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Each
crop planted in Alabama has its own set of management requirements.
Among the similarities in management of all crops is the importance of
getting a good start. Soybeans are no exception. As planted fields begin
to show signs of emerging crops, growers should begin evaluating stands
and scouting for limiting factors. Development during the first six
weeks provides the foundation for successful and profitable production
during the rest of the season. Eliminating limiting factors during this
period is critical to maximizing yield potential.
The
first decisions will revolve around the adequacy and uniformity of plant
stands. While optimum population varies between crops, stands of each
must have healthy, undamaged plants and be free of frequent skips
between plants. Root establishment is very important early in crop
development, so evaluation of root health should be included in stand
counts. After confirming the establishment of an adequate stand of
healthy plants, the focus will shift to limiting factors such as
fertility, weeds, disease, insects and nematodes.
According
to Larry Heatherly with the USDA Agricultural Research Service,
"Data collected in the midsouthern U.S. indicate a density of
80,000 to 120,000 uniformly distributed plants per acre is adequate for
maximum yield. Plant populations that fall below this range can be
tolerated with little or no yield loss if the stand is uniform and the
variety branches profusely."
Stands
should be evaluated by determining plants per row foot and converting to
plants per acre based on row spacing or by using the "hula
hoop" method in drilled soybeans.
Soybeans
are legumes and have the ability to acquire or "fix" nitrogen
from the air through an association with the bacteria Bradyrhizobium
japonicum. This bacteria forms nodules on the roots of soybeans and
can provide all the nitrogen needed to produce a crop. Pale green or
yellow soybeans can be a nitrogen deficiency resulting from poor
nodulation. Poor nodulation can occur if sufficient Rhizobium
inoculant is not present in the soil, the micronutrient molybdenum is
deficient, soil pH is low or the soil is extremely dry. Iron chlorosis
can occur on high pH soils.
Seedling
diseases attack roots and lower stems of seedling soybeans, reducing
early season root and shoot growth and possibly causing seedling death.
Common seedling diseases in Alabama are Pythium, Fusarium,
Rhizoctonia and Phytophthora. Symptoms include rotted seeds,
dark lesions on the roots and necrotic lesions on the lower stems. Soil
temperature below 60 degrees F and excessive soil moisture slows down
seedling growth and promotes the development of disease. Slow growth
results in a longer period of seedling exposure to pathogens.
Several
insects attack soybeans early in the season. Thrips injure soybean
seedlings by feeding on small trifoliate leaves as they emerge from the
terminal. Soybean plants normally grow fast enough to outpace thrip
damage, but economic damage can occur with extreme numbers or during
periods of slow growth. Bean leaf beetles damage soybean seedlings by
chewing small irregular shaped holes in the leaves. Defoliation of
seedling plants is rare but possible and bean leaf beetles can transmit
several viral diseases. Threecornered alfalfa hoppers damage seedlings
by girdling the lower stem, causing the plant to lodge later in the
season.
Several
species of nematodes cause economic damage to soybeans in Alabama.
Soybean varieties vary in their resistance or field tolerance to each
species. Since seed was in short supply this year, growers may not have
been able to select varieties based on ability to withstand nematodes.
Reniform, root-knot and soybean cyst nematode are the main species of
nematodes in the state. Soybeans are the only major crop in the state
that serves as a host for soybean cyst nematode, but cotton and soybeans
both host reniform. Corn, cotton and peanuts each increase populations
of various root knot species.
Above
ground symptoms of damage from soybean cyst nematode and root-knot
nematode are similar in soybeans. Plants in affected areas will
typically be stunted and yellow with symptoms showing up in irregular
shaped circles or ovals in fields. The symptoms can be mistaken for
nitrogen deficiency or iron chlorosis. Reniform nematodes will also
cause stunting, but the pattern will be erratic throughout the field
with plant growth varying greatly within the same row and in adjacent
rows. Depending on populations, reniform can cause yield loss with few
noticeable symptoms.
All
three species will stunt roots. The root-knot nematode causes galls on
roots that 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. Soybean cyst nematode will form a cyst
on soybean roots which is much smaller than a gall or nodule. The cyst
will be lemon-shaped and will progress from white in color to a yellow
or tan. Reniform nematode does not form a gall or cyst on soybean roots.
References:
Soybean
Production in the Midsouth;
Larry G.
Heatherly & Harry F. Hodges; CRC Press LLC; 1999.
Brad
Meyer is an agronomist for Agri-AFC. Contact him at bradm@agri-afc.com.
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