wireworms, or seedcorn maggot. Root-knot nematodes typically are more common and most damaging to plants in light, sandy-textured soils. Belowground symptoms include roots that are galled, stunted, and discolored (Fig. 3). Sometimes the corn root system may appear healthy and galls may be small and difficult to notice even though root-knot nematode numbers may be very high. In either case, root growth is often restricted to shallow depths of only 8 or 10 inches below the surface when heavily infected. This reduces yield by restricting access to water and nutrients that are needed for plant growth and development. In field corn, root-knot nematodes may also cause stubby root symptoms because they stop the growth of root tips.
If nematode populations are high and growing conditions are good, damage may not be visible, but yield may be reduced. Severe damage is often visible and yield losses heavy if corn was stressed during the early part of the season. A fourth common species of root-knot nematode, M. hapla (northern root-knot nematode), is not a parasite of corn.
Resistant cultivars are currently unavailable for southern root-knot nematode, M. incognita; however, there are a few commercial cultivars that are resistant to M. arenaria and M. javanica. In order to utilize these resistant lines, an accurate identification to species is necessary because more than one species of root-knot nematodes may occur in some fields.
Chemical: The decision to apply nematicides should be based on root-knot nematode counts from soil samples collected in the fall immediately after harvest. Usually corn production is not adversely affected until an economic threshold of 500 second-stage juveniles per 500 cubic centimeters of soil is detected. At this level, it may be cost effective to apply a nematicide in cornfields with root-knot nematode counts that exceed this economic threshold [http://ipm-www.ento.vt.edu/states/va.html]. The history of a field with nematode problems should be taken into account when a nematicide or other control tactic is being considered. For more details on nematicides and their application rates, refer to the Pest Management Guide: Field Crops, Virginia Cooperative Extension publication 456-016, http://pubs.ext.vt.edu/456/456-016/456-016.html.
Reviewed by Rod Youngman, Extension Specialist, Entomology
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009