Grower knowledge is central to improving PCN management

It is widely accepted that potato cyst nematodes are a serious threat to the viability of potato production and yet despite efforts to promote better management practices the area of infested land continues to increase. There are many explanations for this trend not least the lack of market acceptance to those varieties with good resistance, which is considered essential to reducing populations, but the dwindling supply of clean land is a serious concern.

The situation is especially acute in Scotland where the need for PCN-free land is vital to maintaining the country’s status as a leading producer of clean seed. A Scottish Government report produced by the Plant Health Centre at The James Hutton Institute considered the situation and what was needed to improve it for the long-term.

The report, The future threat of PCN in Scotland, found that the area of land infested with Globodera pallida is doubling every seven to eight years. Should this rate of increase continue, say the authors, “the widespread presence of G. pallida may prohibit the production of seed potatoes on PCN-free land in as little as 30 years”. The authors note that this level of infestation will also limit the production of ware potatoes through loss of yield.

The financial impact of this is significant. On average, PCN is reckoned to incur an opportunity cost to growers of roughly £5,093 per hectare in lost output which equates to a £25m loss for growers in Scotland in 2019. The authors used this per hectare estimate to calculate the income forgone in the year 2040 should the area of infested land continue to increase at the current trajectory. It estimated this loss to be £125m.

With the threat to the long-term production of potatoes in Scotland so apparent, the authors spoke to growers in a bid to understand why the problem was getting worse, not better. Specifically, they sought to identify why growers were not planting resistant varieties or making better use of the other means of control and what could be done to make PCN management more effective.

The findings identify external barriers such as a lack of choice over the variety to be grown which serves to hamper the adoption of resistant varieties and the absence of a climate in Scotland conducive to achieving the dry matter content required of processing varieties, which tend to be the varieties with good resistance to G. pallida.

The unsuitability of some biofumigant and catch crops to Scottish conditions is also identified. “Despite their potential, trap crops are not widely practicable for use in Scotland as they do not establish well in the climate,” notes the report.

The authors note that resistant varieties and long rotations are probably the most effective means of protecting PCN-free status, but “unless groundkeepers are controlled, long rotations are broadly meaningless”.

In the Netherlands, where PCN infestations are higher than in the UK, “crops are monitored, and restrictions placed on land where too many ground keepers are recorded.” If the tolerance is exceeded the land is treated as if a potato crop has been planted.

The report notes that PCN is not the dominant issue when considering which variety to grow. Most of the growers surveyed view blackleg as the main concern as this is responsible for most down-gradings. PCN came third on the list of concerns with late blight control second.

Perhaps most concerningly, the report identified a lack of knowledge and understanding about PCN or the threat it posed to the long-term production of the crop.

The authors noted the following:

• The need for greater information sharing. “From interviews with growers it became clear that knowledge about PCN, how it multiplies, how it is spread and how to control it is limited. This is particularly true for ware growers who see little economic impact of PCN, as well as for landowners who rent out land to potato growers.”

• Guidance on how to interpret soil sampling results. “Results of PCN testing does not influence the choice of varieties planted on infested land. Lack of appropriate knowledge/advice on the best course of action and, secondly, a lack of choice when it comes to selecting resistant varieties are the main limiting factors.

“Where a PCN test returns a ‘none found’ result, there is a belief that the land is PCN-free and therefore there is no need to manage the land for PCN. This appears to be an important knowledge gap for growers and where increased education is required.

“Farmers who do not have PCN or who have only lost a few fields to PCN don’t seem to understand that there is a risk that their land might become or may already be infested. There needs to be education about sampling and that where a farmer has a patchwork pattern of results there is a low-level infestation across the farm.”

This lack of understanding around PCN and the potential implications that infestations can have on yields also led to a poor grasp of the principles underpinning the EU PCN Directive.

“Most of the growers were concerned about the loss of land to PCN. However, this was mostly perceived because of current government restrictions in place, preventing the use of infested land for seed production, and not because of the effects of the PCN infestation on yields,” says the report.

