Content uploaded by Jonathan Eisenback
Author content
All content in this area was uploaded by Jonathan Eisenback on Dec 16, 2014
Content may be subject to copyright.
Order: Tylenchida
Family: Heteroderidae
Species: Meloidogyne incognita (southern root-knot
nematode), M. arenaria (peanut root-knot nematode),
M. javanica (Javanese root-knot nematode), M. hapla
(northern root-knot nematode; not found in corn)
Size: Adult females are up to 1/16 inch in diameter.
Color: Adult females are a translucent cream color.
Description: Adult females are pear shaped and sed-
ent a r y.
Range: M. incognita, M. arenaria, and M. hapla are
commonly found throughout Virginia. However, M.
javanica is rare and found mostly in the warmer areas
such as near the coast.
Habitat: Root-knot nematodes are obligate endopara-
sites that complete most of their life cycle inside the
host plant. The only stages found outside the host plant
are the egg, the motile portion of the second juvenile
stage, and the adult vermiform male.
Hosts: Root-knot nematodes have a wide range of hosts.
The more than 2,000 host plants include all major eld
crops, most vegetable crops, fruit trees, ornamental
plants, and weeds. Some of the common host plants are
bean, cabbage, cantaloupe, carrot, celery, chard, corn,
cotton, cucumber, dandelion, eggplant, lettuce, mallow,
okra, onion, peach, pepper, plantain, potato, pumpkin,
purslane, radish, rhubarb, sorghum, soybean, spinach,
squash, sugarbeet, sugarcane, sweet potato, tobacco,
tomato, turnip, and watermelon.
Life Cycle: The short-lived adult males are vermi-
form and motile, and move into the surrounding soil
to copulate with nearby females. However, males are
not necessary since female root-knot nematodes are
able to reproduce asexually. Adult females deposit
eggs into a protective gelatinous matrix, near or just
outside the root surface. A single female lays about 500
to 1,500 eggs during her life, which lasts about two to
three months (Fig. 1). Eggs hatch only under favorable
conditions, such as adequate moisture and warm tem-
peratures. The life cycle takes anywhere from 17 to 57
days, depending on the host plant and environmental
conditions. Typically, root-knot nematode development
begins inside the egg. After the completion of embryo-
genesis, the rst-stage juvenile remains inside the egg
until it molts into the second-stage juvenile. Second-
stage juveniles hatch from the egg and move freely in
the soil in search of a suitable host plant (Figs. 2a, 2b).
Root-knot nematode second-stage juveniles undergo
an additional three molts before transforming into the
adults. Root-knot nematodes have a low host plant pen-
etration rate at temperatures below 50ºF, and a reduced
reproduction rate at temperatures below 58ºF.
Root-knot Nematode in Field Corn
Siddharth Tiwari, Department of Entomology, Virginia Tech
Jon D. Eisenback, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech
Roger R. Youngman, Extension Entomologist, Virginia Tech
Fig. 1: Root-knot nematode female and an egg-mass dissected
from a galled root.
www.ext.vt.edu
Produc ed by Com municat ions and Marketin g, Colle ge of Agr icultur e and Lif e Scienc es,
Virgin ia Polyt echnic I nstitute and State Univers ity, 200 9
Virginia Cooperative Extension programs and employment are open to all, regardless of race, color, national origin, sex, religion,
age, disability, political beliefs, sexual orientation, or marital or family status. An equal opportunity/affirmative action employer.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University,
and the U.S. Department of Agriculture cooperating. Mark A. McCann, Director, Virginia Cooperative Extension, Virginia Tech,
Blacksburg; Alma C. Hobbs, Administrator, 1890 Extension Program, Virginia State, Petersburg.
publication 444-107
2
Type of Damage: The second-stage juvenile enters the
plant root behind the root cap, in the zone of elonga-
tion where it then migrates toward the root tip. Once
it reaches the root tip, it turns 180º and travels in the
center of the root cylinder to the zone of differentiation.
In the zone of differentiation, the second-stage juvenile
injects esophageal gland secretions that cause proxylem
root cells to divide, enlarge, and form multinucleate gi-
ant cells. These giant cells act as metabolic sinks that
actively transfer nutrients from host plant to the devel-
oping nematode. Cells around the developing juvenile
also become hypertrophic and more numerous, which
ultimately results in the formation of the characteristic
root galls, associated with Meloidogyne infections.
Although corn is not often damaged by root-knot nema-
todes, it can serve as a host to three of the four common
species of Meloidogyne (M. incognita, M. arenaria, and
M. javanica); therefore, root-knot nematode populations
may be increased during corn production so that the crop
following corn in a rotation scheme may be severely af-
fected if it is sensitive to one of these nematodes.
Corn damaged by root-knot nematodes often is stunted
and has the appearance of moisture and nutrient de-
ciencies. Severe infestations can result in the death
of younger plants. Affected plants typically occur in
patches in the eld and symptoms are often attribut-
ed to other causes such as compacted or acidic soils,
nutrient deciencies, or soil insects, such as annual
white grubs, 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 difcult 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 eld
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 condi-
tions 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-
Fig. 2b. Second-stage juveniles of root-knot nematode,
Meloidogyne sp., inside a plant root.
Fig. 2a. Second-stage juveniles of root-knot nematode,
Meloidogyne sp.
Fig. 3: Comparison between root-knot nematode infected root
(left) and a non-infected root (right).
3
knot nematode, M. hapla (northern root-knot nema-
tode), is not a parasite of corn.
Control Methods
Non-chemical: Rotating corn with a non-host crop such
as alfalfa or oats may be effective in reducing root-knot
nematode populations. Because different species have
different host ranges, it is always good practice to iden-
tify the particular species in the eld before deciding
on crop rotation as a management strategy. Plants that
are non-hosts of M. incognita can serve as good hosts
for M. arenaria or M. javanica. Fields with successive
seasons of corn will suppress populations of northern
root-knot nematode, M. hapla, but at the same time this
scheme may enhance populations of other root-knot,
stubby-root, lesion, sting, lance, and ring nematodes.
Resistant cultivars are currently unavailable for south-
ern 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 identication to species is
necessary because more than one species of root-knot
nematodes may occur in some elds.
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 eco-
nomic 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 corn-
elds with root-knot nematode counts that exceed this
economic threshold [http://ipm-www.ento.vt.edu/states/
va.html]. The history of a eld with nematode prob-
lems 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, htt p://
www.pubs.ext.vt.edu/pmg/.