PCN varietal resistance promise – Crop Production Magazine

Varietal resistance against potato cyst nematode (PCN) will be increasingly important for potato growers following news that it could be the last season of use for Nemathorin (fosthiazate) in Europe, concentrating minds to life without the last mainstay granular nematicide.

While nothing has changed from a GB regulatory perspective for the product, which has a current expiry date of 2029, the impending withdrawal of approval in the EU has led to further nervousness about its future in the UK, said Simon Faulkner, an independent potato agronomist from South Lincolnshire.

Speaking at a recent Hutchisons’ potato meeting, he advised growers that if they own land infested with PCN then they should treat and consider using a resistant variety as well, to make the most of Nemathorin.

On the topic of resistance, Hutchinsons has been conducting potato variety trials supported by Richard Austin
Agriculture trials manager, Michael Rodger, at Worth Farms in Holbeach.

While presenting at the meeting, Michael highlighted that progress is being made with varieties that offer good resistance and/or tolerance to PCN. However, resistance and tolerance aren’t linked, he reminded. “Resistance is the ability of a variety to affect the multiplication of PCN, which ultimately should lower numbers at the end of the season.

“Whereas tolerance is how a crop can withstand an attack from PCN and still yield well. The downside is that a crop can yield positively but have a much higher infestation at the end of the season.”

Cara is a typical example of a potato variety which exhibits excellent tolerance but has no resistance, he suggested.

In terms of the Hutchinsons trial, the work tested 26 varieties across three replicates with and without Nemathorin, with 40-50 soil cores tested for PCN from each plot before and after the experiment.

The population was 100% Globodera pallida and ranged 0.5-20 eggs/g/soil. A PF:PI ratio – which compares the final PCN population with the initial population – of less than one suggests varieties with very good resistance that reduce G. pallida populations, explained Michael, with examples found in packing, processing, chipping and crisping varieties.

Most importantly, there are also varieties that exhibit both good resistance and tolerance to the strains of G. pallida found in the trials field, although this doesn’t necessarily mean there’d be similar results to other G. pallida pathotypes or G. rostochiensis, he added.

Promising options include processing varieties Amanda and Elland, chipping type Lady Luce, and crisping variety
Cinderella, alongside some coded entrants including one suitable for the packing sector, said Michael.

He added that the trial will be repeated next year using a stratification approach to bring more depth to the findings – because stratification helps to minimise the impact from PCN levels in plots varying across the field.

“Instead of randomising the plots after sampling, we’ll purposefully put each variety in three different infestation levels – high, medium and low – so we can see where resistance or tolerance drops off. Sometimes a variety will start performing worse when you have more than 5-10 eggs/g/soil,” he explained.

Hutchinsons has also been investigating varietal tolerance to post-emergence herbicides Shotput (metribuzin), Basagran
(bentazone) and Titus (rimsulfuron), as demonstrated through two years of trials.

In 2023, conditions were perfect for post-emergence herbicides at application and very little damage was seen whereas in 2024, conditions were more marginal with temperatures of 200C when sprayed near midday, said Michael.

That contributed to much higher levels of chlorosis and necrosis with some varieties exhibiting high levels of sensitivity. In particular, processing variety Armedi was very sensitive to all post-em herbicides, he highlighted, while varieties Lady Luce, Amanda and Buster were among those sensitive to metribuzin.

Furthermore, varieties including Lady Luce and Cardyma were also sensitive to full rate Basagran; in some treatments quite severe chlorotic and necrotic symptoms were being found 21 days after application, noted Michael.

In addition, damaged plots were more susceptible to botrytis infections. “We had almost no botrytis in the untreated, but where there was herbicide damage, plots were riddled with it,” he said.

To extend the trial work further, each variety is now being stored under two storage regimes, added Simon.

The pre-pack store is held at 30C with the use of ethylene treatment, Biofresh Safestore, as the sprout suppressant. The processing store is maintained at 80C, with sprout suppressant two-fold – a 20ml/t DMN application followed by ethylene at 5ppm.

“Fry colours were tested before storage and will be re-tested as they come out with the result available at our storage event on 21 May at LFP’s Fleet Lodge farm,” concluded Simon.

Also presented at the Hutchinsons event was work which has been taking place to investigate potato harvesting dates as a means of mitigating wireworm damage.

Independent agronomist, Martyn Cox, shared that Cambridge University Potato Growers Research Association (CUPGRA) trials in two different years at two sites with different varieties have highlighted how damage from wireworm reliably doubles in the six weeks between mid-August and the end of September.

“We achieved a 75% reduction in wireworm damage using a combination of variety and early harvest in 2024, and there wasn’t that much of a yield difference,” he said.

It’s calendar date rather than the duration the crop has been in the ground that’s important with early harvest, he added. “It’s down to the activity of the pest – at that time of the year they have a low period where they don’t eat much,” said Martyn.

Paying attention to application technique for blight sprays is critical, as once the disease is in the crop it’s difficult to control resulting in higher input requirements, warned Hutchinsons root crop technical manager, Darryl Shailes

Sprayers with GPS systems could also be a risk for blight, added Martyn Cox. “We’ve found blight at one end of the field where the sprayer went in and not at the other where it came out of work again.

“The reason was, the sprayer was getting up to speed and the variable rate control was ever so slightly out,” he explained.

The same could also apply if a sprayer is backing into a field corner and then driving off with the GPS not switching quite fast enough, said Martyn. “My advice to growers is to be on manual as much as you can and if you’re backing into corners, take your time.”

With only stocks of mancozeb already in farm stores available to use this season, more pressure is likely to come on blight control products, noted Darryl. “We have to reduce inoculum sources and have a good anti-resistance strategy.

“Be aware of the different groups and alternate products with different modes of activity – don’t trust singular actives and stick to seven-day intervals,” he advised.