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Winning the war against PCN: Latest developments in PCN control


Published in The Potato Review, March (2024) edition


In this issue of British Potato Review, we're bringing you exclusive insights from Dr Matthew Back, reader in nematology at Harper Adams University and a renowned authority on PCN management, and Nick Winmill, head of potato R&D at advisers, Agrii. We learn how the latest advancements are combating a pest that causes up to £31M in damage and lost output annually.


Over the last 20-30 years, there has been significant progress in our understanding of the biology and management of potato cyst nematode (PCN), yet it is still regarded as one of the main threats to potatoes.


Dr Back explains that the prevalence of the pest is concerning, especially in previously untouched seed land in Scotland.


“The James Hutton Institute has warned that at the current rate of spread, there could be in the region of five potato seed crops left that aren’t impacted by the pest,” he says.

He explains that the species, Globodera pallida, has become the most prevalent of the two species infecting potato crops in the UK with the latest research showing occurrence at 95% of sites that have PCN present (Dybal, 2018).

“The dominance of G. pallida has been driven by the trend of cultivating varieties with the H1 gene; a major gene which confers resistance to G. rostochiensis,” he says.

“For example, Maris Piper has the H1 gene and continues to be the most widely grown variety of potato in the UK.”


PCN tolerant varieties

Dr Back explains that although there are many more resistant varieties, such as Elland, Innovator, and Lanorma, available than 25 years ago, these varieties are mainly used for processing and few options exist for fresh prepack markets.

However, while varietal resistance offers a solution to lowering PCN populations, Dr Back explains it does not protect the tuber yield of infected plants. This is where PCN tolerance is important.


“Varieties with PCN tolerance have extensive root systems that compensate for PCN root parasitism, meaning that they continue to produce acceptable yields.

“Essentially tolerance needs to be used alongside resistance to avoid unwanted PCN multiplication,” he says.


Nick Winmill, Agrii agronomist and member of The Potato Partnership steering group, explains how The Potato Partnership has been investigating IPM control of PCN through resistant and tolerant varieties.


The Potato Partnership is an industry collaboration initiated by Agrii and founded on the model provided by the former AHDB Potatoes SPot Farms. Other partners include plant breeders, growers, manufacturers, and a wide range of agronomists.


“Within the Potato Partnership there is a recognition that we urgently need practical solutions to the pest and disease challenges facing the sector. For PCN, our trials have sought to evaluate the resistance and tolerance of a range of potato varieties grown under commercial conditions in the presence of Globodera pallida,” says Mr Winmill.


The 2023 trial was located on a sandy loam site at Sutton, Suffolk. The field was sampled and tested for PCN on a one-hectare grid basis, with the trial finally located within a 1 ha block with a population average of 9.56 eggs/g soil.


“The trial found that certain varieties were better at coping with the PCN burden,” he says.

“The maincrop variety Buster gave the lowest mean rate of multiplication of PCN, followed by the white maincrop Lanorma, then the early maincrop dual resistor Cinderella.

“Iodea a dual resistor salad/ baby type variety performed well followed by two recent introductions, Lady Luce and Bruar, both of which offer dual resistance to PCN. 

“The partial resistant varieties Sensation and Decibel increased the level of PCN but to a lower level than the non-resistant variety Maris Peer. In contrast, the other non-resistant maincrop variety, Markies, had the highest rate of multiplication reflecting its low resistance rating to G. pallida,” says Mr Winmill.


He explains the results suggest that breeders are introducing a range of varieties with good levels of both tolerance and resistance to G. pallida.

“This offers the prospect, subject to market acceptability, of helping in a strategic approach to managing PCN.

“The uncertain regulatory future facing granular nematicides and the high PCN infestation levels seen across much of the growing area, make it clear that varieties exhibiting both resistance and tolerance to G. pallida represent the most effective and sustainable means of managing populations for the long term.


“It’s reassuring that breeders and seed houses have recognised this reality and are introducing varieties with these genetic traits but also with the potential to deliver commercially competitive yields.

“If breeders are to find the confidence needed to increase seed production, the industry first needs to convince consumers that these varieties are as good as the current market leaders such as Maris Piper, Maris Peer and Markies,” says Mr Winmill.


Granular nematicides

Granular nematicides, applied to the seedbed, help protect the yield of potatoes by preventing infective PCN juveniles from reaching the roots, limiting penetration and feeding damage.

Dr Back says: “Granular options have dwindled in the past 5 years with Mocap 15G® (a.s. ethoprophos) and Vydate 10G® (a.s. oxamyl) being withdrawn in 2019 and 2020, respectively.

“Current granular options consist of Nemathorin 10G® (a.s. fosthiazate), which is widely regarded as the most effective nematicide. The product is registered until July 2029, but this could change with increasing emphasis on greener crop protection measures.

“Additionally, there are no longer any soil fumigants for treating infested soil post potato crop,” he adds.


Patrick Mitton, chair of the Nematicide Stewardship Group (NSP) explains that with such limited granular nematicides available to support growers in managing PCN it is important that growers and operators follow the NSP Protocol to support the last remaining product’s future.

“Granular nematicides are an essential tool in any integrated management strategy to reduce yield losses, and it is safe and responsible use which will help retain them for the future,” he says.

