RESEARCH UPDATE
1993
SOYBEANS
Effects of a Damage Threshold 
of
Sicklepod on Soil Seed Reserves
Research indicates that one sickle-
pod per 9.6 ft. of rows spaced 30 in.
apart will not adversely affect soy-
bean yields. However, at this density
soil seed reserves of the weed increase
200-250%, resulting in unmanageable
sicklepod problems in following years.
Experiments at the E.V. Smith Re-
search Center in Tallassee and the
Prattville Experiment Field from 1989-
92 investigated the frequency with
which one sicklepod per 13 ft. in con-
ventional planting of soybeans can re-
turn seeds to the soil without increas-
ing the degree of infestation. Allow-
ing one sicklepod per 13 ft. to reseed
all four years resulted in decreased
soil seed reserves at both sites. Infesta-
tion was reduced by 58%, at Tallassee
and 52% at Prattville.
Infestation at Tallassee for plots that
remained weed free for four years was
reduced 87%. Infestation in plots al-
lowed to reseed one year was reduced
90%; two years, 88%; and 
three years,
64%,.
At Prattville, infestation was re-
duced 70% in weed-free plots; 64%
with one year of reseeding; 69%, two
Continued on page 2
PERCENT OF THE INITIAL SICKLEPOD SEED INFESTATION IN THE SOIL AS AFFECTED BY THE FREQUENCY OF RESEEDING
OF A DAMAGE THRESHOLD, 1989-1992
Tallassee
0/41 1/4 2/4
1989
Spring .......... 100
Fall............ 52
1990
Spring .......... 27
Fall............ 20
1991
Spring .......... 15
Fall............. 11
1992
Spring ......... 13
Prattville
3/4 4/4 0/4 1/4 2/4 3/4 4/4
100 100 100 100 100 100 100 100 100
68 67 88 70 71 83 80 80 72
34 31 31 27 61 75 68 66 65
22 25 21 41 43 42 42 34 95
14 14 10 28 42 46 37 35 90
12 9 29 42 21 31 28 41 72
10 12 36 42 30 36 31 35 48
1 Years that reseeding occurred: 0/4 = none; 1/4 = 1989; 2/4 = 1989,
1992; 4/4 = 1989, 1990, 1991, 1992.
1992; 3/4 = 1989, 1991,
Crop Rotation May
Aid Stem Canker
Control in Reduced
Tillage Systems
Disease reduced the South's 1991
soybean crop by approximately 
12.6%,
a loss of almost $382.5 million. One
major yield-reducing soybean disease,
southern stem canker, apparently can
be managed by crop rotation, espe-
cially under tillage systems that leave
crop debris on the soil surface.
An AAES study was conducted to
determine the effect of crop rotation
and its interaction with tillage on two
soybean varieties: Essex, which is
moderately resistant to stem canker;
and Forrest, which is moderately sus-
ceptible.
A severe outbreak of stem canker
occurred during an ongoing cropping
system and tillage experiment at the
Sand Mountain Substation in Cross-
ville. Using Essex and Forrest, three
tillage systems were compared in three
cropping systems. Plots were rated for
stem canker development and yield.
Averaged over the 
two cultivars,
stem canker incidence was higher un-
der strip- and no-till tillage - with 35
and 25%, respectively, of plants dead
or dying - than for conventional till-
Continued on page 2
AA AA E
Low~ T. FtoIIS11 DROCOW MURNVNV1E$1*,'A8AM
Stem Canker Control, continued
age, with less than
10% of plants af-
fected. Incidence
averaged 60 % for
plots planted con-
tinuously with soy-
beans and 25%/
where soybeans
followed corn. No
stem canker was
detected in soy-
beans planted after
wheat.
However, the
effect of crop rota-
tion was highly de-
pendent upon till-
age system. Under
conventional tillage,
stem canker ratings
were not affected
Cropping
system
Continuous ..................
Soybean-corn ..............
