Meloidogyne chitwoodi
(columbia root-knot nematode)

M. chitwoodi was first described from the Pacific Northwest of the USA in 1980, its common name deriving from the Columbia River between Oregon and Washington states. It is not clear whether this is its area of origin. It was first detected in the EPPO region in the 1980s, in the Netherlands, but a review of old illustrations and old specimens of Meloidogyne suggests that it may have occurred earlier (in the 1930s) and may have been present throughout the intervening period (OEPP/EPPO, 1991). It is possible that M. chitwoodi has a wider distribution, undetected, in Europe than is currently known; the question is now actively under investigation.

A record for Wyoming, USA, was incorrectly included in CABI/EPPO (2000). The record for Virginia, USA, has been changed to 'Absent, confirmed by survey' following surveys conducted by Virginia Department of Agriculture and Consumer Services (2007-2010) and the Department of Plant Pathology, Physiology and Weed Science, Virginia Tech.

M. chitwoodi has been added to the EC Plant Health Directive 77/93/EEC.

A comparison of the air temperatures found in Finland with those found across the present distribution of the pest indicated that the nematode could survive and produce two generations per year in the southern part of Finland. Baker and Dickens (1993) concluded from their PRA that M. chitwoodi would be likely to produce three generations in the UK. They did not feel capable of predicting the likely economic impact of the pest, as this could depend on a number of other unknown factors such as soil wetness, varietal susceptibility and quality control thresholds. A recent PRA for Germany (Braasch et al., 1996) led to the conclusion that M. chitwoodi was most likely to be damaging on potatoes in the north-west of the country, in the area adjoining the Netherlands. Countries further south in the EPPO region would provide climatic conditions suitable for as many as four generations per year.

Potato crops would be most at risk from M. chitwoodi in the EPPO region. For a number of reasons, it represents a greater threat than other Meloidogyne species already widespread in the EPPO region, in particular M. hapla with which it often forms mixed populations. M. chitwoodi is less easily controlled by nematicides, it has a wider host range, it produces more severe tuber symptoms and is tolerant of lower soil temperatures. In fact, the soil temperature requirements for population development of M. chitwoodi are similar to those of Globodera rostochiensis (Tiilikkala et al., 1995), suggesting that the former species could occupy the same geographical distribution as the latter.

M. chitwoodi has a wide host range among several plant families (Santo et al., 1980; O'Bannon et al., 1982), including crop plants and common weed species. Potatoes and tomatoes are good hosts, while barley, maize, oats, sugarbeet, wheat (Triticum aestivum) and various Poaceae (grasses and weeds) will maintain the nematode.

Moderate to poor hosts occur in the Brassicaceae, Cucurbitaceae, Fabaceae, Lamiaceae, Liliaceae, Umbelliferae and Vitaceae.

Fourie et al. (1998), working with a South African population of M. chitwoodi, reported Lycopersicon esculentum, Brassica rapa, Eragrostis tef and Lolium multiflorum as supporting high populations, whereas Eragrostis curvula, Arachis hypogaea and Zea mays were poor hosts.

Korthals et al. (2000) assessed host plant suitability at two sites in the Netherlands. Population growth of M. chitwoodi was greatest in potato, rye and summer wheat while nematodes in sugarbeet, hemp, perennial ryegrass and Tagetes patula remained equivalent to those of fallow. Because of poor reproduction, they recommended perennial ryegrass as a cover crop after cereals, maize, carrots or potatoes.

Lucerne is a good host for race 2 but not for race 1 (see Biology and Ecology for information on races), whereas carrots are a non-host for race 2 but a good host for race 1. Ferris et al. (1994a,b), investigating suitable crops for rotation with potato in the presence of race 1 in the USA, recommend Amaranthus, lucerne, rape (Brassica napus var. oleifera), Raphanus sativus var. oleifera and safflower (Carthamus tinctorius). In the Netherlands, host crops recorded to be attacked by M. chitwoodi are carrots, cereals, maize, peas, Phaseolus vulgaris, potatoes, Scorzonera hispanica, sugarbeet and tomatoes (OEPP/EPPO, 1991).

Recent studies have expanded information on the host range of M. chitwoodi (e.g. Brinkman et al., 1996; Griffin and Rumbaugh, 1996). Nijs et al. (2004) presented the results of experiments performed in the Netherlands and provided information on the host status of many plants together with an assessment of their potential phytosanitary risk.