The report lists several recommendations, including reviewing the testing arrangements for land destined for seed, revising the PCR threshold, and permitting seed production on infested land under certain conditions, including the use of resistant varieties. Other suggestions include increasing the availability of and demand for resistant varieties as well as wider efforts to promote greater knowledge and understanding of PCN among growers.

The case for action

The report’s findings led the Scottish Government to establish a working group with the purpose of identifying actions that could be put into practice. The report has been submitted to the minister for consideration, but as of January 2021 a response is yet to be published.

For Eric Anderson, senior agronomist with Scottish Agronomy and a member of the working group, the challenge in facing up to Scotland’s PCN problem can be summed up as “a focus on short-term profitability and an over-reliance on rented ground”.

“The ambition has to be to preserve the land base for future generations while managing the epidemic. The industry can do much of this for itself, but it cannot continue to deny the threat posed by PCN. The gravity of the situation is compounded by the uncertainty surrounding granular nematicides and, in the longer term, that of glyphosate which is relied on to control groundkeepers,” he says.

Putting the industry on a course to fulfil this ambition, however, will require the leadership and strategic resources of government.

“Some of the short-term measures will come at a cost, which given the significance of seed potatoes to the rural economy of Scotland, should be met by the public purse. The cost of other actions will be shared across the sector,” says Mr Anderson.

An enhanced breeding programme intent on developing G. pallida-resistant ware varieties suited to the Scottish climate is one example of what government could fund. While achieving better control of groundkeepers is a responsibility the industry has overlooked for a long time, says Mr Anderson.

“Groundkeepers are an ignored problem because of the reliance on rented land. The Dutch system of applying a threshold above which the land is considered to have been cropped with potatoes holds appeal as it incentivises the landowner to exercise control.

“Several measures are under consideration that promote control of groundkeepers across the rotation, including the risk of losing a greening payment based on co-conditionality to control groundkeepers or maintain good soil health based on undetectable PCN in a land used for seed production. While the risk of losing the rent from a crop of seed potatoes until the land has completed another rotation should also focus attention,” says Mr Anderson.

Enabling growers to make more informed judgements about cropping decisions will require technological innovations and the development of effective decision support systems.

“Soil testing is likely to remain the basis of a management policy for seed crops not least because the requirement is enshrined in legislation, but there is the opportunity to develop complementary systems that promote more informed decisions. This area warrants further consideration,” says Mr Anderson.

The focus of attention may be on Scotland, but many of the measures proposed could equally be applied in England and Wales, he says.

“Were it not for Scotland’s status as producer of clean seed it is doubtful that the situation would receive the attention it is, but PCN, and G. pallida especially, is not an issue that is unique to those north of the border: Scotland’s issue with G. pallida is relatively minor compared with the area of infested land in the East of England. It is in the interests of growers across Great Britain that this threat to the industry is confronted, and measures to address it implemented,” says Mr Anderson.

Access the Bayer knowledge library

The potato page of the Bayer website contains a range of information to help growers better understand PCN and the management actions that support effective control.

Why not test your knowledge of PCN with one of our two quizzes. Our first quiz – Are you a potato cyst nematode expert? – tests your knowledge of PCN while our second – Understanding nematodes for stronger, healthier, higher-yielding crops – tests your grasp of the relationship between potato cyst nematodes and free-living nematodes while Nematode control: six steps that will protect potatoes from PCN and FLN describes some of the actions involved in developing a management strategy.

To understand how soil sampling results should be interpreted read our article, Potato Cyst Nematode Results: How Reliable Are Yours?

To see how other growers are developing an integrated approach to PCN management read our series of case studies:

• Twin approach to PCN control delivers yield increases in Shropshire trial

• New nematicide holds great appeal for this Norfolk grower

• Velum Prime offers PCN control for salad crop growers

• Encouraging results from new nematicide in Produce Solutions trials

• Velum Prime completes integrated approach for PCN management

Achieving control, however, can be balancing act. Read our article on the considerations involved in developing a strategy: Actions to protect soils may make nematode control more difficult