15 September 2000

How to beat PCN menace

A pest the size of a pinhead threatens much of the UK

potato crop. Andrew Swallow asked Harper Adams

University College nematologist Pat Haydock how growers

should get to grips with the potato cyst nematode problem

POTATO cyst nematode incidence in the UK is so widespread that all growers should be soil testing for the pest as a matter of routine, according to Harper Adams University College nematologist Pat Haydock.

"There is little good quality potato land that is not infested now. The aim is to identify populations at very low levels and keep the population low if PCN is found," he says.

That requires more intense sampling than the traditional 4ha (10 acre) blocks, he stresses.

"Growers should be sampling a 1ha block with at least 50 small samples a block, bulked up to provide a 1-2kg sample, of which 200-500g will be used by the laboratory for testing."

When sampling is carried out depends upon the objective of the sampling, he continues.

If the aim is to establish whether or not a field contains PCN, then testing samples of soil from under the grading line, or deliberately taken from the rows in the field, immediately after lifting, is most likely to pick up a low level population. Targeting gateways, field perimeters, or areas where soil washings or gradings have been dumped in previous years will help.

Where establishing the multiplication rate of a known PCN problem is the goal, and hence monitoring its subsequent decline between potato crops, then 1ha (2.5 acre) block samples should be collected one year after the crop is lifted.

"Sampling on a grid basis immediately after lifting may give a misleading result. If you know a field has PCN in it then it is better to wait until after cultivations for another crop have spread the cysts evenly throughout the soil," he says.

Between potato crops control of volunteers is crucial to maximise the rate of decline of the pest. Growers should consider using Fazor (maleic hydrazide) on the crop before lifting, aim to take as many tubers out of the field as possible, and tackle any volunteers that do get through within five weeks of emergence. This strategy is particularly useful with low nematode population densities.

In the absence of volunteers, PCN populations decline between 10% and 40% a year, but volunteers can slow that reduction rate, he warns.

Determining that rate of decline for individual fields is important for planning the rotation length. Annual intensive sampling of the same 1ha (2.5 acre) block would give a good indication of decline, but at a cost. As a minimum, Dr Haydock suggests testing the same block both one year after lifting, and in the early autumn prior to planting the next potato crop.

"The PCN problem is mainly due to too tight rotations, but commercial constraints have driven growers to plant potatoes as often as possible," he says. But now some supermarkets are specifying a minimum five-year rotation for potatoes grown on contract to minimise the use of nematicides, he adds.

Where intensive post-lifting testing produces a clean result there is no need to re-test pre-planting of the next potato crop. "If you are confident that the field is clean then grow the highest margin cultivars."

Elsewhere, varieties resistant to the type of PCN present should be grown. If only Globodera rostochiensis is present then that is little constraint, but resistance to the increasingly common G pallida species is only available in a few varieties such as Sante, Nadine or Valor, for which markets may be limited.

Where PCN is only identified at one end of a large field, then growers should avoid the temptation to grow a susceptible variety at the apparently clean end of the field. The chances are high that PCN will be present but at such low levels that sampling failed to detect it.

"You have to remember that you are only testing a 200g sample of soil from tonnes of soil in a hectare." Planting a non-resistant variety, especially if it is tolerant to damage, could lead to an explosion in the population, he warns.

Where PCN is identified, then nematicide use is justified even at very low populations on the grounds of long-term control.

"Below 2-5 eggs/g there will often not be an economic return to nematicide use in that crop. But if the grower is thinking about growing potatoes in that field in the long-term then he should be using a granular nematicide at the first sign of the pest."

Whether Temik (aldicarb), Vydate (oxamyl) or Nemathorin (fosthiazate) is used is less important than the method and time of incorporation, he says.

"They are all effective nematicides, but need to be used as close to planting as possible, as they are not persistent products and do degrade rapidly. The key is to get the incorporation right." (see panel.)

Repeat sampling and testing the year after the potato crop should be used to establish how effective the nematicide and resistant variety combination was at limiting the multiplication of the pest.

When populations reach a level where granular nematicides fail to prevent yield loss Telone (1,3 dichloropropene) fumigation should be considered to reduce the viable egg count. For PCN tolerant varieties that is typically at 50-60 eggs/g, but may be as low as 30 eggs/g for intolerant varieties.

As Telone has to be contractor applied in the autumn sampling should be done in late summer if growers expect egg counts approaching these levels. &#42

Spot sample immediately after harvest in the field or under the grader to identify PCN at an early stage, says Harper Adams Pat Haydock. But where a known problem exists, leave testing to a year later.


&#8226 484 fields sampled.

&#8226 64% contained PCN.

&#8226 Of which:

&#8226 92% contained G pallida.

&#8226 33% contained G rostochiensis.

&#8226 25% contained both species.

Survey carried out by Harper PhD Student Stephen Minnis, sponsored by Dow AgroSciences.


&#8226 Most potato fields infested.

&#8226 Use resistant varieties AND nematicides.

&#8226 More monitoring needed.

&#8226 Identify species.

&#8226 Nematicides £300-£340/ha.

&#8226 Fumigation £570/ha.

Tips for most effective incorporation

The most effective method of mixing nematicide and soil is broadcasting the pesticide and rotovating it in to 15cm (6 in) depth. But if a webbed stone and clod separator is used, such as a Grimme Colt, after rotovation the nematicide tends to be redistributed deeper than required, and will be much less effective, warns Dr Haydock. With such machines nematicide should be applied on the front of the separator, relying on the soil mixing action of the webs to spread the granules through the soil profile. Star-based systems, such as the Pearson Megastar, tend to replace the soil in a similar profile to the original mix, so broadcasting and rotovating in front of these machines is still effective, he says. Whatever combination of equipment is used, it is essential that soil conditions are suitable to produce a fine tilth, otherwise thorough mixing of granules and soil is impossible, he adds.

Resistance or tolerance?

Varieties of potato with partial resistance to PCN reduce the multiplication of the pest on the roots of the plant, while full resistance prevents nematode reproduction completely. Tolerance is simply a measure of the varieties ability to withstand damage from the pest. Hence a tolerant variety may appear the best option in the short-term, producing the highest yield from a field with a low PCN population. But unless the variety is also resistant egg numbers will multiply rapidly forcing longer rotations or expensive control methods to be used in future.