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Root-knot nematode
Root-knot nematodes are plant-parasitic nematodes from the genus ''Meloidogyne''. They exist in soil in areas with hot climates or short winters. About 2000 plants 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 larvae infect plant roots, causing the development of root-knot galls that drain the plant's photosynthate and nutrients. Infection of young plants may be lethal, while infection of mature plants causes decreased yield.
==Economic impact==
Root-knot nematodes (''Meloidogyne'' spp.) are one of the three most economically damaging genera of plant-parasitic nematodes on horticultural and field crops. Root-knot nematodes are distributed worldwide, and are obligate parasites of the roots of thousands of plant species, including monocotyledonous and dicotyledonous, herbaceous and woody plants. The genus includes more than 90 species,〔Moens, Maurice, Roland N Perry, and James L Starr. 2009. “Meloidogyne Species: a Diverse Group of Novel and Important Plant Parasites.” In Root-knot Nematodes, ed. Roland N Perry, Maurice Moens, and James L Starr, 1–17. Wallingford, UK: CABI Publishing.〕 with some species having several races. Four ''Meloidogyne'' species (''M. javanica, M. arenaria, M. incognita, and M. hapla'') are major pests worldwide, with another seven being important on a local basis.〔Eisenback, J. D. & Triantaphyllou, H. H. 1991 Root-knot Nematodes: ''Meloidogyne'' species and races. In: Manual of Agricultural Nematology, W. R. Nickle. (Ed). Marcel Dekker, New York. pp 281 – 286.〕 ''Meloidogyne'' occurs in 23 of 43 crops listed as having plant-parasitic nematodes of major importance, ranging from field crops, through pasture and grasses, to horticultural, ornamental and vegetable crops.〔Stirling, G. R., Stanton, J. M. & Marshall, J. W. 1992 The importance of plant-parasitic nematodes to Australian and New Zealand agriculture. Australasian Plant Pathology, 21, 104 - 115.〕 If root-knot nematodes become established in deep-rooted, perennial crops, control is difficult and options are limited. ''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 ranges from negligible to seriously damaging.〔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.〕
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