Late Season Soybean Management
Key Points
 Full pod (R4) marks the
beginning of the critical period
for determining soybean yield
potential.
 Stress during seed fill can
cause more reduction in yield
potential than at any other
growth stage.
 Insects, diseases, weed
escapes, unfavorable weather,
low moisture level, and fertility
challenges all have the
potential to result in
detrimental yield losses.
By this time of the season, many decisions have already been made about your soybean crop
that will affect its yield potential. However, there are still ways to help protect the number of
bushes per acre you will achieve at harvest time. Full pod, or reproductive stage R4, marks the
beginning of the critical period for determining soybean yield potential. Preserving leaf surface
area is of primary concern during the seed filling period to avoid yield loss. Defoliation decreases
light interception, therefore decreasing the plant’s photosynthetic activity, which in turn reduces
crop growth rate, and ultimately results in yield loss. Stress during this period can cause more
reduction in yield potential than at any other growth stage. Insects, diseases, weed escapes,
unfavorable weather, low moisture level, and fertility challenges all have the potential to result in
detrimental yield losses during this stage.
During this crucial time, it is very important for growers to understand the impact of plant stress in
order to make timely decisions that may help improve soybean yield. We all know the first step to
good decision making is to gather all relevant data; therefore, getting out to see and touch your
fields during this time is very important. Scouting for signs of potential plant stress, when done
regularly, can help to mitigate the effects of a treatable problem. If the issue in question is not
treatable for this season, you can still learn lessons to help make more informed decisions in the
future. Outlined here are some things to watch out for during late-season scouting and what
management decisions may be available to help reduce the impacts of those stresses. Be sure to
contact your Channel Seedsman for help identifying issues and considering recommendations.
Insects
Insect damage to soybean can occur in the form of leaf and vegetative tissue destruction as well
as direct pod and seed feeding. Some insects can act as vectors to introduce pathogens into
plants causing further destruction. Treatment threshold levels vary for insects from location to
location, so work with your Channel Seedsman and Agronomist to determine the threshold for
your region and what treatment, if any is recommended.
 Bean leaf beetle (Figure 1A) can cause defoliation throughout all soybean growth stages. While they may also feed on pods, they rarely damage
seed, but may cause enough damage to allow entry points for fungal infection. They are also a known vector for bean pod mottle virus (BPMV).
 Grasshopper (Figure 1B) damage can include leaf feeding as well as aggressive pod and seed feeding. Grasshopper damage is generally more
likely during a hot dry summer and in weedy fields and border rows. If July and August scouting finds numbers exceeding economic
thresholds, spot spraying is usually recommended.
 Soybean podworm (corn earworm) (Figure 1C) small caterpillars feed on foliage while older ones feed on flowers and pods. Scouting is best
done from R1 through R5 and by estimated defoliation by growth stage.
 Stink bugs may attack all aboveground plant material (stems, pods, and seeds). Seeds may become dry and shriveled and pods may appear
flattened. Scouting is best done from R1 through R5, and if extensive damage is seen, foliar insecticides are available for use.
 Two-spotted spider mite (TSSM) (Figure 1D) injury can resemble herbicide damage or foliar disease. Injury appears as tiny, yellow spots, or
stipples, on leaves. As injury becomes more severe, leaves turn yellow, then brown and finally die and drop off. Spider mite injury can reduce
soybean yields and cause pod shattering, wrinkled seed and early maturity. Scouting should be done R1 through R5 and especially during hot
and dry weather. TSSM are not insects and pyrethroids are not very effective, so organophosphates may be considered if warranted.
 Soybean aphid (Figure 1E) can cause significant injury and economic losses if left untreated. Piercing-sucking mouthparts allow them to feed on
the sap of all aboveground plant parts. Scouting should be done throughout July and August and should focus on the newest emerged foliage.
1E
1A
1B
1C
1D
Figure 1. L to R: 1A) bean leaf beetle, 1B) grasshopper, 1C) soybean podworm, 1D) two-spotted spider mite injury, and 1E) soybean aphid.
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Late Season Soybean Management
Diseases
Some fungal diseases that may affect soybean late in the season include Cercospora leaf blight (Cercospora
kikuchii), brown spot (Septoria glycines), frogeye leaf spot (Cercospora sojina), pod and stem blight
(Diaporthe phaseolorum), and anthracnose (Colletotrichum truncatum). These diseases are generally favored
by warm and humid or wet conditions.
Symptoms of Cercospora leaf blight, brown spot, and frogeye may appear as spots on the leaves (Figure 2).
