Meloidogyne incognita (root-knot nematode)

Meloidogyne species may be difficult to identify with certainty due to their inherent variability. Adult females and infective juveniles should be examined before making an identification. Male characters are also useful if they are available for examination, because some populations may not produce males, but they may be stimulated to produce males by putting the host plant under stress ( Snyder et al., 2006 ). 

Female

Endoparasitic, body pear-shaped, pearly white. Stylet knobs usually rounded, sometimes drawn out laterally. Distance of the dorsal esophageal gland to the base of the stylet is short, 2-3 µm. Excretory pore at level of or posterior to stylet knobs, 10-20 annules posterior to the anterior end. Posterior perineal pattern variable. Typical pattern 'incognita type'; with striae closely spaced, very wavy to zig-zag especially dorsally and laterally. Dorsal arch high, squarish. Lateral field not clear, sometimes marked by breaks in striae, broken ends often forked, pattern merging into body striae. Alternate 'Acrita type'; with striae smoother, more widely spaced (or with coarse, widely spaced striae separated by fine, closely spaced striae visible for short distances). Dorsal arch variable, may be flattened at top with the overall appearance of a trapezoid. Striae often forked along a 'lateral line'. Limits of pattern more or less well-defined. Aberrant patterns occur.

Male

Vermiform, 1-2 mm in length. Head not offset, distinct head cap on top of a large head annule that may be further divided into 1-3 incomplete, smaller annules appearing stepped in outline in lateral view. Body clearly annulated. Conus of stylet longer than shaft, stylet knobs prominent, usually of greater width than length, with flat, concave anterior margins. Excretory pore at level of posterior end of isthmus with hemizonid usually 0-5 annules anterior. Lateral field with 4 incisures, outer bands areolated, inner band rarely cross-striated except at posterior end. Testes 1 or 2. Tail bluntly rounded, terminus unstriated. Phasmids at cloaca level or just posterior. Spicules slightly curved, gubernaculum crescentic.

Second-Stage Juvenile

Vermiform, less than 0.5 mm in length. Head not offset, truncate cone shape in lateral view, sub-hemispherical in dorso-ventral view. Head-cap wide containing 2-3 annules. Stylet knobs prominent, rounded. Hemizonid 3 annules long just anterior to excretory pore. Lateral field with 4 incisures, outer bands areolated. Rectum inflated. Tail tapering to subacute terminus, annulations coarsening posteriorly.

Measurements (after Whitehead, 1968)

20 females: L = 500-723 (609) µm; width = 331-520 (415) µm; stylet (10) = 13-16 (14) µm; width stylet base (10) = 3-5 (4) µm; dorsal oesophageal gland orifice (8) = 2-4 (3) µm behind stylet base; length median bulb (10) = 37-63 (46) µm; width median bulb (10) = 31-49 (39) µm; length median bulb valves (10) = 13-16 (14) µm; width median bulb valves (10) = 11-13 (12) µm.

14 males: L = 1108-1953 (1583) µm; a = 31.4-55.4 (46.3); length head (13) = 6.8-8.6 (7.9) µm; stylet (13) = 23.0-32.7 (25.0) µm; width stylet base (13) = 4.7-6.8 (5.8) µm; dorsal oesophageal gland orifice (4) = 1.4-2.5 (2.1) µm behind stylet base; b' (12) = 13.8-20.5 (17.4); c (13) = 97-255 (146); length median bulb (12) = 14.4-25.2 (20.0) µm; width median bulb (12) = 8.6-15.8 (11.2) µm; length median bulb valves (12) = 5.8-9.0 (7.2) µm; spicules (length of arc) (12) = 28.8-40.3 (35.2) µm; gubernaculum (7) = 9.4-13.7 (11.2) µm.

25 second-stage juveniles: L = 337-403 (371) µm; a = 24.9-31.5 (28.3); b (21) = 2.02-3.14 (2.36); b' (24) = 6.4-8.4 (7.1); length tail = 38-55 (46) µm; d = 4.0-5.6 (4.9); c = 6.9-10.6 (8.1); length body to middle of genital primordium = 212-247 (230) µm; stylet = 9.6-11.7 (10.5) µm; length median bulb (24) = 10.1-12.9 (11.3) µm; width median bulb (24) = 5.8-8.3 (7.3) µm; length median bulb valves (22) = 3.6-6.47 (5.2) µm.

M. incognita has also been recorded from the following areas of protected agriculture; for example, around temperate research stations and other areas of intensive horticulture. In some cases the nematodes have been able to overwinter due to mild winters and could be expanding their geographical range.