Corn-wheat-soybean ...
Tillage, avg ..................
Conventiona
Bu./a
37.0
41.2
33.1
37.1
Yield, by tillage systems
I Strip No-till
Bu./a Bu./a
25.5 26.9
36.9 36.8
37.1 37.6
33.1 33.7
Avg.
Bu./a
29.8
38.3
33.7
TABLE 2. EFFECTS OF TILLAGE SYSTEMS AND CROP ROTATION
ON FORREST SOYBEAN YIELDS IN 1992
Cropping Yield, by tillage systems
system 
Conventional 
Strip No-till 
Avg.
Bu./a Bu./a Bu./a Bu./a
Continuous ............... 23.1 14.1 12.4 16.5
Soybean-corn ........... 34.0 18.5 12.4 26.4
Corn-wheat-soybean ... 29.8 31.1 32.0 32.0
Tillage, avg ............. 29.0 21.2 23.6
by rotation, but under strip- and no-
till, rotation with corn greatly reduced
stem canker incidence. Conventional
tillage was effective enough in con-
trolling stem canker that rotation pro-
vided no added benefit.
Effect of tillage on yield was de-
pendent upon crop rotation, with crop
rotation having less effect under con-
ventional tillage. See tables 1 and 2
TABLE 1. EFFECTS OF TILLAGE SYSTEMS AND CROP ROTATION
ON ESSEX SOYBEAN YIELDS IN 1992
Cultural Practices That Aid Weed Control and Yield
Because of low yields TABLE 1. E
and declining prices, 
com- ON
plete reliance on herbicides
for weed control in soy- 
Row spaci
beans will quickly drive a
producer out of business. Rotill, 30 ir
Recent research has fo- 
Drill, 8 in..
cused on using 
narrow row 
1 Aver
spacing and early planting Stonewall
to benefit yield and 
reduce 2 Dual
species inc
production costs. weed, pric
Experiments at the
Plant Breeding Unit of the E.V. Smith
Research Center showed a slight ad-
vantage for soybeans planted in eight-
in. rows with a grain drill. Compari-
sons were made with soybeans planted
in conventional 30-in. rows, and both
planting techniques were observed
with minimum herbicide inputs and
without. However, yields were not
impressive regardless of treatment,
suggesting that drill planting is not a
cure all (Table 1).
Later experiments in Tallassee con-
tinued these comparisons but also in-
EFFECTS OF Row SPACING AND HERBICIDES
YIELD OF SOYBEAN WHEN PLANTED
EARLY JUNE 1991, TALLASSEE
1
ng No herbicide herbicide'
Bu./a Bu./a
n. ..... 20.5 22.9
21.6 24.3
age of three experiments with the
cultivar.
+ Sencor (2 + 0.375 lb. ai/A). Weed
:luded large crabgrass, smooth pig-
kly sida, and sicklepod.
cluded an early planting
variable. Table 2 shows
that April 30 planting in
narrow rows plus herbicides was the
best combination. Even without herbi-
cides, April planting in narrow rows
produced good yields. Lack of ad-
equate control of sicklepod in the tests
probably prevented yields above 40
bu. per acre.
Similar experiments also were con-
ducted in Prattville. Planting April 16
in 10-in. rows and controlling weeds
produced an average of more than 45
TABLE 2. EFFECTS OF Row SPACINGS, HERBICIDES, AND PLANTING DATES
ON 1992 SOYBEAN YIELD, TALLASSEE 1
Drill-8 in. rows
Planting No
date herbicide
Bu./a
herbicide
2
Bu./a
Rotill-30 in. rows
No
herbicide herbicide
Bu./a Bu./a
April 30 ........... 31.5 37.2 26.1 29.7
June 22 .......... 24.21 29.2 27.0 30.1
1 Average of two experiments. Cultivars were Hutcheson (4/
30) and Stonewall (6/22).
