Globodera pallida
(white potato cyst nematode)

Pictures

Picture	Title	Caption	Copyright
Title Second stage juvenile (anterior portion) Caption Second stage juvenile, anterior portion showing stylet and rounded knob shape. Copyright Janet A. Rowe	Second stage juvenile (anterior portion)	Second stage juvenile, anterior portion showing stylet and rounded knob shape.	Janet A. Rowe
Title Field symptoms Caption Globodera pallida (white potato cyst nematode); field symptoms on potatoes. Note yellowed and stunted plants. Copyright ©Florida Division of Plant Industry Archive/Florida Dept. of Agriculture & Consumer Services/Bugwood.org - CC BY 3.0 US	Field symptoms	Globodera pallida (white potato cyst nematode); field symptoms on potatoes. Note yellowed and stunted plants.	©Florida Division of Plant Industry Archive/Florida Dept. of Agriculture & Consumer Services/Bugwood.org - CC BY 3.0 US
Title Cyst Caption Slide of whole cysts. Copyright Janet A. Rowe	Cyst	Slide of whole cysts.	Janet A. Rowe
Title Vulval cone of G. pallida Caption G. pallida vulval cone showing vulva, cuticular ridges, anus and punctations in cutical. Copyright Janet A. Rowe	Vulval cone of G. pallida	G. pallida vulval cone showing vulva, cuticular ridges, anus and punctations in cutical.	Janet A. Rowe

Identity

Preferred Scientific Name

• Globodera pallida Stone 1973

Preferred Common Name

• white potato cyst nematode

Other Scientific Names

• Globodera pallida (Stone, 1973) Behrens, 1975
• Heterodera pallida Stone, 1973

International Common Names

• English: pale potato cyst nematode; potato cyst nematode; potato root eelworm
• Spanish: nematodo quiste blanco de la papa
• French: nématode blanc de la pomme de terre

Local Common Names

• Germany: Aelchen, Cremefarbenes Kartoffelzysten-; Nematode, Weisser Kartoffel-

EPPO code

• HETDPA (Globodera pallida)

Summary of Invasiveness

G. pallida originates from the Andes and is known to be present in 55 countries. It is found predominantly in temperate regions as is the related species, Globodera rostochiensis. It is possibly more difficult to manage than G. rostochiensis because there is currently less resistance available in commercially-grown potato cultivars.

Egg-laden cysts are the most environmentally resistant and easily transportable stage in the parasites life cycle, and are found in soil particles, on host roots, stolons or tubers. The microscopic size of the cyst makes it difficult to detect, and it can successfully establish new infestations when an appropriate climate and host plant are available.

Machinery used on infested land followed by use in otherwise uninfested areas is a common method of spread, for example, fumigation equipment that has not been cleaned before use in another area. Disinfection of farming tools, transport and clothing helps to keep uninfested land free from G. pallida. Wind, rain and flood water are also capable of redistributing viable cysts to create new infestations.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Nematoda
•             Class: Secernentea
•                 Order: Tylenchida
•                     Family: Heteroderidae
•                         Genus: Globodera
•                             Species: Globodera pallida

Notes on Taxonomy and Nomenclature

Heterodera pallida was first described by Stone in 1972 (published in 1973), originally considered to be a pathotype of Heterodera rostochiensis Wollenweber 1923, and has only white or cream, not yellow, females. Other workers had also noted this difference (Guile, 1966, 1967, 1970) and that many of the distinguishing features of the cream and white pathotypes were generally larger than those of the yellow (Evans and Webley, 1970). H. pallida was described from two localities: Epworth in Lincolnshire, England, representing the Pa3 pathotype, and Duddingston, Scotland, representing the Pa1 pathotype. H. pallida is thought to have originated in the Andes mountains of South America and has been detected growing on Solanum acaule on the pre-Columbian agricultural terraces of Peru (Jatala and Garzon, 1987).

To accommodate the potato cyst nematodes and related species having round cysts, Skarbilovich (1959) erected the subgenus Globodera which was later elevated to generic status by Behrens (1975).

Most literature, records and data referring to Heterodera rostochiensis before 1972 could also have been referring to H. pallida.

Description

Eggs

The eggs of G. pallida are retained within the cyst and no egg sac is produced. The surface of the eggshell is smooth; no microvilli are present.

Measurements of the egg fall within the range: 108.3 ± 2.0 µm × 43.2 ± 3.2 µm.

Female

Female measurements: stylet length = 27.4 ± 1.1 µm; head width at base = 5.2 ± 0.5 µm; stylet base to dorsal gland duct = 5.4 ± 1.1 µm; head tip to median bulb valve = 67.2 ± 18.7 µm; median bulb valve to excretory pore = 71.2 ± 22 µm; head tip to excretory pore = 139.7 ± 15.5 µm; mean diameter of the vulval basin = 24.8 ± 3.7 µm; length of vulval slit = 11.5 ± 1.3 µm; anus to vulval basin = 44.6 ± 10.9 µm; number of ridges on the anal-vulval axis = 12.5 ± 3.1.