Defoliation is the primary concern with these diseases as healthy, green plant material is needed for
photosynthesis and converting sugars for pod fill.
2A
Symptoms of pod and stem blight and anthracnose are similar and present as irregularly-shaped brown
blotches found on stems, petioles, and pods in the early reproductive stages. While neither disease usually
causes severe yield loss, it is important to scout and assess overall plant health throughout the reproductive
stages. Infection on pods may result in few or small seeds per pod. Leaf defoliation may also be an issue if
infestation is severe.
The first step in disease management is identification. Proper identification can help you save time and
money as you learn how to treat a disease and to prevent its occurrence or spread in the future. Once you
know what diseases your fields are prone to, selecting resistant products based on tests conducted in
closest geography with your operation can help to prevent problems. Learning the management practices
associated with specific diseases in your cropping system is also important. Finally, consider using
fungicides to help control those diseases that are still treatable in-season. Deciding whether to spray can be
a difficult decision and should be based on disease severity and timing. Fungicide applications for lateseason diseases are generally made during pod development stages R3 and R5. Pod set through seed fill
stages (R3 through R6) are the most critical period for seed yield. Leaf loss can significantly reduce yield if
diseases attack during early seed filling. Spraying fungicides after R6 is generally not recommended.
2B
Weeds
While generally weed control should be completed before mid- to late-season, field scouting should still
involve an observation for any weeds that have escaped control. Generally weed escapes have little impact
on yield potential and chemical control is not warranted. However, there are some things to consider when
observing late-season weeds. While these weeds may not have much affect on yield, they do have the
potential to interfere with harvest and introduce foreign matter to the harvested soybean as well as adding
weed seed to the soil. In some situations, weed management may become necessary. If this is the case,
you should carefully select herbicide chemistry based on the type and size of weed present in the field,
growth stage of the soybean, and preharvest interval. Be sure to follow labeled recommendations for the
selected herbicide. Finally, try to identify the cause for weed escapes. Those of particular concern are weeds
that may have developed glyphosate or ALS resistance.
2C
Figure 2. Top to bottom: 2A)
Cercospora leaf blight, 2B) brown spot,
and 2C) frogeye leaf spot.
Water Stress
Water demands are highest during the soybean reproductive period. Drought stress from beginning bloom through the pod-fill stage can greatly
reduce yields. This loss in yield may result from production of fewer seeds, smaller seeds, or a combination of the two. Good soil moisture
conditions need to exist from rainfall or irrigation to ensure adequate moisture from beginning bloom until the beans are touching in the pods.
Irrigation during this time may help reduce potential yield loss.
Nitrogen Fertility
A soybean plant’s demand for nitrogen and water is greatest during pod fill, particularly after the R3 growth stage. High-yielding soybeans, such as
those grown under irrigation, may not be able to fix enough nitrogen (N) in nodules to supply all the N necessary for optimum grain fill. Both soil N
and fixed N may be necessary to achieve maximum soybean yield, particularly under high-yield environments. Research done in Kansas has shown
an average yield increase of nearly 12% when supplemental N was applied at the R3 soybean growth stage under irrigation.1
Sources:
1
Wesley, et al. 1998. Effects of Late-Season Nitrogen Fertilizer on Irrigated Soybean Yield and Composition, Journal of Production Agriculture, Vol. 11: 331-336.
Compendium of Soybean Diseases, Fourth Edition. APS. 1999.
Dorrance, A., et al. 2007. Using Foliar Fungicides to Manage Soybean Rust. Plant Health Initiative. NCERA publication SR-2008.
Field Crop Insects. 2012. Iowa State University Extension. CSI 0014.
Knezevic, S.Z. et. al. 2003. Yield penalty due to delayed weed control in corn and soybean. Plant Management Network. http://www.plantmanagementnetwork.org, (verified 7/18/2013).
Kranz, W.L. and J.E. Specht. 2012. Irrigating soybean. NebGuide. University of Nebraska-Lincoln Extension. G1367. http://extension.unl.edu/publications (verified 7/18/13).
Pedersen, P. 2006. Brown spot—Septoria leaf blight. Iowa State Univ. Extension. http://extension.agron.iastate.edu (verified 7/18/13).
Sisson, A., et al. 2012. Late season soybean scouting. Iowa State University Extension. CSI 0008.
Sweets, L. Late season soybean diseases. University of Missouri Integrated Pest & Crop Management. http://ipm.missouri.edu (verified 7/18/13).
Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and
weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Leaf
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