Armenia ( Pogosyan and Karapetyan, 1976 ), Heilongjiang and Ningxia in China ( Yang et al., 1991 ), Hokkaido in Japan ( Yamada and Takakura, 1975 ), Belarus ( Gladkaya, 1981 ), Estonia ( Pallum, 1972 ), Latvia ( Rasina, 1970 ), Lithuania ( Rudzyavichene et al., 1975 ), Macedonia ( Krnjaic, 1977 ), Moldavia ( Batyr and Kozhokaru, 1985 ), Poland ( Brzeski et al., 1978 ), Romania ( Romascu et al., 1974 ), European part of Russia ( Kozhokaru, 1972 ; Gushchin and Efremenko, 1975 ; Mar'enko, 1989 ), Siberia ( Shiabova, 1977 ), Ukraine ( Timchenko, 1981 ) and UK (IIP, 1991).

M. incognita is widely endemic in subtropical and tropical regions, thus quarantine regulations would be superfluous and none are known to be in place.

M. incognita is probably the most widely distributed and economically important species of plant parasitic nematode in tropical and subtropical regions. Two-thirds of the root-knot nematode samples obtained from a number of tropical countries were of M. incognita ( Sasser, 1979 ). In India alone, 232 plant genera have been reported as hosts to M. incognita ( Krishnappa, 1985 ) and worldwide the species is a parasite of a wide range of crop plants.

Field symptoms are typically of stunted, poorly growing plants with yellowing leaves. Infected root systems show characteristic knots or galls, the severity of which varies with the degree of nematode infection and species and variety of plant parasitized (see Dropkin, 1989 ).

General identification of root-knot nematode infestation can be conducted by competent nematologists with basic equipment. Identification of species within the genus Meloidogyne requires the services of specialized taxonomists. In addition to classical taxonomic techniques, biochemical methods have been developed ( Esbenshade and Triantaphyllou, 1985 ), and molecular techniques are standard by sequencing the ribosomal DNA 18S-ITS-5.8S, 28S D2/D3 and a mitochondrial DNA fragment flanking cytochrome oxidase gene subunit II ( Ye et al., 2019 ).

The situation regarding M. incognita is complicated by the existence of morphologically indistinguishable races differentiated by their ability to reproduce on resistant tobacco and cotton (Hartman and Sasser, 1985).

The morphology of M. incognita is similar to other species of Meloidogyne and often confused with other species such as M. enterolobii, M. floridensis, M. hispanica, etc. Confusion within this group is common as the characters used tend to be variable; however molecular techniques have made species identification very reliable ( Ye et al., 2019 ).

M. incognita is found worldwide in tropical and subtropical regions, in particular in the warmer areas. For example, in California (USA), M. incognita is found more commonly in the hot valleys of the interior ( Ferris and Van Gundy, 1979 ) and in East Africa it is restricted to altitudes below 2000 m above sea level ( Whitehead, 1969 ). M. incognita is found on many soil types. Damage and yield losses caused are generally more severe on coarse-textured sandy soils ( Van Gundy, 1985 ). Meloidogyne spp. are generally intolerant of flooded soil conditions.

The life cycles of Meloidogyne spp. are well studied and in their essentials differ little between the major species ( De Guiran and Ritter, 1979 ). At 21°C M. incognita took 37 days to complete its life cycle on Antirrhinum majus, a similar time to that reported on soyabeans (temperatures not published) ( Ibrahim and El-Saedy, 1987 ). Juveniles penetrate root tips, occasionally invading roots in the zone of root elongation. Invaded nematodes initiate the development of giant cells in the meristematic, cortical and xylem tissues of the root and galling of roots occurs. Third- and fourth-stage juveniles and young females occur after about 6-8 and 15 days, respectively. Adult females were observed after 20 days and egg laying commenced after 25 days ( Ibrahim and El-Saedy, 1987 ).

A considerable amount of work has been devoted to the biological control of root-knot nematodes in general and M. incognita in particular ( Kerry, 1987 ). Most research has concentrated on the endoparasitic microorganism Pasteuria penetrans and the egg-parasitic fungus Purpureocillium lilacinum (syn. Paecilomyces lilanicus ). Experimental work has been encouraging, for example, P. penetrans in the control of M. incognita on tomatoes ( Sayre, 1980 ) and P. lilanicinum in the control of M. incognita on potatoes ( Jatala, 1985 ). The development of P. lilanicinum has reached the stage of multi-local field trials ( Jatala, 1985 ). However, there are considerable problems in the preparation and incorporation of inocula of putative biocontrol agents and the practical application of such technology remains to be developed ( Kerry, 1987 ).

