Root-knot nematode

The degree of damage depends upon the population density of the nematode, taxa present, susceptibility of the crop, and environmental conditions, such as fertility, moisture and presence of other pathogenic organisms, which may interact with nematodes. In sweetpotato an estimated annual yield loss of 10.2 % was reported. In susceptible varieties pathogenicity of Meloidogyne incognita showed a 50% storage root reduction at a population density of 20,000/cm³.  Aside from yield loss, cracking  can make storage roots unmarketable.

The root-knot nematode, Meloidogyne incognita, is worldwide in distribution. It is widespread in Asia, Southeast Asia and usually occurs in warmer areas.  In some countries, M. javanica is more dominant.

Above-ground symptoms exhibited by sweetpotato plants due to root-knot nematode include poor shoot growth, leaf chlorosis and stunting.  Galling of rootlets and severe cracking of storage roots on some varieties or formation of small bumps or blisters on other varieties are important below-ground symptoms in sweetpotato. There may also be brown to black spots in the outer layers of flesh which are not evident unless the storage root is peeled. 

Presence can be diagnosed by the pearl-like swollen female nematodes in flesh of storage roots or in fibrous roots, within the galls or dark spots.

M. incognita is sexually dimorphic. The female is saccate to globose, 0.4-1.3 mm. long, and usually embedded in root tissues which are often swollen or galled.  Its body is soft, pearl white in colour and does not form a cyst. The neck protrudes anteriorly and the excretory pore is anterior to the median bulb and often near the stylet base. Its vulva and anus are terminal, flush with or slightly raised from the body contour, the cuticle of the terminal region forms a characteristic perineal pattern, which is made up of the stunted tail terminus, phasmids, lateral lines, vulva and anus surrounded by cuticular striae; the pattern is often characteristic for individual species. The female stylet is shorter, 10-24 µm usually 14-15µm, and more delicate with small basal knobs. The paired gonads have extensive convoluted ovaries that fill most of the swollen body cavity. There are six large unicellular rectal glands in the posterior body which produce a gelatinous matrix, which is excreted via the rectum to form an egg sac in which many eggs are deposited.

The male has long, thin, cylindrical shape of a worm but the lip region has a distinct head cap, which includes a labial disc surrounded by lateral and medial lips. The head skeleton is usually weaker and the stylet less robust and shorter, 18-24 µm long for many species. Infective second stage juveniles, often free in the soil, are usually 0.3-0.5 mm long; they are less robust, the stylet is delicate with small basal knobs, under 20 µm long, and the head skeleton weak. The median oesophageal bulb is well developed and the oesophageal glands are extensive, overlapping the intestine for several body widths, mainly ventrally; the tail is conoid, often ending in a narrow rounded terminus, but tail length is variable, 1.5-7 anal body widths between species, it often ends in a clear hyaline region, the extent of which can help to distinguish species. (Sasser and Carter, 1985).

Life cycle

In addition to the adult and egg, there are four juvenile stages and four moults in the life cycle of M. incognita. The first stage juvenile develops in the egg, and the first moult usually occurs within the eggshell, giving rise to the second-stage juvenile, which emerges free into the soil or plant tissue. Once the nematode begins feeding on tissue of a favourable host, the second, third and fourth moults occur giving rise to the third, fourth and fifth or adult stages, respectively. Between moults, there is further growth and development of the nematode, with concurrent development of the reproductive systems in the two sexes. Upon maturity, the female deposits eggs and the life cycle is repeated. Its life cycle is similar to Heterodera but the generation time, 4-8 weeks, is shorter.

M. incognita has a very wide host range including weeds. Meloidogyne spp. attack virtually all plants.

Crop rotation. Non-host crops or resistant crops can be planted when nematode population is high.

Addition of organic amendments. Chicken manure is very effective reducing nematode egg masses by 56%.

Use of trap crops and antagonistic crops. Planting Tagetes erecta and Crotolaria spectabilis in nematode infested soil is effective against the root-knot nematode

Paecilomyces lilacinus, a fungal egg parasite, was found effective against root-knot attacking sweetpotato. The parasite reduced egg masses by about 50%.

There are many varieties of sweetpotato found resistant to the root-knot nematode.  Some of these are: W-86, L4-89, BPA-4 and Sinibastian, Jasper, Jewel, Miracle, Georgia Red, Garcia Yellow and Travis. However, some populations of M. incognita can infect even some of the resistant cultivars.

Several nematicide have been very effective against the root-knot nematode in sweetpotato. Examples are Nemagon, Mocap, Dasanit, Nemacur, Furadan, Temik, Vydate.

Evans, K, D. L. Trudgill and J. M. Webster. 1993. Plant Parasitic Nematodes in Temperate Agriculture. University Press, Cambridge. 648 p.

Galano, C. D., R. M. Gapasin and J. L. Lim. 1996. Efficacy of Paecilomyces lilacinus isolates for the control of root-knot nematode (Meloidogyne incognita (Kofoid and White) Chitwood) in sweet potato. Annals of Tropical Research 18: 4-12.

Gapasin, R. M. and R. B. Valdez. 1979. Pathogenicity of Meloidogyne spp. and Rotylenchulus reniformis on sweet potato. Annals of Tropical Research 1:20-26.

Gapasin, R. M. 1984. Resistance of fifty-two sweet potato (Ipomoea batatas (L.) Lam.) cultivars to Meloidogyne incognita and M. javanica. Annals of Tropical Research 6: 1-19.

Sasser, J. N. and C. C. Carter. 1985. An Advanced Treatise on Meloidogyne. Vol. I: Biology and Control. North Carolina State University Graphics. 422 p.

Sasser, J. N. 1989. Plant Parasitic Nematodes: The Farmer’s Hidden Enemy. University Graphics, North Carolina State University, Raleigh, N. C. 115 p.

Contributed by:  Ruben Gapasin

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Life cycle

Host range

Management

References

Galls and egg masses of root-knot nematodes on fibrous roots (left).  Galls on sweetpotato roots are often much smaller and difficult to see (W. Martin, APS). 

Increasing yield reduction and storage root damage with increasing nematode population.

Cracking on storage roots due to root-knot nematode (G. Lawrence, APS).

Dark spots in the outer layers of the flesh, each containing a nematode surrounded by dark cork tissue (G. Lawrence).

Subcortical spots which were not evident until the roots were peeled (W. Martin, APS). 

Small blister-like bumps are sometimes apparent on the storage root surface, where nematodes are embedded (G. Philley)

Egg masses of M. incognita (R. Gapasin).