The female has an almost spherical body from which the neck and head protrude. G. pallida females are either white or cream, depending on pathotype, when they break through the cortical root cells, and this phase lasts for 4-6 weeks. There is never a golden or yellow stage as in Globodera rostochiensis. The head bears one to two annules and the neck is covered in ridges between which tubercles are found when viewed by SEM. The head skeleton is hexaradiate and weak. The stylet is equally proportioned (50% conus and 50% shaft), and the basal knobs reflex in an anterior direction. The median bulb is very well developed and has a large crescentic pump. The oesophageal gland lobes are often displaced into a forward position as the large paired ovaries expand in the body cavity. The excretory pore is located at the base of the neck. The vulval slit of the female is found in the vulval basin, a round depression at the opposite pole of the body to the neck. The vulval slit is surrounded by a translucent area of thin cuticle which bears papillae.

Cyst

The tough, hardened cuticle of the dead female tans to a deep brown colour and acts as a protective bag around the embryonated eggs, which will form the next generation of G. pallida. The dimensions of the cyst are almost identical to those of the female, although the head is usually lost. The features of the cyst fenestrae are important in morphologically-based identification. The fenestrae have normally been lost by the mature cyst stage so that only a hole remains. The anus is generally conspicuous. New cysts may retain the remnants of a thin subcrystalline layer. All Globodera species are abullate, but occasionally small rounded brown pigmented bodies are found, and these are termed vulval bodies.

Cyst measurements: width = 534 ± 66 µm; length, excluding neck = 579 ± 70 µm; neck length = 188 ± 20 µm; mean fenestral diameter = 24.5 ± 5.0 µm; anus to fenestra distance = 50 ± 13.4 µm; Granek's ratio (Granek, 1955) = 2.2 ± 1.0.

Male

The male is vermiform and usually assumes an open C shape upon death and fixation, the short rounded tail twisting through 90-180°. The body annules are regular along the body and there are three bands in the lateral field, which narrows both posteriorly and anteriorly. The head skeleton of the male is heavily sclerotised and hexaradiate, and the head is offset and bears 6 or 7 annules. The anterior cephalids are located at head annules 2 and 4 and posteriorly at annules 6 to 9. The stylet is strong with backward sloping knobs. The median bulb is rounded and the crescentic valve strong. The oesophagus is encircled by the nerve ring. The dorsal oesophageal gland nucleus is the most prominent of the three gland nuclei and the lobe itself extends almost to the excretory pore. The hemizonid is two body annules wide and two body annules behind the excretory pore. There is a single testis which extends for about 60% of the body length. The arcuate spicules have single points and some workers have separated the species G. pallida and G. rostochiensis using spicule measurements (Behrens, 1975). The gubernaculum is small and without ornamentation.

Male measurements: body length = 1200 ± 100 µm; body width at excretory pore = 28.4 ± 1.0 µm; head width at base = 12.3 ± 0.5µm; head length = 6 ± 0.3µm; stylet length = 27.5 ± 1.0 µm; stylet base to dorsal gland duct opening = 3.0 ± 1.0 µm; head tip to median bulb = 66 ± 7.1 µm; median bulb to excretory pore = 81.0 ± 11 µm; head tip to excretory pore = 176.4 ± 14.5 µm; tail length = 5.2 ± 1.4 µm; tail width at anus = 13.5 ± 2.1 µm; spicule length = 36.3 ± 4.1 µm; gubernaculum length = 11.3 ± 1.6µm.

Juvenile

The second-stage juvenile, the infective stage in the life cycle, hatches directly from the egg where it has already undergone a moult. The juveniles of G. pallida and G. rostochiensis are very alike, but G. pallida juveniles are generally larger: the body length is greater and the stylet is longer and more robust, with anteriorly facing stylet knobs as opposed to those of G. rostochiensis, which have smaller, backward-sloping knobs. The juvenile is folded four times within the egg and the tail tapers to a rounded point. The body cavity extends half way along the tail, ending at the anus. The hyaline tail region is about 20 µm in length. There are four incisures (i.e. 3 bands) along the body length. The head is offset, rounded and bears 4-6 annules. The head skeleton is strongly sclerotised and hexaradiate. The cephalids are located at body annules 2 and 3 and posteriorly at annules 6 to 8. The stylet is well developed as are its basal knobs, which project anteriorly in lateral view. The nerve ring encircles the oesophagus and the excretory pore is about 110 µm from the head. The hemizonid is found just before the excretory pore and is about 2 annule widths long. The hemizonion is 5-6 annules behind the excretory pore. The genital primordium is located 60% of the body length from the head tip.

Juvenile measurements: body length = 486 ± 2.8 µm; body width at excretory pore = 19.3 ± 0.9 µm; stylet length = 23.0 ± 1.0 µm; stylet base to dorsal gland duct = 5.3 ± 0.9 µm; head tip to median bulb valve = 68.7 ± 2.7 µm; head tip to excretory pore = 108.6 ± 4.1µm; tail length = 51.1 ± 2.8 µm; tail width at anus = 12.1 ± 0.4 µm; length of hyaline terminus = 26.6 ± 4.1µm.