2 Dual + Lexone and Dual 
+ Canopy (2 = 0.375 lb. 
ai/A). Weed
species included large crabgrass, smooth pigweed, prickly sida,
and sicklepod.
3 Low yields in one experiment 
due to poor stand with the 
drill
planting.
bu. per acre. Even without complete
weed control, yield was above 35 bu.
per acre.
Early planting in narrow rows not
only shows potential to improve weed
control with fewer herbicides, it also
makes soybeans less vulnerable to
drought during the critical bloom and
pod filling stages and allows the crop
to be harvested earlier.
Contact: R.H. Walker, 844-3994
for further analyses of how the vari-
ous treatments affected yield for each
cultivar.
Contact: J.H. Edwards. 844-3979
Effects of Sicklepod, continued
years; and 65',, three years (see 
the
table). Lesser declines in seed reserves
at Prattville may have been due to diffi-
culty in fully inserting the six-in. soil
probe used in sampling for these stud-
ies. Also, fewer weed flushes were
observed at Prattville.
The AU tests indicate that produc-
ers should base weed control programs
on damage thresholds to help reduce
weed control cost. However, damage
threshold of weeds needs to be based
on more than just short-term yield
losses. If allowing the damage thresh-
old to reseed causes substantial in-
creases in soil seed reserves, lowering
the damage threshold may be required
to realize greater economic benefits.
Contact: R.H. Walker, 844-3994
A Flower and Pod
Staging System for
Soybean
Most soybean flower and pod
shedding, which can greatly limit
yield, occur within 14 days after flow-
ering, a time that includes the critical
period of pod set and early pod exten-
sion. A system for quantifying flower
and pod development based on the
morphological appearance of the
flower before and after flowering has
been developed by Auburn University
and USDA researchers.
Changes in the appearance of the
flower petals are used to distinguish
the different stages of the system. Ex-
ternal dimensions of the pistil, or
young pod, have been correlated with
internal features of the immature seeds,
or ovules, for each stage of develop-
ment. See the figure showing flowers at
several stages of development.
From flower opening to pod set,
which is four to five days under typi-
cal field conditions, pistil length and
weight increase about twofold and
fivefold, respectively. Ovule develop-
ment progresses from unfertilized egg
cells to embryos surrounded by cellu-
lar endosperm. Pod lengths are corre-
lated with ovule dimensions and em-
brvo cell number.
Stages can be determined without
removing flowers or pods from the
plant, thus permitting observations or
experiments to be completed without
impeding pod development. The sys-
tem identifies precise stages when pod
set occurs and when young pods cease
growing and eventually shed. This sys-
tem of standardized descriptions of
flower and pod development is useful
in studies assessing effects of stress and
genetic factors that influence pod set
and abortion.
Contact: C.M. Peterson. 844-1632
Reproductive Stage of
structure development
-- -- r~'
Bud
Flower
Pod -
i i U
-Anthesis-f
Post
anthesis
Visible
pod set
I Continued I
Ipod
expansion
AO
-
Al
- A2
- PO
- P1
- P2
- P3
-P4 Pod 
length
-P5 (mm)
15
-P6 20
25
I i
This figire show's the stages of soy bean reprodiuctive deveilopment oli m the flower
bud to earlY pod extension: (A) BI bud w ith tile tip of the banner petal (arrow)
slightl y extended two days betore flowering: (B) Al flower with completely opened
and reflexed banner petal at fidl flowI'er. (C) PI pod showing collapse and owithering
of tlhe petals two to three daus qaftcr /lowering.: (D) P3 or pod set stage with ithe stigmia
(arrowO) beyond the wi'ithered petals at /0lo to five days after flowering,. and (E) P5
pod stage showing firther pod extelnsion beyond the wiithered corolla .fie to eight
days aier flowlering. The bar on eahi photograph is tlwo mn.
Flower and Pod Development in Water-Deficient Soybeans
Droughts during flowering and
early pod development decrease soy-
bean yield, primarily due to an in-
crease in flower and pod abortion.
