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Root-knot nematodes are plant-parasitism from the genus Meloidogyne. They exist in soil in areas with hot climates or short winters. About 2000 species of plants worldwide are susceptible to infection by root-knot nematodes and they cause approximately 5% of global crop loss.Sasser JN, Carter CC: Overview of the International Meloidogyne Project 1975–1984. In An Advanced Treatise on Meloidogyne. Edited by: Sasser JN, Carter CC. Raleigh: North Carolina State University Graphics; 1985:19–24. Root-knot nematode infect plant , causing the development of root-knot that drain the plant's photosynthate and nutrients. Infection of young plants may be lethal, while infection of mature plants causes decreased yield.
Meloidogyne spp. were first reported in cassava by Neal in 1889.Neal, J. C. 1889. The root-knot disease of the peach, orange and other plants in Florida due to the work of Anguillula Bull. I.S. Bur. Ent.20.31pp. Damage on cassava is variable depending on cultivar planted, and can range from negligible to serious.Jatala, P., bridge, J. 1990. Nematode parasites of root and tuber crops. In Plant parasitic nematodes in sub-tropical and tropical agriculture., pp 137-180. Luc, M., Sikora, R.A., Bridge, J., CABI Publishing, Wallingford, UK. Early-season infection leads to worse damage.Makumbi-kidza, N. N., Speijer and Sikora R. A. 2000. Effects of Meloidogyne incognita on growth and storage-root formation of cassava ( Manihot esculenta). J Nematol.; 32(4S): 475–477. In most crops, nematode damage reduces plant health and growth; in cassava, though, nematode damage sometimes leads to increased aerial growth as the plants try to compensate. This possibly enables the plant to maintain a reasonable level of production. Therefore, aerial correlations to nematode density can be positive, negative or not at all.Gapasin, R.M. 1980. Reaction of golden yellow cassava to Meloidogyne spp. Inoculation. Annals of Tropical Research 2:49–53. Vegetable crops grown in warm climates can experience severe losses from root-knot nematodes, and are often routinely treated with a chemical nematicide. Root-knot nematode damage results in poor growth, a decline in quality and yield of the crop and reduced resistance to other stresses (e.g. drought, other diseases). A high level of damage can lead to total crop loss. Nematode-damaged roots do not use water and fertilisers as effectively, leading to additional losses for the grower. In cassava, it has been suggested that levels of Meloidogyne spp. that are sufficient to cause injury rarely occur naturally. However, with changing farming systems, in a disease complex or weakened by other factors, nematode damage is likely to be associated with other problems.Theberge, R. L. (eds). 1985. Common African Pests and Diseases of cassava, Yam, Sweet Potato and Cocoyam. International Institute of Tropical Agriculture (IITA). Ibadan, Nigeria 107 p.
An excellent model system for the study of the parasitic behaviour of plant-parasitic nematodes has been developed using Arabidopsis thaliana as a model host. The Arabidopsis roots are initially small and transparent, enabling every detail to be seen. Invasion and migration in the root was studied using M. incognita.Wyss, U., Grundler, F.M.W. & Munch, A. 1992 The parasitic behaviour of second stage juveniles of Meloidogyne incognita in roots of Arabidopsis thaliana. Nematologica, 38, 98–111. Briefly, second stage juveniles invade in the root elongation region and in the root until they became sedentary. Signals from the J2 promote parenchyma cells near the head of the J2 to become multinucleateHussey, R. S. & Grundler, F. M. W. 1998 Nematode parasitism of plants. In: The Physiology and Biochemistry of free-living and plant-parasitic nematodes. Perry, R. N. & Wright, D. J. (Eds), CABI Publishing, UK. pp 213 – 243. to form feeding cells, generally known as giant cells, from which the J2 and later the adults feed. Concomitant with giant cell formation, the surrounding root tissue gives rise to a gall in which the developing juvenile is embedded. Juveniles first feed from the giant cells about 24 hours after becoming sedentary.
After further feeding, the J2s undergo morphological changes and become saccate. Without further feeding, they moult three times and eventually become adults. In females, which are close to spherical, feeding resumes and the reproductive system develops. The life span of an adult female may extend to three months, and many hundreds of eggs can be produced. Females can continue egg laying after harvest of aerial parts of the plant and the survival stage between crops is generally within the egg.
The length of the life cycle is temperature-dependent.Madulu, J. & Trudgill, D. L. 1994 Influence of temperature on Meloidogyne javanica. Nematologica, 40, 230–243.Trudgill, D. L. 1995 An assessment of the relevance of thermal time relationships to nematology. Fundamental and Applied Nematology, 18, 407–417. The relationship between rate of development and temperature is linear over much of the root-knot nematode life cycle, though it is possible the component stages of the life cycle, e.g. egg development, host root invasion or growth, have slightly different optima. Species within the genus Meloidogyne also have different temperature optima. In M. javanica, development occurs between 13 and 34 °C, with optimal development at about 29 °C.
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