“The NSP best practice steps bring together practical advice on how to use granular nematicides safely and forms part of the Red Tractor standard for growers.

“The Protocol consists of six easy steps to protect operators, the environment, and consumers. These include: gaining the necessary qualifications for application, ensuring machinery is correctly calibrated, applying in a single pass, ensuring shut-off before the end of the row, making sure any spillages are buried immediately, and carrying out field checks 12-24 hours after application.


“If you are using granular nematicides this season, make sure you are up to date with the protocol. More information and helpful videos can be found on the NSP website,” says Mr Mitton.


Biofumigants

As well as established IPM methods, such as extended rotations and volunteer management, Dr Back says populations can be lowered further by the inclusion of biofumigant brassica cover crops.

“Performed correctly,” he says. “These measures have the potential to reduce populations to manageable levels and shorten rotations.

“Brasscica biofumigants, such as Indian mustard, are grown during the summer months, typically behind crops such as winter barley, vining peas or carrots.

“Their activity on PCN comes from volatile and toxic compounds, mainly isothiocyanates, produced from chopped stems and leaves of plants that have reached early flowering.

“The aim here is to grow maximum biomass and to accomplish this I recommend achieving optimal sowing (date and method) and adequate supply of nitrogen (typically 80-100 kg N ha).


“Glucosinolates can be maximised by appropriate supply of sulphur (25 kg S/ha) and correct date of chopping (green bud to early flowering),” he says.

He adds that work undertaken by William Watts (Watts, 2018) showed that chopping using a rotary flail and incorporation via a rotavator or spader gave better results in terms of PCN suppression.

“Water is also vital for the conversion of glucosinolates to volatile compounds (hydrolysis), so soil moisture at 60-80% of field capacity is ideal,” he says.

Dr Back warns that biofumigation should not be practiced on acidic soils (pH lower than 5.5) as this influences the effectiveness of the process. Low pH leads to lower concentrations of isothiocyanates.

“An area receiving greater interest is partial biofumigation,” he says.

“This type of biofumigation is associated with isothiocyanate production from growing crops and is related to the secretion of glucosinolates from brassica roots.


“Glucosinolates released into the soil can be broken down into toxic isothiocyanates by microorganisms (fungi and bacteria) that produce myrosinase – the enzyme used by brassica plants to convert glucosinolates into isothiocyanates. The breakdown of root secreted glucosinolates results in continuous release of isothiocyanates, albeit at a much lower concentration than traditional biofumigation.

“Partial biofumigation doesn’t cause as much suppression as traditional or complete biofumigation but provides greater flexibility to growers in rotations as there is no requirement for chopping and incorporation, meaning that cover crops can be sown later or even overwintered.


“Additionally, partial biofumigation can be achieved with other brassica species such as oilseed radish (Raphanus sativus),” he says.

There is extensive work on biofumigation, and researchers around the world have recorded suppression against a range of pests, weeds and diseases.

“In potatoes, there’s evidence[1] that brassica biofumigants can lower soil borne pathogens such as Rhizoctonia solani (the cause of stem canker and black scurf), Spongospora subterranea (powdery scab) and common scab (Streptomyces spp.),” he says.  

“A number of UK studies have focused on biofumigant activity against PCN and have shown reductions of the pest typically ranging from 35-70%.

Dr Back says the key to success with this approach is growing the right species and variety, performing the practice at the right time – typically when the temperatures are higher and daylength is longer – and using the correct agronomy.


Trap crops

Solanaceous trap crops offer another tool for lowering PCN populations in potato rotations.

Prickly nightshade (Solanum sisymbriifolium) is the most well-known trap crop and was used more commonly in the mid-2000s.

Trap crops, typically native to South America, are close relatives to potatoes, which stimulate PCN hatch by producing hatching factors such as the glycoalkaloids alpha-solanine and alpha-chaconine.

Infective PCN juveniles can penetrate the roots of these plants but not fully complete their lifecycles, due to the plants’ defence responses.

The trapping activity of these ‘dead end plants’ leads to reductions in PCN populations as high as 85%.


“In the last 10 years, there’s been greater interest in other solanum species such as African nightshade (S. scabrum) and tall/velvet nightshade (S. chenopodioides),” explains Dr Back.

“Work in Kenya highlighted that African nightshade could provide a tool for reducing PCN and root knot nematodes (RKN).

“However, the challenge of growing trap crops is achieving consistent results with establishment and plant development,” he says.


“The key to overcoming these inconsistencies appears to be around ensuring seed is sown at a shallow depth of 0.5-1cm, with sufficient moisture to enable seed-to-soil contact.”

Research conducted on the Innovate UK project, DeCyst - Factors affecting trap crop success against PCN (Produce Solutions, CHAP, Harper Adams University, VCS Potatoes and Curious Raven), has indicated that standard or precision drilling are likely to provide better results with plant ground cover and biomass than broadcasting.

In summary, Dr Back explains there is a good opportunity for trap crops and biofumigants to be used within integrated control programmes for the management of potato cyst nematodes.


“Trap crops and biofumigants can reduce the PCN challenge, while allowing growers greater flexibility with rotations. Embedding these crops and biofumigants within environmental stewardship schemes, would be a further step forward and would help incentive their usage,” he says.   


 

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