Other measurements can be found in Granek (1955), Spears (1968), Green (1971), Greet (1972), Golden and Ellington (1972), Hesling (1973, 1974), Mulvey (1973), Behrens (1975), Mulvey and Golden (1983), Othman et al. (1988) and Baldwin and Mundo-Ocampo (1991).

 

Distribution

See also CABI/EPPO (1998, No. 161). The centre of origin of the species is in the Andes Mountains in South America, from where it has spread with the introduction of potatoes to other regions. The present distribution covers temperate zones down to sea level and in the tropics at higher altitudes. In these areas, distribution is linked with that of the potato crop.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

Potato cyst nematodes are indigenous to the Andean regions of Peru and Bolivia. Their centre of origin is purported to be in the area surrounding Lake Titicaca. Most hosts of PCN are from the family Solanaceae as well as sources of resistance to it: these too are found usually in the same geographic areas, which would support this theory.

How the transfer of PCN to Europe and beyond was expedited is considered most likely to be from contaminated seed potatoes and soil adhering to them during the 1850s. Although potatoes had previously been imported to both England and to Spain in the mid sixteenth century by different routes, no records or reports exist of damage by PCN at that time. Potato became an important food source particularly in the warmer regions of Europe. Pre-seventeenth century the potato yield was smaller and the plants preferred a shorter day length, but by the eighteenth century potatoes had become commonly grown throughout Europe, parts of Asia and the British colonies. The potato famine caused by potato blight brought about a search for resistance to it and involved the revisiting of South America to search out new genetic stock resistance to blight, and it is thought most likely that with the new consignment of potato stocks came the potato cyst nematode.

In 1881, Kühn recorded finding cyst nematodes on potatoes, which he referred to as Heterodera schachtii; this is a time lag of about 30 years from the introduction of the new genetic material from South America to the distribution and damage of PCN reaching noticeable levels. In1923, Wollenweber diagnosed the potato cyst as a separate species Heterodera rostochiensis, and in 1972 Stone identified a second species of potato cyst nematode having a white female form and a significantly different biology and morphology, described as Heterodera pallida. Prior to this date all PCN were referred to as Heterodera rostochiensis.

With travel to all parts of the globe, various introductions at different points in time must have occurred, and are still happening to-day. For example a recent outbreak of Globodera pallida has been recorded from Idaho, USA (O’Dell and Hoffman, 2006).

Potato cyst nematodes are still being transferred from country to country. This most resistant pest takes time to build up, usually from one point in a field to a small patch, then if unnoticed by dispersal mechanisms such as wind, water, or soil movement usually by machinery or human intervention. This demonstrates that continued vigilance is required to keep this pest under control.

Introductions

Risk of Introduction

Potato cyst nematodes are A2 quarantine pests for EPPO. They are also of quarantine significance for APPPC and NAPPO. Virtually all areas within the EPPO region that grow potatoes are already contaminated with potato cyst nematode. These areas are generally closely monitored. It is important to keep seed potato areas free of potato cyst nematode. Domestic measures and import controls are justified as they help to reduce spread and introduction of new pathotypes into already colonised areas. Globodera rostochiensis still seems to be the dominant species throughout Europe with the exception of England, UK, where G. pallida is common.

Phytosanitary measures

To prevent further spread of potato cyst nematode into uninfested areas, several methods are used. These include international and national legislation on the movement of seed potatoes, nursery stock, flower bulbs and soil (CEC, 1969).

The specific EPPO quarantine requirements (OEPP/EPPO, 1990) for these nematodes are that fields in which seed potatoes or rooted plants for export are grown are inspected by taking soil samples according to an EPPO-recommended method (OEPP/EPPO, 1991; EPPO, 2007) and must be found free from viable cysts of both species of potato cyst nematode. The sampling must be performed after harvest of the previous potato crop.

Habitat List

Hosts/Species Affected

The major hosts of G. pallida are restricted to the Solanaceae, in particular potato, tomato and aubergine (Ellenby, 1945; 1954; Mai, 1951; 1952; Winslow, 1954; Stelter, 1957; 1959; 1987; Roberts and Stone, 1981; Sullivan et al., 2007). Oxalis tuberosa has been extensively tested in host range tests by Sullivan et al. (2007) with constant negative results and has been declared a non-host on this basis.

Host Plants and Other Plants Affected

Growth Stages

Symptoms

List of Symptoms/Signs

Biology and Ecology

A cyst of G. pallida (PCN) contains as many as 500 eggs, which will form the next generation. The eggs can remain viable for many years before gradually deteriorating. Factors affecting hatching are very similar to those that affect Globodera rostochiensis. Soil moisture content has an important effect on both the movement of the second-stage juveniles and, consequently, their use of lipids; adequate soil moisture and lipid levels are both necessary for root infection. Various hatching stimuli are known, for example, root diffusate (Perry and Beane, 1988) and certain organic and inorganic chemical compounds (Clarke and Hennessy, 1987). G. pallida hatches at around 10°C or less and is adapted to develop at cool temperatures between 10 and 18°C, whereas G. rostochiensis seems to be adapted to a temperature range of 15 to 25°C (Franco, 1979). Day length also influences egg hatching, which is faster where the host has continuous light rather than prolonged hours of darkness (Hominick, 1986).