Failure to set pods may indicate an
inherent sensitivity to low tissue wa-
ter potential at critical stages.
Water status and pod set of flow-
ers exposed to water deficits at various
stages of development from bud (BO)
to early pod expansion (PS) were mea-
sured in a joint AAES/USDA project.
See the figure with "A Flower and Pod
Staging System for Soybean" for a de-
scription of flower and pod stages.
Results indicate that sensitivity of
soybean flowers to low water potential
varies with flower cluster position and
100
80
60
0
0
0 40
P3 P5 P6P3 P5 P6
stage of development. Flowers in mid-
cluster positions are particularly vul-
nerable. Early ovary expansion is the
critical stage in soybean reproductive
development under drought condi-
tions.
Water potential of leaves and re-
productive structures decreased when
water was withheld for two days, and
then returned to normal values within
one day. The water potential of pods
that had just formed (P2) but were not
set recovered more slowly. This brief
water deficit decreased pod set on ter-
minal flower clusters by as much as
70%.
The effect of low water potential
on pod set varied with flower position
on the cluster and stage of develop-
ment. In general, low water potential
during flowering and prior to rapid
pod expansion (A to P2 stages) caused
the largest decrease in pod set. See the
figure with this report.
Reproductive structures at basal
nodes of flower clusters (1-4) were
least affected, regardless of when stress
was imposed. Flowers borne on middle
nodes (5-8) were far more vulnerable,
and low water potential eliminated pod
set almost completely at the last-
formed nodes (9-12) of the clusters. Pods
destined to shed ceased to develop at
the P2 stage within three to four days
of flowering.
Understanding the stage of devel-
opment when soybeans need water
the most can help producers time ir-
rigation to this critical period and may
also provide information useful in the
selection of more drought-resistant
strains.
Contact: C.M. Peterson 844-1632
Stimulation of Pod
and Ovule Growth
of Soybean by BAP
A staging system developed by
Auburn scientists was used to study
the influence of the plant growth regu-
lator 6-benzylaminopurine, 
or BAP,
on soybean flower and pod shedding.
Previous studies showed that
shedding of last-formed flowers on
flower clusters is preceded by starch
depletion within the flowers or young
pods three to four days after flowers
fully open during the P2 stage of devel-
opment. Loss of starch may signal the
onset of abortion that leads to shedding.
Some growth regulators have been
shown to delay this abortion and in
some cases actually increase the starch
deposition generally associated with
pod set.
Studies were performed to deter-
mine the influence of BAP on pod devel-
opment at the last-formed flowers on
terminal clusters. Three daily applica-
tions of lanolin or BAP in lanolin paste
were made to the base of fully opened
flowers at the four most distal flower
nodes of flower clusters 
(D, D-1, D-2,
and D-3).
Continued on page 5
Alternative
Rotation Crop for
Nematode Control
Previous AAES research found
that one of the best ways to control
root-knot or soybean cyst nematodes is
rotation with a nonhost crop combined
with a nematode-resistant cultivar.
Grass crops, such as bahiagrass, corn,
and sorghum, are effective in reducing
nematode populations and increasing
soybean yields. During 1991-92, some
less well-known crops were evaluated
for their effectiveness in providing
these benefits.
Velvetbean, an African legume
that was once widely grown, in Ala-
bama as a nitrogen source, was shown
to differ from most other legumes in
Continued on page 5
This figure shows the effect of brief wiater deficit on the percentage of the
total flower population that set pods on the last-foried flower clusters of sovbeans.
Water was withheld for two days when flowers/pods at transition nodes (5-8) wvere at
A-P2, P2-P3, or P4-P5 stages of developnient. Three stages of pod development (P3
= pod set; P5 = pods 10-20 nini; and P6 = pods greater than 20 nn) were present
10-12 days after rewatering, and the numbers of each stage are presented as a
percentage of the total population. See the figure with a "A Flower and Pod Staging
systemftor Soyhbean" for a description offlower and pod stages.
