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PCN under the microscope in 2017

Publication Date: 
23 May 2017
Author/Contact :
Jimmy Phillips and Anne Stone

“PCN is one of the biggest threats facing the industry. Testing has improved in recent years and as we take more soil from field we find low levels of potato cyst nematodes (PCN) more frequently” Says agronomist John Sarup of SPUD Agronomy.

Yield losses caused by infestation by potato cyst nematodes (PCN) (eelworms) depend on initial pest populations and potato variety, with a range from 1-35% in trials with infestations at 10-20 eggs per gram of soil.

At low levels of infestation, no symptoms are seen in the field, which creates the danger that populations can go untreated and therefore multiply. The first visible signs are usually patches of crop with weak vigour that typically don’t meet in the rows.

These stunted plants have poor root structure, reducing nutrient uptake and are more susceptible to drought. When the roots are examined carefully in situ, cysts can sometimes be seen with the naked eye or a hand lens.

The spread of PCN and the withdrawal of many pesticide controls has necessitated changed approaches to control. Since the MAFF guide to PCN management was issued in 1999 there have been advances in knowledge of PCN development alongside the introduction of novel methods of control.

New Guide and Spot Farm Focus in 2017

AHDB is providing fresh focus on the management of PCN in 2017 and beyond. The new Strategic Potato (SPot) Farm West, will be specifically PCN focused. A range of complimentary demonstrations will take place on farm in order to help visitors to make informed decisions at their own businesses. An updated PCN management guide has been drafted and will be released in July. This will be distributed at SPot West events and can be requested directly from AHDB’s Knowledge Exchange team.

This guide presents a summary of the latest national prevalence survey and describes control methods. None of these methods used in isolation will prevent infestation or eliminate established infestations, so recommendations are given for ‘best practice’ measures that will offer a sustainable strategy for PCN management in the long term, in an Integrated Pest Management Programme.

Relevant regulations have changed since the introduction of Directive 2007/33/EC, the PCN Control Directive

Spread and danger to crops

In the absence of a host, eggs hatch over an extended period, so that the numbers left viable decline each year. When potatoes are grown, specific chemicals exuded by the roots of both susceptible and resistant cultivars stimulate most (up to 90%) of the eggs to hatch. Juvenile nematodes are attracted to potato root tips, invade them and feed within them. This causes much of the damage to the crop, by stunting root growth and reducing uptake of nutrients and water from the soil. This feeding can cause considerable yield loses without necessarily producing large numbers of cysts, especially in resistant, intolerant varieties.

Juvenile nematodes develop within the root. On susceptible cultivars they develop and small (0.5mm) round females eventually protrude through the surface of the root to form a cyst. In July and August they are visible to the naked eye on the outside of the root.

Tolerant varieties can withstand or recover from damage and produce a yield. Resistant varieties can prevent or restrict nematode reproduction although the yield may be severely impaired.

PCN Management

There is no single method of PCN control. Most experts will recommend an ‘integrated approach’. Local and historical conditions will have a large impact on the final mix of methods chosen.

John Sarup says: “The key is to stay ahead of the game. In much of our industry, potatoes are grown on rented land where often the grower has little influence in controlling volunteer potatoes in subsequent rotations thus allowing the PCN lifecycle to continue. It becomes vital to test for PCN in this situation, and I would normally recommend testing to 40 cores per hectare grid.”

The list of options below would normally be considered in most PCN control plans, however there are a range of other options available which will be discussed in the forthcoming grower guide and at SPot Farm events.


Buy certified seed rather than using farm saved seed. In Scotland land for growing farm saved seed must be tested and be free from PCN. If planning to grow seed for use in the same business, test the soil to the same standard as if growing for seed.


Increased rotation length, together with control of volunteers, makes a contribution to PCN control. Where opportunities arise to irrigate previously unirrigable land, or where scab resistant varieties can be grown without irrigation allowing use of clean land, growers have been able to make major increases to rotation length.


Clean and wash machinery before and after use where PCN transfer is a risk. Dispose of waste grading soil to minimize risk of PCN transfer.

Varietal resistance

SPot West emphasises this aspect of control above all others. In most situations of severe infestation use of resistant varieties is crucial to control of PCN. To improve sustainability, retailers and processors should move to resistant varieties, where possible. When growing a resistant variety for its long term benefit, the current crop yield and how to conserve it becomes the crucial concern. While resistance is scored by breeders and in Independent Variety Trials, tolerance depends partly on the environment and is less easily assessed. The combination of different resistance and tolerance traits can be seen for many varieties in the demonstration field at SPot West.


For optimal incorporation, apply nematicides after de-stoning and before planting, using a rotavator that works in the vertical plane. Use a depth control so the nematicide is mixed only to the same depth that the seed will be planted.

Use product stewardship guides produced by the chemical company suppliers for advice on the harvest interval for products, as well as sampling protocols for tuber residue testing.

Next Steps

Nematicide Stewardship

The Nematicide Stewardship Programme (NSP), initiated in 2015, identifies and promotes best practice. The full programme, which will probably evolve, and a list of BASIS-approved advisers specialising in PCN management and Nematicide Stewardship, can be viewed at

Requirements include:

·         Operators have to be qualified to apply nematicides (NPTC PA4 or PA4G certification);

·         By March 2017 staff applying nematicides must have completed the Industry Stewardship Training Module;

·         By March 2017 all applicators must be fitted with a device in cab that allows the operator to shut off nematicide granule flow at least 3 meters from the end of each row.

PCN Management Demonstrations at SPot Farm West

We have two major events planned at Heal Farms, Shropshire during the season. The events are a chance to discuss the detail of control methods with scientists, growers, agronomists and other experts. The first event is an afternoon field walk on the 6th June.

PCN Management Guide 2017

An updated PCN management guide has been drafted and will be released in July. This will be distributed at SPot West events, including the August Open Day. It can be requested directly from AHDB’s Knowledge Exchange team.

Video content

Learn more about the science of PCN with Dr Matt Back here:

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