As with other cyst nematodes, the second-stage juvenile is the infective stage and, upon hatching, invades the host just behind the root tip. The juveniles move up or down the root until they receive a specific signal, which is likely to be of a chemical nature, to set up a feeding site in the form of a syncytium. The ultrastructure of the feeding site and nematode interaction with the syncytium have been studied intensively (Wyss and Zunke, 1986; Golinowski et al., 1997; Endo, 1998). The life cycle takes around 45 days to complete, during which time the second-stage juvenile develops into a male or a female whose survival depends on environmental factors such as available nutrients. Juveniles that penetrate the pericycle cells of the plant are more likely to become males, whereas those that penetrate the procambial cells tend to become females (Golinowski et al., 1997). Studies of G. rostochiensis juveniles have shown that, after a few hours of inactivity, the juvenile probes the selected cell and inserts its stylet into it while remaining motionless for several hours; the stylet is withdrawn and re-inserted into the same cell. A secretory product from the oesophageal glands is injected via the stylet into the selected cell. The initial syncytial cell is altered to provide large amounts of nutrients to the developing nematode. The syncytium undergoes major changes, for example lysis of inner cell walls, formation of cell wall ingrowths next to plant conductive tissue, formation of numerous lipid bodies and enlarged amoeboid nuclei are present. Some of these events are still not fully understood. In resistant plants, the juvenile may try to form a syncytial feeding site but the walls of cells involved usually thicken and cells may die. This prevents the ready movement of nutrients to the juvenile (Rice et al., 1986; Robinson et al., 1988).

Males are relatively more abundant when environmental conditions are poor as they require less food than females and do not feed during the free-living stage (Evans, 1970). The vermiform male has a short life span of ten days or so and, during this time, it will mate with as many available females as possible. At this point in the life cycle, the white or cream females have become obese and broken through the root cortex, exposing their genitalia. The females secrete pheromones which attract males (Green and Miller, 1969; Green and Plumb, 1970; Mugniery, 1979; Mugniery et al., 1992).

Numerous studies on the biochemistry of both G. rostochiensis and G. pallida have been published since isoelectric focusing was first used to display different protein profiles for G. rostochiensis and G. pallida (Fleming and Marks, 1983). This is a quick, dependable and relatively inexpensive method of identification of the species of potato cyst nematodes. Other techniques are capable of greater discrimination, such as pathotypes within a population (Hinch et al., 1998). Monoclonal antibodies have also been used to develop diagnostic procedures based on ELISA (Curtis et al., 1998). DNA-based techniques are used routinely with a range of methods and species-specific primers (Mullholland et al., 1996; Fullaondo et al., 1999) now being used with diagnostics applications such as RAPD, RFLPS, AFLPS and multiplex PCR, which have become less expensive and more user friendly. With the many kits now designed to help with the methodology, even greater discrimination is possible. Sequencing of DNA is now undertaken in many laboratories not only to identify to species, but also in studying plant-nematode interactions with a view to developing better control methods (Perry and Jones, 1998).

Disruption of the nematode life cycle may be possible through the insertion of genetic constructs coding, for example, enzyme inhibitors into the plant by genetic engineering (Burrows, 1996; Burrows and De Waele, 1997). Studies have also been made of nematode secretions and their function and interaction within the host plant (Jones and Robertson, 1997; Jones and Harrower, 1998). The role of cuticular secretions in plant parasitic nematodes is not yet fully understood (Forrest et al., 1989; Jones et al., 1997). Changes occur in the cuticle of second-stage juveniles when they begin to feed: in addition to changes associated with natural growth patterns and the onset of moulting, changes occur that appear to be linked to interaction with the plant host (Jones and Robertson, 1997).

Molecular techniques have now advanced so far as to enable the functions of individual genes to be studied. Gp-FAR-1 has been identified from the surface of G. pallida and  is considered to be involved in eluding the defence system of the host plant (Prior et al., 2001). More recent studies on other genes identified from G. rostochiensis have highlighted the complexity of host penetration and the function of pectate lyases utilised by PCN for parasitism (Kudla et al., 2007). RNA interference (RNAi)  has been used in a range of studies and a recent review of progress in this area (Lilley et al., 2007) provides details of target genes in plant parasitic nematodes including PCN. Other biochemical studies have investigated the action of nematode proteinases and their possible involvement in extracellullar digestion (Koritsas and Atkinson, 1994; Lilley et al., 1996). Proteinase genes have been found in Caenorhabditis elegans (Sarkis et al., 1988) and Haemonchus contortus (Pratt et al., 1990). Proteinase inhibitors expressed in plants may have the potential to control nematodes (Hepher and Atkinson, 1992). There are marked differences in the secretions from the two species of potato cyst nematodes, namely G. pallida and G. rostochiensis (Duncan et al., 1997) and lectin blotting with WGA (wheat germ agglutinin) shows further differences: there were no differences between populations of G. rostochiensis, but there were between populations of G. pallida.

