Nematology

The Nematology Laboratory at SASA provides technical support through the detection and identification of plant-parasitic nematodes (eelworms). Our Laboratory work supports the Scottish Government's Plant Health Service in the implementation of plant health regulations and through meeting the requirements of the Seed Potato Classification Scheme (SPCS). The Scottish potato industry benefits from the production of seed potatoes that meet high standards of plant health, minimising the loss of quality and yield that these pests can cause.

The Nematology Laboratory is able to offer the following services in relation to the diagnoses and control of Potato Cyst Nematodes:

What are nematodes?

Take a handful of soil from almost anywhere in the world, from the Arctic to the Tropics, from the tops of mountains to the depths of seas, from deserts to swamps, extract the living organisms in water, and among the other forms of life you will find elongate, threadlike, active animals – these are nematodes (or eelworms, or roundworms).  Many of them will just be visible without magnification, but others will only be seen with a good magnifying lens or microscope.

Or, if you catch a fish, bird, or mammal, dissect out its stomach or intestine, in most cases you will find some nematodes living there.

Nematodes (the name is derived from the Greek word for thread) are elongate, tubular organisms that move like snakes or eels.  They are aquatic and live in marine or fresh water, and in films of water within soil/compost/forest litter/moss etc.  They are one of the most successful and adaptable of animal groups, being rivalled only by insects as regards range of habitats or number of species.

Most species are free-living, with their food consisting of micro-organisms – fungi, bacteria, and algae, and these play an important role in decomposition and recycling of nutrients.  Nematodes occur in soil in great numbers-rich arable soil may contain 3 billion (3,000,000,000) nematodes per acre (0.4 hectares), calculated to a depth of 20 cm.

Many nematodes are highly successful parasites.  The first recorded ones were, not surprisingly, human parasites.  Passages in the “Papyrus Ebers” (dated 1550 BC) are said to contain reference to the human intestinal parasite, Ascaris lumbricoides, and the tissue parasite, Dracunculus medinensis, (the Guinea Worm), both of which are large parasites which may exceed a foot in length.  The Guinea Worm is believed to have been the fiery serpent of the bible which plagued the Children of Israel after their journey through the swampy regions near the Red Sea.  Certainly this pest is still found there today.

Many of these animal parasites have intricate life cycles during which the parasites alternate between their vertebrate hosts and invertebrates such as arthropods and crustacea.

Although nematodes have undoubtedly been associated with plants for thousands of years, the first plant-parasitic nematode was reported in 1743 by an English clergyman, Needham.  He was examining some dry, diseased wheat kernels in a drop of water, and after a time, to his amazement, he saw a mass of fibres that began to twist.  He wrote:

“I am satisfied that they are a species of aquatic animal and may be denominated worms, eels, or serpents, which they much resemble.”

Whereas animal parasitic nematodes may be large, (the largest of all being Placentonema gigantissima, a monster nine metres long found in the placenta of sperm whales), the plant parasites are mainly microscopic.  The largest, the longidorids, are 5-10mm long, but many nematodes are shorter than 1 mm.  Being slender and transparent, they cannot often be seen by the naked eye.

Other groups of worms may be confused with nematodes.  These include flatworms, (Platyhelminthes), e.g. liver flukes, which are flattened and oval in shape; tapeworms (Cestodes), most of which are adapted for living in the gut of vertebrates and have a body of rectangular flattened segments with suckers and hooks at the anterior end; and earthworms, which are long and segmented.  The white, wriggling worm-like organisms, commonly seen in rotting plant material or compost, are seldom nematodes.  With a few exceptions, if you can see an organism, with the naked eye, it is not a plant-parasitic nematode.

Although nematodes are typically worm-shaped, in one particular group of plant-parasitic nematodes the cuticle of the females develops into a tough wall creating a spherical, or near-spherical, cyst protecting the female’s fertilised eggs.  These cyst nematodes are the most economically important nematode pests of temperate agriculture.  The potato cyst nematodes, Globodera pallida and Globodera rostochiensis, introduced into Europe with potatoes from South America, have subsequently spread throughout most of the potato growing areas of the world.  They are treated as quarantine pests because of the economic damage they cause and the difficulties faced when eradication is attempted.

Potato Cyst Nematodes (PCN)

World-wide Distribution

Potato cyst nematodes (PCN) comprise of two very closely related species (Globodera rostochiensis, Globodera pallida) which co-evolved with the potato in South America, but have subsequently been introduced elsewhere with the production of potatoes.  Outbreaks of potato cyst nematodes have now occurred in most of the potato growing areas of the world.  Probably the only country in the world growing a large acreage of potatoes that has yet to report an outbreak of PCN is China.  Reports of PCN remain scarce from some countries with extensive potato acreages, most notably Australia, Canada, USA, India and, probably, some parts of the former USSR.  PCN continues to be a quarantine pest throughout the world. 

