Breeders believe they have the tools to stay one step ahead of late blight in potatoes, despite concern about new and more aggressive strains emerging across Europe.

Since 2000, researchers have had technology that allows rapid identification of any genetic changes – or new “clones” – in populations of Phytophthora infestans (see box).

See also: Why potato sprout management now starts in the field

Sometimes these clones have little influence on how the disease is managed in the field, but in some instances, they can make life much more difficult for growers.

British blight population

David Cooke and the team at Dundee’s James Hutton Institute, who run the AHDB’s monitoring initiative Fight Against Blight, reported a relatively uneventful year in 2020, as the UK blight population largely remained in status.

However, there were some findings of note, including the fact that previously dominant foe Blue 13_A2 – a clone resistant to fungicide active metalaxyl-M – was largely non-existent and responsible for just one outbreak.

The aggressive Pink 6_A1 is still holding on to a significant share, being the cause of 36% of outbreaks sampled and sent in to the institute by Blight Scouts. EU 36_A2 (29%) and EU 37_A2 (10%) also increased as a proportion of the population, although 37_A2 only marginally.

So, what does this mean? Dr Cooke says that when there is a shift in dominant strains, the clones rising to prominence always have a particular characteristic that allows them to gain a foothold.

6_A1 is still hanging on because it is known to be very aggressive, produce large lesions that sporulate profusely, whereas 37_A2 is resistant to fluazinam and was managing to escape control programmes reliant on the active when it arrived in this country.

Where European growers still cling to regular use of fluazinam in fungicide programmes, 37_A2 has increased more quickly, but in Great Britain its spread has slowed after experts advised new guidelines for more careful use in most situations.

Control

“It’s a good news story in terms of the response to that advice. There have been fewer blocks [of fluazinam sprays] applied, more mixing and less reliance on it for tuber blight control at the end of the season.

“If you haven’t got that umbrella of fluazinam sprays, EU_37_A2 isn’t that aggressive and will decline naturally,” he says.

It is a similar story to when Blue 13 exploded, and growers heeded advice to stop using metalaxyl-M.

With the clone almost out the picture, Dr Cooke says valuable actives such as metalaxyl-M could be used with caution in the future and the same might be true of fluazinam as EU_37 fades away.

However, clone EU_36_A2 may be more problematic, as it is also very aggressive and fungicide laboratory efficacy testing by the James Hutton’s Institute’s Alison Lees in 2019 showed hints of poorer control when using very low rates of some common fungicides.

This led some industry observers to hypothesise that the EU_36 clone can detoxify itself when exposed to fungicides, making it harder to kill. However, in similar tests in 2020 these observations were not so clear.

“The observations [in 2019] were at very low doses, so at field rates there shouldn’t be a problem, but what happens four to five days after application? It could be a chink in the armour that EU_36 is exploiting,” says Dr Cooke.

European clones

In Europe as a whole, similar trends in population dynamics have been seen to those in Great Britain, with newer clones including EU_36, EU_37, EU_41_A2 and EU_43_A2 now making up 40% of the population.

What seems to be causing some alarm is EU_41, which has not yet made it to our shores, perhaps aided by prevailing westerly weather patterns and a drop in trade between here and the continent.

A presentation by researchers at France’s INRA institute at the EuroBlight Workshop at York in 2019 suggested that it was much more virulent on resistant varieties than other clones.

Some resistant varieties contain one or more R genes from Black’s differential set (Black was a researcher who identified and characterised 11 different R genes commonly used in breeding programmes).

The virulence of EU_41 was tested on detached leaflets of four varieties suitable for organic production, including Carolus, Alouette, Sarpo-Mira and Kelly.

Isolates of the clone managed to overcome several of the R genes – both individually and in combination – within Black’s differential set and was frequently virulent on Kelly.

The clone was first detected in Denmark in 2013, where it is now dominant (40% in 2019), and has since spread into Norway, Sweden, Finland, Poland and Germany.

New clone

At a recent EuroBlight technical meeting in March, Jens Hansen of Aarhus University presented information on another strain, EU_43, which has only been found in Denmark so far.