Climate

Latitude/Altitude Ranges

Air Temperature

Natural enemies

Means of Movement and Dispersal

Potato cyst nematodes are microscope in size and are most easily dispersed with the movement of seed potato and /or soil from one place to another, both locally and internationally. The distribution of this pest via contaminated machinery, animal movement from field to field or human means can have disastrous consequences. Phytosanitary measures are vital to integrated pest management schemes, and inspection of seed potato is particularly important to stop the spread of PCN to “clean” areas and countries. 

Natural Dispersal (Non-Biotic)

Natural dispersal is generally slow as cyst nematodes tend to grow in patches and are only moved around by soil disturbance. The most common means of natural dispersal are via run-off from flooded fields, the water carrying the very resilient cysts to adjoining fields, where given favourable conditions and a host plant new infestations will occur. Wind during dust storms can lift soil and cysts and deposit them into new areas spreading infection. 

Vector Transmission (Biotic)

Cysts can survive unfavourable conditions for some years due to the hard cuticle which protects the eggs. Cysts can pass through the gut of animals without damage and once excreted, provided conditions are favourable, they can begin a new infestation. 

Accidental Introduction 

The unintentional infestation of new introductions was probably the way most PCN was and still is transferred. Potato has continued to be both a nutrionally and economically important crop. Potato tubers are still imported into many countries and without stringent regulations and personnel that are well trained to recognise potential pathogen problems there will always be a risk of new infestations arising.

Pathway Causes

Pathway Vectors

Plant Trade

Impact Summary

Economic Impact

Introduction

For potato cyst nematodes (PCN) in the UK, loss is estimated at around £50 million per annum and for Europe, several times this amount. Potatoes are one of the top five important food and cash crops. PCN cause extensive damage, particularly in temperate areas and particularly when virulent pathotypes occur and any resistance has failed. The situation is worse with G. pallida, where commercial cultivars with good resistance are still few and often have undesirable characteristics. Damage is related to the number of viable eggs per unit of soil, and is reflected in the weight of tubers produced. One of the first models to describe this relationship was that of Seinhorst (1965). This model has subsequently been improved upon and adapted to take into consideration the various soil types, environments and population densities that occur in field situations (Been et al., 1995.) Studies by other workers (Elston et al., 1991; Phillips et al., 1991) have also provided information on how nematodes cause yield losses.

Direct and Indirect Losses

In the UK, losses due to G. pallida and Globodera rostochiensis were estimated by Brown and Sykes (1983). At densities of 0-24 eggs/g soil, losses were 6.25 t/ha per 20 eggs/g soil. At 40-160 eggs/g soil, losses were 1.67 t/ha per 20 eggs/g soil. The maximum crop loss was 22 t/ha.

In the Netherlands, potato yields decreased from 45 t/ha at pH 4.5 to 33 t/ha at pH 6.5. Nematode densities decreased from ca. 18 to 9 juveniles/g soil. In a container experiment, tuber yields were ca. 11% lower at pH 6.5 than at pH 4.5 in the absence of nematodes, but ca. 44% lower when an initial population of 27 juveniles/g soil was present (Haverkort et al., 1993).

In Norway, continuous cropping of susceptible potato cultivars on land heavily infested with G. pallida and G. rostochiensis resulted in an average yield loss of 50-60%. Yields were increased by the use of resistant crops even in the first year of cropping (Oeydvin, 1978).

In Germany, the use of nematicides reduced nematode populations and lead to increases in yield of nematode-susceptible cultivars. Combinations of nematicides, which cost ca. DM 1000/ha more than individual compounds, lead to further increases of ca 10% in tuber yields (Lauenstein, 1992).

In Italy, the relationship between numbers of G. pallida and yield of potato tubers was investigated in microplot trials in 1981. A tolerance limit of 1.7 eggs/g soil was derived (Greco et al., 1982).

In Southern Spain, G. pallida occurred at 48% of sites when 96 fields over an area of 2400 ha were sampled. Losses in potato yield of nearly 80% occurred as a result (Talavera et al., 1998).

In Russia, problems with Globodera species have been reported on a total area of 41,250 ha. The potential yield losses in areas of high infestation were 70-80% or more (Vasyutin and Yakoleva, 1998).

In Bulgaria, under experimental field conditions, the minimum initial population density affecting yield loss was 1 egg or juvenile/g soil (Samaliev and Andreev, 1998). Crop rotation was also shown to influence population density of G. pallida (Samaliev, 1998). When non-host crops were cultivated for 2 or 3 consecutive years, potato yield increased by 2.7 and 3.4 times. Nematode soil populations declined by 54 and 91%, respectively.

In India, losses due to nematode species including G. pallida are estimated to range between 5 and 10% (Misra and Agrawal, 1988). In Tamil Nadu, a 1987 survey showed that a mean level of G. pallida and G. rostochiensis on susceptible cultivars of 2.7 (3.8 females/2.5 cm root), resulted in about a 30% yield loss (Subramaniyam et al., 1989).