Introduction

The root knot nematode species, M. incognita, is the most widespread and probably the most serious plant parasitic nematode pest of tropical and subtropical regions throughout the world ( Sasser, 1979 ). It occurs as a pest on a very wide range of crops. Most estimates of yield loss come from the use of nematicides and it should be noted that these can possibly cause other beneficial growth effects.

Cotton

The most accurate and detailed estimates of cotton yield losses due to nematodes have been undertaken in states of the USA where nematicides are regularly used. In Arkansas, where M. incognita is by far the most important nematode of cotton, yield losses after the application of nematicides are estimated at 1.5% annually; in South Carolina, M. incognita with other nematodes account for an estimated 5% loss annually ( Kirkpatrick, 1988, 1989). In Texas over a 16-year period of field experiments, the average cotton yield increase was 26% when fields infested with M. incognita were fumigated with nematicides. In six Texan counties, where over 47% of field soils were infested with M. incognita, it was calculated that the total annual yield loss was 10.2% or 85,600 cotton bales ( Orr and Robinson, 1984 ). Disease complexes involving M. incognita and vascular Fusarium wilt on cotton are well recognized. Where the two organisms occur together, increasing the nematode population has a greater effect on wilt incidence and cotton yield loss than an increase in the Fusarium population ( Starr et al., 1989 ; Hillocks and Bridge, 1992 ; Hillocks, 1997 ).

Tobacco

Root knot nematodes, mainly M. incognita and M. javanica, are always pests of economic importance in tobacco culture, wherever the climate favours them ( Barker et al., 1981 ; Shepherd and Barker, 1990 ). In North America, losses to tobacco from root knot nematodes, mainly M. incognita, have been estimated to range from 1 to 14% annually in areas where control measures are the norm ( Shepherd and Barker, 1990 ). Root knot nematodes, probably M. incognita, have been estimated to cause 50 to 60% yield losses in parts of Turkey and losses of 25% from field infestation and 50% if the infestation started in the seedbed in India ( Shepherd and Barker, 1990 ). The percentage loss in tobacco yield is estimated to be 8 to 9% for each ten-fold increase in the initial population of M. incognita ( Barker et al., 1981 ). In Cuba, in the region where the highest quality tobacco for cigars is produced, the losses due to M. incognita are estimated at 27.5% of the crop potential of flue-cured tobacco ( Garcia and Perez, 1987 ; Fernandez and Ortega, 1998).

Food Legumes

M. incognita is a major economic pest of food legumes in the tropics and subtropics (Sikora and Greco, 1990). Moderate soil populations of 1000 nematodes per plant in pathogenicity pot experiments can reduce pod yields of chickpea ( Cicer arietinum ) by 27% and very high soil populations of 10,000 per plant reduce pod yields by 87%. In M. incognita -infested field plots treated with nematicides, yield of chickpea can increase by 15 to 33% (Reddy, 1976). Common bean (Phaseolus vulgaris) is very badly damaged by Meloidogyne species in the tropics. Significant bean yield increases of 45 to 60% have been achieved in Kenya by reducing mixed soil populations of M. incognita and M. javanica with the application of nematicides ( Ngundo and Taylor, 1974 ). Similarly in Peru and Colombia, in fields infested with mixed populations of Meloidogyne species, mainly M. incognita, common bean seed yield can be reduced by 26 to 63% depending on the bean cultivar grown and other factors ( Mullin et al., 1991 ). M. incognita is a serious pest of Phaseolus vulgaris in the USA where there is a reduction of 66% in crop value in fields where the nematodes are not controlled. The reduced crop value is a result of a combination of reduced plant stands and a 27% lower average pod production by surviving plants ( Smittle and Johnson, 1982 ). Cowpea ( Vigna unguiculata ) is another very susceptible host crop of M. incognita . In Nigerian soils where M. incognita is the predominant nematode parasite, application of nematicides can increase cowpea yields by 95 to 222% ( Babatola and Omotade, 1991 ). Soyabean ( Glycine max ) is another legume severely damaged by root knot nematodes and in Brazil yield can be reduced by over 55% in the presence of M. incognita ( Antonio, 1988 ). In India, nematicide application to farmers' fields infested with a mixture of M. incognita and Rotylenchulus reniformis increased yield of soyabean by 19% ( Prasad, 1999 ).

