol binding protein inhibitor (OSBPI) – updated March 2024 – and carboxylic acid amide (CAA) fungicides to control blight.
What has testing revealed?
One concern is that testing of a sub-population suggests that some isolates are resistant to more than one fungicide group, which is a first.
Bayer testing shows EU43 isolates collected in 2023 have developed resistance to OSBPI fungicides as well as the CAA resistance that emerged in 2022.
This double mutation is possibly the result of block applications which put extra selection pressure on the two groups of fungicides.
Albert points to the Dutch practice of having three applications of Revus (mandipropamid) followed by three applications of Zorvec (oxathiapiprolin) with a low rate of amisulbrom.
Dr Juergen Derpmann, Bayer blight specialist and also member of Frac, says the double mutation is quite worrying in terms of resistance management.
“If you use one of these with a mixing partner that isn’t strong enough, you could end up selecting for strains with resistance to both,” he says.
There is an additional related strain, EU_46_A1, that appeared in the Netherlands/Germany border area in 2023.
Resistance summary
- 43_A1 has demonstrated resistance to CAA fungicides: Revus (mandipropamid), Versilus (benthiavalicarb) and Paraat (dimethomorph); and OSBPI fungicides: Zorvec (oxathiapiprolin). It has also demonstrated intermediate resistance to metalaxyl
- 46_A1 has demonstrated resistance to the OSBPI fungicides
- Isolates of 36_A2 collected in Denmark and the Netherlands were found to demonstrate resistance to OSBPI fungicides. The mechanism that conferred resistance is believed to be the same as that found in 46_A1.
What was the Danish experience?
In contrast, the Danish didn’t see the same blight problems as in the Netherlands, despite having the resistant strains. Albert says it’s because they have tighter rules limiting CAA use to one spray a season.
This lower CAA use has also resulted in a reduction in the resistant EU_43 strain from 62% in 2022 to 20% in 2023.
“This shows that the situation can get better.”
What is the advice?
With this in mind, Frac has updated its guidelines aimed to take away the selection pressure.
The double mutation means there is a risk those using a CAA fungicide early in the season will select for OSBPI resistant strains, as seen in NL/Denmark. This would render OSPBI fungicides to be ineffective later in the season, warns Juergen.
So the general recommendation is to always use both CAA and OSBPI fungicides with effective mix partners with a different mode of action at a sufficient dose to manage disease on their own.
Frac is also advising farmers not to overuse CAAs or OSBPI fungicides over the season:
OSBPI
- Make no consecutive applications of OSBPI fungicides
- No more than three applications per season
- If the total number of applications targeting late blight is 6-10, make no more than two applications per season
- If the total number of applications targeting late blight is five or fewer, make only one application per season
- In countries where OSBPI resistance has developed, make no more than two applications per season.
CAA
- Should total no more than 50% of the total number of late blight control applications
- No more than two consecutive applications
- In regions with reported cases of resistance: CAA fungicides must be used in mixtures, with not more than two consecutive applications.
However, Juergen goes further by saying CAA or OSPBI use at the start of spraying programmes is not advised at all, to avoid pre-selecting populations.
Bayer also urges growers to use ready mixed co-forms or tank mixes and use actives at a minimum rate of 75%.
Juergen highlights that the company’s Infinito (fluopicolide + propamocarb) is fully effective against the double mutation strains. He says its unique mode of action makes it a valuable tool for managing resistance.
In the UK
In the UK, where the resistant strains have not been seen, the advice is to base programmes on diverse modes of action.
Farmers should be alternating different modes of action and use of the correct mixing partner is also important.
David Cooke, research lead at the James Hutton Research Institute, adds that the two groups of fungicides (CAA and OSPBI) are good products and still work in the UK. And following this guidance will mean they continue to do so.
UK strain update
The EU_43_A1 strain that is resistant to CAA fungicides, including mandipropamid, is edging closer to the UK with testing revealing its presence in County Carlow in Ireland.
David Cooke, research lead at the James Hutton Research Institute, says the past four-to-five years have seen a marked increase of the strain in European outbreaks.
In 2022, there was rapid expansion with it spreading into the Nordic and low countries. Last year, it increased by 36% in the Netherlands.
In the UK, the good news is that the 43_A1 genotype was not attributed to any outbreaks in Great Britain last season.
However, the two cases in Ireland are a concern, says David. Not only could the disease spread from the Netherlands to the East, but it could spread on the prevailing winds from the West.
Fungicide resistance testing in 2023 shows no change in the UK, with no issues with the five key fungicides tested (amisulbrom, propamocarb, oxathiapiprolin, zoxamide and ametoctradin).
So there is no change apart from the prior EU_13 resistance to metaxly and EU_37 resistance to fluazinam.
In fact, fluazinam resistance is reducing in the UK. “With hindsight, fluazinam was overused, but practice has changed and we have seen population declines in EU_37 which is good news and will help.”
Looking ahead, there is concern with green bridges after such a warm wet winter and there are also pockets of unlifted crops.
He says planning ahead for 2024 will be more complex with careful planning needed. There will also need to be vigilance for when EU_43 appears, which is why the Fight Against Blight initiative is so valuable, he says.
Syngenta advice
Syngenta’s advice for UK growers using Revus (mandipropamid) is that it should always be used in a mix with a complementary blight fungicide with an alternative mode of action.
Furthermore, the partner must be used at a rate that will give protection equal in duration to Revus in the blight programme schedule, says Syngenta technical manager Andy Cunningham.
He cited suitable partners for Revus in the UK for the 2024 season including mancozeb, fluazinam, propamocarb and cyazofamid, as well as the leading contender, amisulbrom.
“There is potentially a role for cymoxanil to give kick-back activity, in the event of a weather delayed application, but in our Eurofins blight trials there was indication that it may not give the desired duration of protection – so intervals would have to be tightened,” he suggested.
The combination of mandipropamid and amisulbrom could prove especially valuable, targeted at the reduction of tuber blight risks.
And what about mancozeb?
Potato growers still have multisite mancozeb and independent agronomist Denis Buckley of Highfield Lodge Agronomy sees the fungicide having a key role in his 2024 programme.
“Whatever programme I go with, mancozeb will be a large part of it. You need to be applying mancozeb, certainly in the first half of the season, with every spray.
“You don’t know what will turn up on your farm, and the last thing you want is blight.”
Denis explains that mancozeb is the only potato blight fungicide growers have access to with true multisite activity, meaning selection for resistance is highly unlikely.
However, the present withdrawal timelines mean the sale and supply of any plant protection product containing mancozeb ends on 31 October this year, says Geoff Hailstone, potato technical lead for UPL in the UK.
An additional year would be allowed for the storage, disposal and use of these products.