In Panama, average crop losses due to G. pallida, G. rostochiensis and Meloidogyne were estimated to be 10-30% (Pinochet, 1987).

In Canada, the discovery of G. pallida in 1977 followed that of G. rostochiensis in 1962 in Newfoundland. Since then, about $Can 800,000 a year has been spent on control measures and research (Miller, 1986). 

In the USA, G. pallida was identified for the first time from several fields in Bonneville and Bingham Counties, Idaho, during the spring and summer of 2006. This finding has prompted an eradication programme that will be in place for 5 to 7 years to prevent the spread of PCN to other potato-growing areas. Some $11 million have been allocated to financing all the measures required to eradicate and control the nematode.

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Highly mobile locally
• Long lived
• Fast growing
• Has high reproductive potential
• Has propagules that can remain viable for more than one year
• Has high genetic variability

• Changed gene pool/ selective loss of genotypes
• Host damage
• Increases vulnerability to invasions
• Modification of nutrient regime
• Negatively impacts agriculture
• Negatively impacts cultural/traditional practices
• Negatively impacts human health
• Negatively impacts animal health

• Causes allergic responses
• Pest and disease transmission
• Interaction with other invasive species
• Parasitism (incl. parasitoid)
• Pathogenic

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

Uses List

General

• Laboratory use
• Research model

Genetic importance

• Test organisms (for pests and diseases)

Detection and Inspection

Detection based on host plant symptoms and identification by morphological and molecular methods are detailed in OEPP/EPPO (2009).

Surveys

Field samples are taken to check if potato cyst nematode is present or not, and to determine which species is present and in what quantities. This information is required in order to plan the management of any infestation. In the past, nematodes were thought to have a random distribution throughout a field, whereas it now seems likely that distribution is aggregated. Statistical models have been used to assess the existing and potential distribution of potato cyst nematode in fields, but geostatisical models and techniques may provide more definitive information on spatial distribution. See Been and Schomaker (2006).

Similarities to Other Species/Conditions

G. pallida, a sibling species of G. rostochiensis, can be distinguished by the colour of the mature female: most other species of the genus have yellow to golden females, whereas G. pallida has white or cream females, depending on the pathotype. In general, the second-stage juveniles of G. pallida are longer, have a more robust-looking stylet with forward pointing basal knobs, and have longer true tail lengths than other species in the genus.

G. pallida, G. rostochiensis and the G. tabacum complex, which includes G. tabacum (Lownsbery and Lownsbery, 1954), G. solanacearum (Miller and Gray, 1972) and G. virginiae (Miller and Gray, 1968) plus the subspecies G. 'mexicana', incompletely described by Campos-Vela (1967), share an extensive host range mainly composed of solanaceous species, with the G. tabacum complex more commonly found on the spiny species whereas potato cyst nematodes have a preference for tuber-forming species. The G. tabacum complex is mainly confined to North, South and Central America. G. 'mexicana' appears to have greatest affinity with G. pallida and these species are able to interbreed (Mugniery, 1978).

Other Globodera species, such as G. artemisia (Eroshenko and Kazachenko, 1972), are found only on Compositae hosts. G. artemisia can be distinguished morphologically, having a very small Granek's ratio and second-stage juveniles with backward sloping stylet knobs. Globodera leptonepia (Cobb and Taylor, 1953), a species found just once in soil with a consignment of potatoes, has seven folds in the second-stage juvenile within the egg, whereas G. pallida has only four.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

SPS Measures

Legislation provides a definitive set of rules (MAFF, 2000) designed to prevent the spread of potato cyst nematodes. The movement of soil is a common factor in the spread of potato cyst nematode, whether this is between countries, between farms or between sites in one field. Potato cyst nematodes probably spread to Europe through trade and the movement of potato tubers and soil adhering to them from other continents. Most countries have quarantine organizations that inspect potato shipments to protect their own countries from inadvertent importation of pests and diseases.

Physical barriers also tend to isolate pests in local areas, for example, deserts and rivers may isolate pests. Trade is the most common probable simple cause of new infestations of plant parasitic nematodes (Parker, 2000). Farmers are made are made aware of the risk of moving contaminated matter from one site to another on machinery, and of the need for preventive measures such as not sharing machinery between farms without thorough washing, cleaning and disinfestation first. Some natural movement cannot be controlled, for instance topsoil blown by strong winds from field to field, or infected topsoil moved by flood water onto clean sites. Animal manure has also been shown to contain viable cysts; the ingestion of the cyst appears not to impair its reproductive ability so can provide the means for new infestations to arise.

The following prevention methods can be used:

1. Check that machinery is thoroughly clean and free from plant debris.
2. Do not return soil that may contain potato cyst nematode to fields.
3. Clean the soil from potato tubers and have the soil tested to be sure that potato cyst nematodes are not transferred.
4. Make sure that the agency that tests the soil is competent and tests 500 g of soil per sample.
5. Grow susceptible and resistant potato cultivars alternately, thus reducing the possibility of selecting a highly virulent or new pathotype.