Vegetables

M. incognita is a major pest of vegetables throughout the tropics and subtropics. Many crops grown as vegetables are susceptible to the nematode particularly tomato, aubergine, okra, cucumber, melon, carrot, gourds, lettuce and peppers. Estimates of vegetable crop losses due Meloidogyne species, mainly M. incognita and M. javanica, have ranged from 17 to 20% for aubergine ( Solanum melongena ), 18 to 33% for melon and 24 to 38% for tomato ( Sasser, 1979 ). In India, yield losses of okra ( Abelmoschus esculentus ), tomato and aubergine field crops are estimated at 91%, 42 to 54% and 18%, respectively ( Bhatti and Jain, 1977 ; Subramaniyan et al., 1990 ). Other trials in India using nematicides showed that in fields naturally infested with mixed populations of M. incognita and M. javanica the avoidable yield losses in peas, okra, tomato and bottle gourd ( Lagenaria siceraria ) crops is 46% to 56% ( Sharma and Baheti, 1992 ).

Yams

The primary damage to yams ( Dioscorea species) by M. incognita is in the reduction in quality and marketability of the tubers due to the extensive surface galling caused by the root knot nematodes ( Jatala and Bridge, 1990 ). It is estimated that there is a reduction of 39 to 52% in the price of galled tubers compared with healthy ones ( Nwauzor and Fawole, 1982 ). However, there are instances were the nematode is known to actually reduce yields, for example in Nigeria significant yield reduction can occur in D. alata due to M. incognita ( Adesiyan and Odihirin, 1978 ). In Martinique, M. incognita is reported to have completely destroyed a crop of the yam D. trifida ( Kermarrec, 1974 ). In India, pot experiments have demonstrated that even low nematode populations of 100 M. incognita juveniles per plant can cause a highly significant reduction in yam ( D. rotundata ) tuber weights of 27% and larger nematode populations over 55% yield loss ( Mohandas and Ramakrishnan, 1997 ).

Potatoes

Losses of potatoes due to Meloidogyne species, mainly M. incognita and M. chitwoodi, are estimated at 25% or more ( Mai et al., 1981 ).

Spices

M. incognita is an important pest of black pepper ( Piper nigrum ) in many countries particularly in Brazil and India where infestation levels of over 90% have been reported ( Koshy and Bridge, 1990 ). As few as 10 juveniles per plant can reduce black pepper growth by 16% under potted conditions ( Koshy et al., 1979 ). Cardamom ( Elettaria cardamomum ) suffers yield losses of 32 to 47% in the presence of M. incognita in India ( Ali, 1986 ). In Australia, M. incognita and M. javanica are pests of ginger and severe infestation of rhizomes can reduce yields by 57% ( Pegg et al., 1974 ). In pot experiments, a reduction of 74% in ginger rhizome weight has been recorded with initial M. incognita soil populations of 10,000 nematodes ( Sukumaran and Sundararaju, 1986 ).

Coffee

M. incognita is found in many coffee-growing areas in the world but it is in Brazil where its effects have been the most catastrophic. In Brazil, the nematode has caused the complete destruction of coffee plantations and resulted in major agricultural changes as farmers were forced to adopt alternative crops ( Campos et al., 1990 ).

Above-ground symptoms are non-specific (see Symptoms): examination of roots can reveal the presence of swollen roots and galls. However, some hosts such as maize do not form galls. Closer examination, for example with a hand lens, can show the presence of egg masses on the root surface.

Methods for the control of M. incognita vary with the production system used and the value of the crop. Chemical control may be used on high-value crops, and a wide range of chemicals are available (Johnson, 1985a; Hague and Gowen, 1987 ). More recently, seed treatments have become widely utilized to minimize the amount on active ingredient applied or for the application of biological control agents. Often these tactics are reserved for the most important row crops like soyabean, maize and cotton ( Monfort et al., 2006 ). Strategies of cultural control are less well developed and crop rotations are difficult to design because of the wide host range of M. incognita . Groundnuts or maize, which are both poor or non-hosts to M. incognita, have been evaluated for use in cropping systems designed to manage this nematode ( Raymundo, 1985 ). Systems of integrating cropping sequences and chemical control have been evaluated in the USA (Johnson, 1985b). Resistance to M. incognita exists in a number of commercial crop varieties, principally vegetables ( Lehman and Cochran, 1991 ). These have generally been developed in the USA, but have been evaluated in other parts of the world (for example, see Krishnappa, 1985 ).