Fig. 15.3. A female and male Pratylenchus species (photo by Edward Oyekanmi).
Pratylenchus species infect soybean in most growing areas. Lesion nema-todes cause stunting, leaf yellowing and yield losses, depending on soil densities at planting. Yield losses are linearly related to P. brachyurus densities in a sandy-clay loam soil (Schmitt and Baker, 1981).
The lesion nematode is normally a parasite of the root cortex, but it may also harm phloem and sometimes xylem. Reproduction is sexual. Although it is not possible to give precise threshold populations above which losses occur, five to ten nematodes 100 g-1 of soil may be considered damaging (Whitehead, 1998). For most Pratylenchus species infections, infested roots show typical lesions that may eventually girdle the root. These root lesion nematodes are obligate root endoparasites and are injurious to many crops. They feed primarily on the root cortex, through which they migrate, creating cavities and channels in which their eggs are deposited singly or in small groups. Before penetrating the roots, Pratylenchus species sometimes browse on the root surfaces and root hairs. Small lesions, at first yellow and then brown to black, develop where the nematodes enter and feed in the roots. These often enlarge and may eventually girdle the roots. Cells on which the nematodes feed and cavities and channels through which they pass become necrotic and secondary invading bacteria and fungi may rot the root. The loss of functional feeder and sometimes main roots results in leaf chlorosis, twig die-back and yield loss. Pratylenchus species are known to increase damage caused by root-rotting fungi, which may further reduce yield (Sikora et al., 2005a).
The management of Pratylenchus species is affected by wide host ranges and the presence of multiple species in a field. However, the use of resistant varieties in the management of lesion nematodes is an effective means of management. Most of these cultivars are also resistant to other nematodes, such as reniform and root-knot nematodes.
Cultural methods such as crop rotation do not adequately control Pratylenchus species, although some reduction in soil infestation may be achieved by growing a poor host (Whitehead, 1998). Nematicides may be used as a control measure for Pratylenchus species. If this is being considered, an economic analysis should first be conducted to ascertain profitability (Zirakparvar, 1985).
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