Crop Rotation

Crop rotation is frequently used to reduce population densities of potato cyst nematode. The major commercial hosts of the two potato cyst nematode species are in the plant family Solanaceae, namely potato, tomato and aubergine, all important cash crops. Where these crops are grown in monoculture for several seasons in infested soil, nematode densities increase to extremely high levels and crop yields become uneconomic. To reduce nematode population densities, non-host crops such as barley are grown between host crops (Whitehead, 1995). The length of the rotation and the crops used can have variable effects on the yield and the potato cyst nematode density. In South American countries, slightly different regimes are used and may include fallow, with intervening crops of lima beans, maize, barley or wheat. The annual decline rate in soil of G. pallida is, in general, slower than that of Globodera rostochiensis. When the reduction of potato cyst nematode is too slow by rotation alone, other additional methods can be used; for example, trap cropping can hasten the reduction of population densities.

Trap Cropping

Trap cropping is a simple method that has been used successfully for the reduction of cyst nematode populations (Halford et al., 1999). Sufficient crop growth time is allowed for the nematodes to penetrate the roots and develop into young adults (5-6 weeks), but not enough time for them to form new eggs. Potato cyst nematode populations can be reduced very quickly as long as the grower removes and destroys the crop, including the nematodes in the roots. If left too late, the nematode density will increase but, if the crop is removed in time, the nematode density is reduced and there will be a significant yield benefit for any subsequent potato crop. G. pallida can be reduced by as much as 80% per annum and even greater reductions were found when ethopropos was used in the soil before the first of two potato trap crops; almost 100% control was achieved (Mugniery and Balandras, 1984). The use of other non-tuberous solanaceous plant species to stimulate hatching has proved very efficient in Dutch studies. Non-hosts that are well adapted to temperate conditions, in this case Solanum sisymbrifolium, proved capable of inducing large hatches of potato cyst nematode juveniles. Solanum sisymbrifolium is fully resistant to potato cyst nematode, therefore eliminating the risk of increasing potato cyst nematode density (Scholte, 2000). 

The use of 10 potato clones as trap crops has been tested in field trials in Northern Ireland (Turner et al., 2006) and their potential for the organic market has also been shown. Non-hosts that are well adapted to temperate conditions, in this case S. sisymbriifolium, proved capable of inducing large hatches of potato cyst nematode juveniles. S. sisymbriifolium is fully resistant to potato cyst nematodes, thus eliminating the risk of increasing potato cyst nematode density (Scholte, 2000). A note of caution: it is important to use the correct seed accession number as Stelter (1987) recorded lines no.72 and 121 as poor hosts, although producing fewer than 5 cysts per pot.

Soil Solarization

Solarization is a good method of killing nematodes in hot climates. The soil is covered with two layers of polyethylene sheeting, allowing the soil underneath to heat up to temperatures of 60°C or more. In cooler climates solarization is much less effective.

Resistant Cultivars

Certain European cultivars of potato have resistance (often only partial) to European pathotypes of potato cyst nematode but some South American populations are more virulent than European populations and are able to overcome the resistance in European cultivars (Kort and Jaspers, 1973; Turner et al., 1995). Thus, strict quarantine measures remain essential. Studies at the International Potato Centre (CIP) in Lima, Peru, on 3000 accessions of potato, have focused on the two locally predominant pathotypes of G. pallida: P4A and P5A. A more virulent pathotype (P6A) has emerged and been selected in some areas. Variety of cultivars (tolerant, resistant and partially resistant) has an important part to play in maintaining an acceptable balance of pathotypes. For example, Maris Piper is a popular potato cultivar in the UK and has full resistance to UK populations of G. rostochiensis but no resistance to G. pallida. Its widespread cultivation has selected G. pallida and it is now recognized that other types of cultivars (e.g. tolerant ones) should be included in the rotation to avoid this problem. Some potato cultivars with high resistance to G. pallida are grown in Europe and others are currently being developed. For example, Karaka (Anderson et al., 1993) and Gladiator (Genet et al., 1995) both have high resistance to both species of potato cyst nematode. Other cultivars with desirable qualities are listed in Whitehead (1998). UK populations of G. pallida are genetically more diverse than UK populations of G. rostochiensis. Resistance to G. pallida is usually polygenic and only partial, as in the cultivars Santé and Morag. Another problem in introducing new cultivars of potato is persuading retailers and consumers of the value and desirability of those cultivars.

Biological Control

Biological control agents active against potato cyst nematode are currently the subject of intense study. Although several parasites of eggs and females have been identified, none has given consistent control (Crump, 1987). Soils suppressive to potato cyst nematode have been identified (Roessner, 1986; Crump, 1998) and the fungal causal agents isolated. Selected isolates of Verticillium chlamydosporium, Paecilomyces lilacinus and Acremonium sp. show considerable potential and methods for their production, formulation and application are being evaluated. Natural parasites and biological control are being studied in order to identify natural agents for potato cyst nematode control, without needing to use the toxic chemicals currently in use. These methods will integrate with a variety of strategies such as trap cropping and rotation in sustainable management systems. This work began in the late 1930s (Linford et al., 1938) and still continues (Crump and Flynn, 1995; Segers et al., 1996).

