|
From
the Field
By
Brad Meyer |
|
Plant
parasitic nematodes are an economic problem in corn, cotton, peanuts and
soybeans in Alabama. While few management practices provide control
after crops have been planted, the month of June is a good time to
diagnose problems. Crops in affected areas typically exhibit stunting
and slow growth, which is in sharp contrast to the rapid growth of
healthy plants in the early part of the season. Several species of
nematodes damage crops in the state. Reniform and root-knot nematodes
are responsible for most of the economic loss, but damage from soybean
cyst nematode in soybeans and lesion nematode in corn is possible with
the increased acres of each.
|
|
The reniform nematode is
a pest of cotton and soybeans.
|
Reniform
Nematode
The
reniform nematode, Rotylenchus reniformis, is a pest of cotton
and soybeans. Reniform is found throughout the state and is not limited
to a particular soil type. It is capable of causing severe yield loss in
these crops and has spread at an alarming rate over the past decade.
Aboveground symptoms of reniform infestations include stunting and
irregular patterns of growth within a field. In no-till or recently
infested fields, population densities can vary greatly within the row
and in adjacent rows. Plant growth varies with population densities,
resulting in erratic patterns visible from the turnrow. Foliar symptoms
may include chlorosis in soybeans and yellowing between the veins of
mature leaves in cotton. In cotton with high reniform numbers, a
sulfur-deficiency can be induced by side-dressing nitrogen. Nitrogen
placed beside the row is readily taken up, but a root system stunted by
reniform is unable to fully explore the soil profile. The plant is not
able to take up sufficient sulfur to maintain the proper N:S ratio.
|
|
The
reniform nematode stunts both cotton and soybean roots, with root death
possible in severe infestations. Unlike root-knot or soybean cyst
nematodes, the reniform nematode does not form a gall or cyst on roots.
The female feeds with her head inserted into the root and lays eggs in a
sticky gel at the root surface. Soil sticks to these egg masses, making
them visible with a hand lens when roots are carefully removed from the
ground. No cotton varieties are commercially available with resistance
to the reniform nematode, but several resistant breeding lines exist in
soybeans. Nematicides are available and can be effective at low to
moderate infestations, but crop rotation is the most effective
management practice. Corn, peanuts and grain sorghum are non-hosts for
reniform and are effective in lowering nematode numbers. Low populations
in the first six to eight weeks of crop development are critical to
avoiding yield loss. Unfortunately, nematode populations rebound by the
end of the first year following rotation.
|
|
Root-Knot
Nematode
Several
species of root-knot nematode attack row crops. The southern root-knot
nematode, Meloidogyne incognita, is a pest of corn, cotton, and
soybeans in Alabama and the peanut root-knot nematode, Meloidogyne
arenaria, is a pest of peanuts and soybeans. Root-knot is typically
found in fields with lighter soils or in sandy areas within a field. It
is capable of causing economic yield loss and has taken on new
significance as fields are rotated with corn for management of reniform
nematode. Southern root-knot nematode populations increase on corn and
rotation with it for reniform control in cotton and soybeans provide a
continuous host. Aboveground symptoms of root-knot infestations include
stunting, chlorosis and premature wilting. In contrast to the irregular
growth within the row and in adjacent rows found with reniform nematode,
symptoms of root-knot appear in irregular shaped circles or ovals in
fields. The worst symptoms typically appear in the sandiest spots in the
field.
The
distinguishing symptom of root-knot infestations is galling on the
roots. Root-knot nematode stunts the roots of the host crop and causes
the root to form a gall which ranges in size from small (<1mm) on the
roots of corn and cotton to large (<10mm) on soybeans. Galls on the
roots of soybeans can be mistaken for Rhizobium nodules. To
distinguish between the two, lightly pull on the gall or nodule. Galls
from root-knot will not detach from the root, while nodules will easily
come off and are bright pink inside. Cotton varieties are commercially
available with field-tolerance to root-knot and resistance is available
in certain soybean varieties, but currently no corn hybrids
|
The southern root-knot
nematode is a pest of corn, cotton and soybeans in Alabama; and the
peanut root-knot nematode affects peanuts and soybeans.
|
|
have
resistance or field-tolerance. Use of nematicides varies by crop and
care must be taken when
considering crop rotation for control of root-knot nematode. Cotton is a
good rotation for peanuts in fields infested with peanut root-knot
nematode and peanuts are a good rotation for cotton in fields infested
with southern root-knot nematode.
Brad
Meyer is an agronomist for Agri-AFC. Contact him at bradm@agri-afc.com.
REFERENCES
Compendium
of Cotton Diseases, Second Edition;
T.L. Kirkpatrick, C.S. Rothrock; APS Press; 2001.
Compendium of Soybean
Diseases, Fourth Edition;
G.L. Hartman, J.B. Sinclair, J.C. Rupe; APS Press; 1999.
|
|