Almost half of the isolates of EU_43 collected between 2018 and 2020 were on the variety Kuras, which has high resistance to foliar and tuber blight, according to the European Cultivated Potato database.

So, has the advice on control changed in Denmark and other countries where one or both clones are present?

Dr Hansen recently published two articles warning Danish growers of the threat and urged them to work closely with their agronomist to sample infected plants as soon as outbreaks occur so strains can be identified by researchers quickly.

© Martyn Cox/Blackthorn Arable

Swedish researcher Björn Andersson of SLU (Swedish University of Agricultural Sciences) tells Farmers Weekly that in neighbouring Sweden they are not so sure about the extent of EU_41’s presence (due to lack of significant surveying) and they are yet to find EU_43.

However, he believes EU_41 will dominate across Sweden – if it hasn’t already – due to its competitiveness and its evident rise as a proportion of samples elsewhere.

“The [general] advice [for blight control] has not been affected by EU_41. We only ask [growers] to be more observant, especially in the more resistant cultivars,” he adds.

British concerns

From a British perspective, Dr Cooke says EU_41 is a concern, particularly as the potato industry moves away from intensive fungicide programmes to increased reliance on varietal resistance.

This is a trend likely to play out over the next five to 10 years as governments across Europe set out plans to reduce overall pesticide use by 50% by 2030.

So long as there is a sexual population, new strains such as EU_41 will continue to emerge and arrive in the UK, and breeders will have to try and stay one step ahead with more robust combinations of resistance genes.

“One thing we are keen on doing as part of EuroBlight is increase the information flow to breeders to help with that. I think everyone would like to see better natural resistance and fewer fungicides in the future,” adds Dr Cooke.

Until then, he urges growers not to lose focus on the basics, particularly after low-pressure seasons like last year, which can provide a false sense of security.

This involves the adequate control of sources such as dumps and volunteers and the use of forecasting systems such as AHDB’s new BlightSpy to make optimal use of fungicides in relation to disease pressure.

In terms of breeding efforts, one company that believes it will be able to stay one step ahead of a dynamic blight population is Wageningen-based Solynta.

Most commercial potato lines have a very complex tetraploid genome, consisting of four sets of chromosomes, making it very difficult to improve genetics through traditional plant-breeding methods.

It is also time consuming, with a 10-15 year wait between a first cross and a commercial cultivar being planted on farm.

Hybrid potatoes

To speed the process up, Solynta has developed a system for potatoes, where diploid lines – which contain just two sets of chromosomes – are cross-bred to produce a hybrid. These hybrids will then be delivered to the grower as true seed, rather than seed tubers.

In a new development, recently published in the journal Nature Communications, Solynta and Wageningen University researchers have identified and characterised the Sli (S-locus inhibitor) gene of diploid potatoes.

Diploid plants have evolved to become self-incompatible, with the Sli gene responsible for this trait.

Now breeders know exactly where it is on the genome, it will help speed up the hybrid breeding and improve their ability to select for better traits such as taste, water use efficiency and disease resistance.

Solynta’s research team lead Michiel de Vries says for blight resistance specifically, institutes like the Hutton and the Sainsbury’s Laboratory have done fundamental work to identify many new resistance genes in wild potato relatives.

It is now up to the breeding companies to apply the knowledge by developing resistant varieties.

Companies such as Solynta can now monitor blight populations and performance of plant resistances. Where there are changes in virulence, they can then stack new combinations of R genes to maintain control.

“With the hybrid system, this can be done in two years or even less if combined with speed-breeding methods available to us. Gene editing could reduce this further and could be another useful tool in the box,” explains Dr de Vries.

When asked about the threat from aggressive strains such as EU_41 and the many clones that will come in future years, he looks excited by the challenge it presents.

“This is the core aim of our business, and it’s what we are good at. The way forward is a combination of resistances and limited use of crop protection agents as part of an integrated pest management strategy.

“We have a future vision to make hybrid potatoes with many resistance genes, similar to what we see in tomato production, where crops can be cultivated with almost no pesticides.”