The majority of studies in the late 1990s have concentrated on the fungal control agents Verticillium, Hirsutella and Arthrobotrys, and the bacterium Pasteuria. Several workers (e.g. Roessner, 1986) have studied biological control of G. rostochiensis in pots and in vitro. This work is difficult to transfer to the field for several reasons. For example, the form and volume in which to apply the control agent must be decided, and how the inoculum reacts to the microbial population already present in the field must be determined. Verticillium chlamydosporium will infect young females in pots but is less effective when potatoes are grown in the presence of low nematode population densities.

Very few data are available on the effectiveness of biological control in the field. This is due in part to the logistics of such operations, such as producing sufficent inoculum. It is also known that some tests do not produce the expected results for reasons as yet undefined, but they are probably related in some way to the physiology and ecology of the host parasite relationship. Progress in the area of biological control requires a better understanding of the population dynamics of potato cyst nematode and their parasites (Davies, 1998; Davies et al., 1991). A number of factors interact, such as plant host, the action of root exudates, soil type and the mode of parasitism of the control micro-organism at any one point in time. Also, potato cyst nematode may be more susceptible to infection by any one fungus at different points in its life- cycle. For example, the three major fungal parasites V. chlamydosporium, Fusarium oxysporum and Cylindrocarpon destructans, have all been detected at different times in the nematode life cycle but the most active of the three depends on the life-stage present (Crump, 1987). 

The fungi, Paecilomyces lilacinis, Pochonia chlamydosporia and Monographella cucumerina may be used to aid control in PCN infested areas, but not used as the only method of control as field data suggests that the reduction in PCN populations is only around 60%. M. cucumaria is available commercially (MeloCon WG and BioAct WG - approved in the USA).

Some isolates of the bacteria Pasteuria penetrans can reduce populations of PCN. Applications are made during rotation of non-potato crops or before sowing potato to trigger a reduction in population density. Pasteuria penetrans is commercially available for root-knot nematodes. (Nematech Ltd., Tokyo).

Integrated pest management schemes benefit from the inclusion of a biocontrol agent, but, to date, no biocontrol agent can offer full protection on its own. Mutualistic bacteria and fungal endophytes are probably common in the agroecosystem (Sikora, 2007) but, to exploit potential candidates, a necessary objective for the future, is to understand the complicated manner in which they interact. The need to identify the mechanisms of control, how they function and then to scale up the technology for use commercial use in the field will take time.

A novel biological nematicide DiTera®, produced by Valent Biosciences Corp., USA, which has already been shown to control other plant parasitic nematodes in the field such as Xiphinema spp. and Radopholus spp., seems to have the capability to prevent hatching of potato cyst nematode in a specific manner. The specificity is linked to the permeability of the eggshell membrane. After testing DiTera® in solutions of 1 to 10 %, all were found to inhibit the hatching of the eggs. Meloidogyne incognita was used as a control and hatching of its eggs was not inhibited by DiTera® (Twomey et al., 2000).

Chemical Control

Kuhn first used chemicals as a nematode control method in 1881, but chemicals were not used extensively for this purpose until 1943 (Carter, 1943). The first attempts used D-D (1,2-dichloropropane - 1,3-dichloropropene mixtures). This led to other chemical control agents, such as chloropicrin.

The oximecarbamates are more effective in the early stages of plant growth, as the nematodes must hatch first. G. pallida is more difficult to manage than G. rostochiensis because it hatches over a longer period of time and can therefore escape as nematicidal activity is gradually lost (Whitehead,1975).

In 1991, Mazin suggested that chemical control is more effective where resistant cultivars are used. Nematicides do not prevent PCN build up, merely avoid yield loss (Tiilikkala, 1991). Fumigant nematicides are toxic and expensive, but their use is sometimes essential to keep PCN population densities low or beneath the damage threshold (ca 2 eggs/g of soil). Soil fumigants can kill large numbers of nematodes, especially in moist sandy soils under polythene sheeting. Soil fumigants are injected into the soil, usually in the autumn. The excessive use of all pesticides is currently discouraged on environmental grounds. It may be better to concentrate chemical applications on hot spots in a field in combination with other methods which, when combined, will lead to a lower population density of potato cyst nematode in the field (Been and Shomaker, 1998).

Non-fumigant nematicides are used in smaller amounts then fumigants but usually persist in the soil as long as fumigants. They are applied as granules in broadcast, row or narrow band treatments at the time of planting. In some cases a second application may be made at mid-season.

For further information, see Whitehead (1998) and Haydock et al. (2006).

 

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Links to Websites

Contributors

18/03/2008 Updated by:

Janet Rowe, IACR-Rothamsted, Rothamsted Experimental Station, Harpenden, Hertfordshire, AL5 2JQ, UK

Distribution Maps