The life cycle of M. chitwoodi takes approximately 3-4 weeks under favourable conditions. Juveniles hatch from eggs in soil or on the root surface. Second-stage juveniles (the infective stage) penetrate root tips through unsuberized epidermal cells or wounds and move into the cortical region. Soon after entry, nematodes stimulate giant cell and gall formation in the host tissue. Necrotic lesions occur in the cortex. Juveniles then swell to become sausage-shaped, stop feeding and undergo three rapid successive moults to become adult males or females. Adult males have slender, vermiform bodies; they leave the root and are found free in the rhizosphere or near the protruding body of the female. However, as in the case of other Meloidogyne spp., it is probable that males are largely functionless and reproduction is nearly always parthenogenetic. Adult females have characteristically pear-shaped, pearly-white bodies and they are found embedded in host tissue. Eggs are laid by the female in a gelatinous sac near the root surface. In potato tubers, modified host cells form a protective layer or 'basket' around the egg mass and the juveniles as they hatch. The egg shell itself is hyaline with no visible markings.

M. chitwoodi passes the winter as eggs or juveniles and can survive extended periods of sub-zero temperatures. M. chitwoodi needs a temperature of 4°C for hatching and penetrating roots, and 6°C for development. Charchar and Santo (2001) reported on the effect of temperature on embryonic development and hatching of M. chitwoodi in Brazil. M. chitwoodi requires 600-800 degree-days to complete the first generation, whilst subsequent generations require 500-600 degree-days. Studies on potato in Oregon, USA, during 1992-1994 indicated that rapid population increase occurred after 1200 degree-days (Ingham and Rykbost, 1995). M. hapla requires a similar number of degree-days for development but does not begin development until temperatures rise above 10°C. Population dynamics in relation to degree-day accumulation have been considered by Pinkerton et al. (1991). M. chitwoodi was found to be more pathogenic at 25°C than at 30°C in glasshouse tests performed on yellow sweet clover (Griffin and Jensen, 1997). Brommer and Molendijk (2001) reported that there were at least two generations per year in fields naturally infected with M. chitwoodi in the Netherlands.

Several races of M. chitwoodi have been described, distinguished by slight differences in host range. The first two, known as race 1 and race 2, were in particular distinguished with regard to carrot and lucerne (Santo and Pinkerton, 1985; Mojtahedi et al., 1988). Race 3 has recently been described in California, USA (Mojtahedi et al., 1994). The nematode found in the Netherlands previously named M. chitwoodi B-type has been redescribed as a new species (M. fallax) (Karssen, 1996). Current research is attempting to clarify the position regarding races, Beek et al. (1999) proposing a pathotype scheme to describe intraspecific variation in pathogenicity.

Predatory mites (Hypoaspis aculeifer) were observed feeding on egg-masses of this nematode on tomato roots in Utah, USA (Inserra and Davis, 1983).

M. chitwoodi was relatively susceptible to the nematode-trapping fungi Monacrosporium ellipsosporum and M. cionopagum, compared with Heterodera schachtii, in laboratory tests (Jaffee and Muldoon, 1995). Jacobsen (2002) reviewed biological control of potato pathogens. Wishart et al. (2004) reported attachment of the spores of Pasteuria penetrans and P. nishizawae to juveniles of M. chitwoodi, M. fallax and M. hapla.

M. chitwoodi has been distinguished from M. fallax by esterase and malate dehydrogenase patterns (Karssen, 1996) and DNA methods (Petersen and Vrain, 1996; Williamson et al., 1997; Zijlstra et al., 1997). A number of molecular based methodologies have been developed to characterize the species and to help differentiate it from related root-knot nematodes (Castagnone-Sereno et al., 1998, 1999; Castagnone-Sereno, 2000; Zijlstra, 2000; Fourie et al., 2001; Wishart et al., 2002; Fargette et al., 2005; Skantar and Carta, 2005).

The presence of M. chitwoodi in infested soil can be determined by sampling and extraction of the second-stage juveniles, using a standard nematode extraction procedure for free-living nematodes of this size. External symptoms on tubers are obvious in the case of heavy infestations but, where nematode numbers are low or in the early stages of infection, such symptoms are not obvious. Clearing and staining of the tissues can show the presence of nematodes (Hooper, 1986) but this can be a laborious procedure. Storage of lightly infested tubers may lead to the development of obvious external symptoms.

Means of Movement and Dispersal

M. chitwoodi has very limited potential for natural movement; only second-stage juveniles can move in the soil and, at most, only a few tens of centimetres. The most likely method of introducing M. chitwoodi into a new area is through the movement of infected or contaminated planting material. Infected host plants or host products such as bulbs or tubers can easily transport the nematode. The movement of non-host seedling transplants, nursery stock, machinery or other products which are contaminated with soil infested with M. chitwoodi could also result in spread. Infective larvae of this genus have been known to persist for more than one year in the absence of host plants. Nematode movement can also be facilitated by contaminated irrigation water.