PCN biology - impact on potato production

Symptoms of PCN attack on potatoes reflect those of plants with an inefficient roots system i.e. poor growth, wilting during periods of water stress, early senescence and reduced tuber yield up to levels in excess of 80%.  Although most PCN development occurs within the root system, the cyst mature outside the roots and then fall into the soil at harvest, making them easily transported within soil attached to tubers, especially in natural crevices such as tuber eyes.  Only under exceptional circumstances, e.g. at very high densities, will cysts be found directly attached to potato tubers. 

PCN control

The main route by which PCN spreads is through the movement of infested material, primarily soil which may be transferred with tubers, plants, waste material or farm machinery.  The higher the population of PCN in a field, the greater the risk of spreading it to other land.  However, if potatoes are grown in soil infested with PCN, the risk of transmission with tubers from such a crop can also be reduced by minimising the quantity of soil associated with those tubers.  Therefore, the key principles of PCN control are targeted at seed potatoes: ensuring that the land on which the seed is grown has been tested and the sample has been found to be free from PCN prior to planting and that a low tolerance (1%) is set for soil associated with seed potatoes for marketing. 

In the EU, one major step forward in PCN control from the old 1969 PCN Directive to the new 2007 Directive (2007/33/EC ) relates to the definition of seed potatoes.  For the purposes of the 1969 Directive, the legislative controls on PCN related only to seed potatoes that were marketed.  The 2007 Directive defines seed potatoes as any potatoes that will be planted, recognizing that the risk of spreading PCN relates to the movement of any planting material. 

PCN Resistance Testing

The 2007 EU PCN Directive requires Member States of the EU to submit annually a list of all new potato varieties which have found to be resistant to PCN by official testing.  Until 2010, all cultivars of potato entered for the UK National List VCU (Value for Cultivation and Use) trials have been tested for resistance to the dominant pathotypes of PCN occurring in the UK. Under the new Directive all official testing for resistance will be harmonised, including assessing resistance by a common scoring system based on a 1 to 9 scale.

In the UK, testing for resistance to Globodera pallida (pathotypes Pal & Pa2/3) is conducted at two centres: Edinburgh (SASA) and Cambridge (NIAB).  Testing for resistance to Globodera rostochiensis (pathotype Rol) is conducted at Edinburgh only.

Soil Testing

Potato cyst nematode (PCN) is the name commonly given to two species of cyst nematode which are serious pests of potato crops world-wide, namely Globodera pallida and Globodera rostochiensis. They feed on the roots of the plant and can cause significant loss of yield, including crop failure.  The cysts can survive in the soil for many years (over 25 years under favourable conditions), multiplying rapidly when a new crop of host plants is planted.

Taking effective action against PCN is vital to maintain supplies of both healthy seed potatoes for the ware industry in Scotland, and uncontaminated land for potato production.

The main methods of control are to ensure seed potatoes are produced in land free of PCN and, where PCN are found, to place restrictions on the use of the land and the disposal of crops, waste and soil to prevent the pest spreading.

The new EU PCN Directive

PCN have been subject to controls under European legislation since 1969. A revised PCN control Directive, came into force on July 1, 2010. This aims to strengthen and harmonise controls against PCN, taking account of changes in the understanding of the biology of the pest, its distribution across the EU and practices within the potato industry.

The Directive is implemented in Scotland by the Plant Health (Scotland) Amendment Order 2010, which was laid before the Scottish Parliament on May 21, 2010. Under the Plant Health Fees (Scotland) Amendment Regulation 2010, fees are now in place for PCN testing.

Under Directive 2007/33/EC, seed potatoes must only be planted on land which has been found to be free from PCN infestation following an official soil test. Failure to pass this test results in the land concerned being ‘recorded' as infested with potato cyst nematode. No seed potatoes may be grown in this land, either for inspection within the SPCS or for farm saved seed.  Ware potatoes may be grown, but only if an Official Control Programme is in place. The land remains ‘recorded’ as infested until such time as a future official tests show that PCN are no longer present.

Growers wishing to produce seed potatoes, either for classification or for farm-saved seed which will be planted other than at the place of production, are requested to make an application for a soil test by 31 August of the preceding year.  The application form and guidance for making an application are set out on the PCN Soil Testing documents page

The new PCN Directive sets out a harmonised protocol for soil sampling for use by all EU Member States.  Fields are sampled at a standard rate of 1500ml/ha or, if certain conditions are met which reduce the risk of PCN infestation, at a lower rate of 400ml/ha.  These conditions relate to history of the land, either in relation to previous potato crops or PCN soil tests.  Sampling is carried out by staff from the Scottish Government Agriculture and Rural Delivery Division Area Offices.