Water Deficient Soybeans, continued
This figure shows mean fresh weight of pod
walls and ovules collected from various positions on
the flower cluster six days after the first lanolin or
BAP application.
Alternative Rotation Crop, continued
that it is a nonhost for both common
species of root-knot nematode and the
soybean cyst nematode.
Velvetbean and soybean were
planted in 1991 in a field infested with
a mixture of these nematodes to com-
pare the response of a soybean crop
planted at the site in 1992. Nematode
populations prior to planting the 1992
soybean crop were much lower in the
velvetbean plots than the soybean
plots.
Yields in plots planted in soybeans
for two consecutive years averaged
13.7 bushels per acre. Yields in plots
planted first in velvetbean averaged
27.9 bushels per acre. Yields were av-
eraged over seven cultivars with a
wide range of nematode resistance (see
the table). The highest yielding treat-
ment was Leflore, a nematode-resis-
tant cultivar, following 
velvet-bean,
250
200
" 150
50
501
which produced 32.7 bushels an acre.
Further experiments are planned
to evaluate the potential of velvetbean
as a crop in Alabama and to determine
the agronomic worth of various velvet-
bean types.
Contact: R. Rodriguez-Kabana, 844-5003
YIELD OF SEVEN SOYBEAN CULTIVARS FOLLOWING
SOYBEAN OR VELVETBEAN AS A PREVIOUS CROP IN A
SOIL INFESTED WITH ROOT-KNOT AND SOYBEAN CYST
NEMATODES, 1992
Previous crop
Cultivar Soybean Velvetbean
Bu./a Bu./a
Braxton .......... 11.3 26.4
Brim .......... 10.0 24.5
Bryan......... 15.2 29.5
Kirby ......... 15.2 27.9
Leflore ............ 18.3 32.7
Stonewall ....... 11.8 24.9
Thomas ....... 13.9 29.6
BAP, continued
BAP increased pod and
ovule length, width, and
weight at all four distal
nodes within clusters when
compared to controls treat-
ed with lanolin (see the fig-
ure). Greater amounts of
starch were found in BAP-
treated ovules, and this in-
creased starchwas associated
with improved pod set.
Results indicate that
BAP prevents shedding
and stimulates renewed
growth of distal pods by
delaying the onset of abor-
tion. Changes detected in
the physical attributes of
BAP-treated pods support
this hypothesis.
Further research can
use this information in two
ways: (1) to pursue the de-
velopment of a spray-on
application of a pod-setting
hormone such as BAP, or
(2) to select for a soybean
cultivar that is efficient
at producing its own
growth regulating hor-
mone.
Contact: C.M. Peterson, 844-1632
Frogeye Leaf Spot:
A Threat to
Alabama
Soybean
Producers
Frogeye leaf spot can have a sig-
nificant negative impact on soybean
yield. Severe outbreaks of the disease
occurred in Alabama and elsewhere in
the Southeast in recent years, but there
are no reliable estimates of the yield
loss caused by it.
The disease, which is caused by a
wind-bome fungus similar to the one
that causes early and late leaf spot of
peanuts, is characterized by dead spots
on the leaves. It gets its name from the
gray color of these spots, which re-
semble a frog's eyes.
Moist conditions favor develop-
ment of the disease, which causes dam-
age by reducing the photosynthetic
capacity of the leaf. AAES tests have
shown that the severity of leaf symp-
toms is directly related to yield loss,
and much of the yield loss is attribut-
able to a reduction in individual seed
weight.
Unlike leaf spot of peanuts, which
is normally controlled by fungicide
sprays, the best way to control frogeye
is by growing soybeans with genetic
resistance. Although resistant cultivars
exist, many cultivars now grown in
Alabama are susceptible.