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

M. chitwoodi closely resembles M. hapla; it can be distinguished from this and other species by the perineal pattern, the appearance of vesicles or vesicle-like structures in the median bulb of the adult female and by the short, blunt, larval tail which has a hyaline, rounded (not tapered) terminus (Golden et al., 1980; Nyczepir et al., 1982; Jepson, 1985).

M. chitwoodi differs from M. fallax by having smaller female and male stylet length, presence of small, irregular outlined male and female stylet knobs, male labial disk not elevated, shorter juvenile body-, tail-, and hyaline tail length, different hyaline tail shape, hemizonid position, esterase and malate dehydrogenase patterns (Karssen, 1996).

Schemes have been described to differentiate M. chitwoodi from M. hapla, M. microtyla and M. incognita by differential host tests (Townshend et al., 1984), and also to distinguish the two races of M. chitwoodi (Mojtahedi et al., 1988). Biochemical methods have also been used. A diagnostic DNA probe which distinguishes M. chitwoodi from M. hapla has been developed by Piotte et al. (1995) and Wishart et al. (2002) used a PCR-based technique involving the ribosomal intergenic spacer. Recently, M. chitwoodi has been distinguished from M. fallax by DNA methods (Petersen and Vrain, 1996; Williamson et al., 1997; Zijlstra et al., 1997; Castagnone-Sereno et al., 1999; Castagnone-Sereno, 2000; Zijlstra, 2000; Wishart et al., 2002).

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.

Chemical Control

Control measures currently used against other root-knot nematodes have proved to be less effective against M. chitwoodi. In north-western USA, crop failures in several potato fields have been attributed to M. chitwoodi despite the use of spring soil fumigations.
Cultural Control and Sanitary Methods

Recent work on sugarbeet suggests that late drilling and reduced irrigation can play a part in reducing damage to crops (Heijbroek, 1996).

Crop rotation with cereals is often used to reduce populations of M. hapla. However, M. chitwoodi reproduces well on wheat, oats, barley and maize. Therefore, rotation with any of these crops will favour a build-up rather than a decrease of M. chitwoodi populations in infested soils. There are, however, other crops which will reduce populations (see Host Range) which can be used in rotations. Brinkman et al. (1996) suggested that chicory cultivars (Cichorium intybus), Dahlia (cv. Vuurvogel) and borage (Borago officinalis) supported low populations of M. chitwoodi and can offer alternatives in crop rotations. However, further studies are necessary to collect more information on crops which can be used in rotation to reduce nematode populations.

Some success has been achieved by the incorporation of green manure into the soil, which reduces population densities of M. chitwoodi (Mojtahedi et al., 1993a, b; Suloiman and Hafez, 1996).

Measures similar to those for potato cyst nematodes (EPPO/CABI, 1992) would appear relevant, i.e. that consignments of rooted plants should come from areas where the pest does not occur or from fields found free from the pest. Suitable survey and test methods have still to be established. Freedom from M. chitwoodi should be specifically assured by certification schemes for seed potatoes.

Host-Plant Resistance

Potato cultivars differ in their tolerance of M. chitwoodi (van Riel, 1993). There is interest in breeding for host resistance to M. chitwoodi, for example, in potatoes, using resistance from Solanum bulbocastanum and other sources (Brown et al., 1991; Austin et al., 1993; Janssen et al., 1997) and cereals (Jensen and Griffin, 1994). Yu (2001) and Yu and Lewellen (2004) registered nematode-resistant sugarbeet germplasm; Zoon et al. (2002) discussed durable resistance against M. chitwoodi and M. fallax whilst Brown et al. (1999, 2003, 2004) investigated resistance to M. chitwoodi in potato and evaluated several wild Solanum species as sources of resistance. Tovar-Soto et al. (1997) reported on the response of five potato genotypes to M. chitwoodi race 2 in Mexico. Berthou et al. (2003) characterized virulence in populations of M. chitwoodi by challenging with Capsicum annuum line PM217. Their results demonstrated great polymorphism in M. chitwoodi populations and the existence of a major gene in pepper controlling a specific resistance against some nematode populations.

IPM Programmes

Guidelines for integrated pest management in potatoes are still being devised in those countries affected by this M. chitwoodi (Ferris et al., 1994a,b; Santo, 1994; Hafez et al., 1997).