If you have any technical enquiries about soil sampling for PCN, please contact Jon Pickup or email the Zoology Team. If you are enquiring about the results of any soil samples that you have applied for, please contact Seed Potato Classification Scheme.

Laboratory testing for PCN

After sampling, the soil is sent to the Nematology Laboratory at SASA. The laboratory is the single Scottish centre for the examination of all official PCN samples.  Cyst nematodes are extracted from the soil samples using a carousel - an automated sieving and flotation technique.

Diagnoses from the extractions produced by the carousel are now carried out using a polymerase chain reaction (PCR) diagnostic technique developed at SASA for use on the floats from soil samples.  This method has the advantage of having been developed for high throughput and consequently provides the laboratory with the ability to provide PCN diagnoses much more quickly than the more traditional method of visual examination.

Certificates of clearance are issued via the team who administer the Seed Potato Classification
Scheme, who also deal with the recording of infested land.

If you have any technical enquiries about soil sampling for PCN, please contact Jon Pickup or email the Zoology Team. If you are enquiring about the results of any soil samples that you have applied for, please contact Seed Potato Classification Scheme

Peat Testing

Testing of Peat used for Seed Potato Production

Under the new PCN Directive 2007/33/EC, there is no longer a requirement to test samples of growing media (usually peat based) in which potato mini-tubers are grown, to confirm the absence of PCN.  The new PCN Directive only relates to the testing of 'field-soil' used for growing seed potatoes. 

No PCN have been found in tests of peat-based growing media carried out at SASA during the past 20 years, and it can therefore normally be considered to be "a pest-free growing medium" as required by the SPCS.  If pre-basic growers involved in mini-tuber production wish to obtain a test on, for example, a new delivery of the growing medium, SASA can provide this service on a commercial basis.  A fee of £14.00 plus VAT per sample of 400ml or less will be charged.

A representative 400ml sample should be submitted by the grower to the Nematology Laboratory at SASA.  At the laboratory, peat samples are dried and then processed by centrifugation in a glycerol, ethanol and kaolin mixture. The supernatant is then passed through two sieves. The residue from the second and finer sieve is transferred onto filter paper and then examined visually for the presence of PCN.  Results will normally be issued within 10 working days of receiving the sample.

If you have any technical enquiries about testing peat for PCN, please contact Jon Pickup or email the Zoology Team. If you are enquiring about the laboratory results for any peat that you have had tested, please contact the Zoology Team.

Consignment Testing for PCN prior to Export

Most countries require imported potatoes, if not all plant material and soil, to be free from Potato Cyst Nematodes (PCN). For seed exports, a phytosanitary certificate issued on the basis of a pre-crop soil test is a generally accepted means of assuring PCN freedom. As PCN cannot be detected by the visual inspection of harvested tubers, standard pre-export inspection for health and quality cannot confirm the presence or absence of cysts. Cyst detection at this stage necessitates laboratory examination.  Information on countries requiring pre-export consignment tests for PCN is available on the Potato Export Conditions part of the Scottish Government website.

The soil for a consignment test is collected from a representative sample of bags of tubers, by staff from the Scottish Government Agriculture and Rural Delivery Division Area Offices. The number of bags chosen is on a sliding scale, from a minimum of 5 bags at 5 tonnes, to 10 at 20 tonnes, with a further bag for each additional 10 tonnes. It is important to keep soil contamination to a minimum, although it can be difficult to reduce the amount of soil to the 'dusting' demanded by some countries. The Scottish Government operates to a maximum soil tolerance of 0.5% by weight.

At SASA, the sample is prepared for examination by using a two sieve method. Larger samples may be processed using a fluidising column, or Trudgill Tower. Detection of any cysts present is achieved by visual examination of the extracts from the sieves.

If PCN cysts are found, the lot concerned cannot be exported or treated as seed. However, further lots from the same crop can be considered for sale, providing they are cleared by further soil testing.

If necessary, consignments from ware crops can be given phytosanitary clearance by a post-harvest field test, taken exactly as the pre-crop soil test, or by a consignment test on soil riddled from harvested tubers.

Results from consignment tests are issued via the agricultural officer from the the local Area Offices who collected the sample.

If you have any technical enquiries about consignment testing for PCN, please contact Jon Pickup or email the Zoology Team.