To determine yield loss due to
frogeye, Auburn researchers con-
ducted a two-year project to test soy-
bean lines that differed only in that
one is susceptible and the other con-
tains genes that provide resistance to
the disease.
At the Gulf Coast Substation in
Fairhope, no disease developed, and
there was no difference between the
resistant and susceptible lines.
At the E.V. Smith Research Center
in Shorter, plots were inoculated with
the disease. Conditions for inoculation
were poor in 1991, and frogeye re-
duced the yield of susceptible plants
by 5%. Inoculation in 1992 was more
Continued on page 6
0 I/-
D D-1 D-2 D-3
Diseases Had Little
Impact on Soybean
Production in 1992
Diseases were not an important
factor in soybean production during
the 1992 season.
Soybean stem canker, which dev-
astated Alabama's soybean industry
in the late 1970s and early 1980s, was
not observed in the state, except in a
few susceptible varieties at the E.V.
Smith Research Center in Tallassee. Use
of resistant varieties, adoption of new
cultural practices, and unfavorable
weather conditions for canker develop-
ment are the principal reasons Alabama
has not experienced the problems with
stem canker as neighboring states have
in recent years.
Sudden Death Syndrome (SDS)
was observed in a few fields in Jackson
County last season, but only one soy-
bean field suffered appreciable losses
to the disease. First observed in a soy-
bean variety test at the Sand Mountain
Substation in 1990, SDS has not been
as serious a problem to soybean pro-
duction in Alabama as it has been in
Mississippi, Arkansas, Tennessee,
Kentucky, and Illinois.
Pod and foliar diseases - Anthra-
cnose and pod and stem blight - were
responsible for an estimated 6-7% re-
duction in yield in 1992. Other foliar
diseases such as frogeye leaf spot,
brown spot, downy mildew, and
cercospora leaf blight accounted for
less than 
2
% yield losses in soybean
last season.
Contact: W.S. Gazaway, 844-5505
EDITOR'S NOTE
This publication presents summaries of recent
research by the Alabama Agricultural Experiment
Station (AAES), which is based at Auburn Univer
sity. Mention of company or trade names does not
indicate endorsement by the AAES or Auburn
University. Any mention of nonlabel uses or appli
cations in excess of labeled rates of chemricals does
not constitute a recommendation. Such use in re-
search is part of the investigation necessary to
evaluate materials and treatments.
This information is available to all persons with-
out regard to race, color, sex, or national origin.
April 1993 1 M
Threecornered Alfalfa Hoppers Grow
Faster in Warmer Temperatures
The Threecornered alfalfa hopper
is considered an economic pest of soy-
bean in several southeastern states, in-
cluding Alabama. They injure plants
by girdling main stems, lateral
branches, and leaf petioles. Informa-
tion on the growth and development
of this insect is needed to better under-
stand when it will become a pest.
Development and survival of
nymphs was assessed at 10 constant
temperatures from 55 to 95
0
F. Twenty-
four nymphs were reared at each tem-
perature while individually caged on
two-in. sections of beans in clear plas-
tic Petri dishes. They were observed
every 24 hours for molting or death.
Development increased with tem-
perature from 55 to 90
0
F. Sixty-two to
100% of nymphs died at 95
0
F. Sur-
vival of nymphs to adulthood was
greater than 50"/ at 80 to 850F. Nymphs
grew slowly or not at all in tempera-
tures ranging from 55 to 60
0
F. These
data indicate that threecomered alfalfa
hoppers will grow and develop best
during summer months, but will not
grow well early in the growing season
or when daily maximum temperatures
regularly exceed 100
0
F.
Contact: T.P. Mack, 844-2558
Frogeye Leaf Spot, continued
effective, and yield of susceptible lines
was reduced by 38%.
Greenhouse reaction to frogeye
leaf spot for most cultivars available in
Alabama will be published in the 1993
Soybean Variety Test Report.
Contact: D.B. Weaver, 844-3982
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