The soybean reproductive stage R4 or full pod is the most crucial period for determining yield potential.
Plant stress that limits seed number and/or seed size during full pod can results in more yield loss than at any other time.
In order to make timely decisions that may help protect soybean yield potential, it is important to understand the impact of plant stress.
Stink Bugs. Several species of stink bugs can infest soybean fields including green stink bugs (Acrosternum hilare) (Figure 1), southern green stink bugs (Nezara viridula), brown marmorated stink bug (Halyomorpha halys), and brown stink bugs (Euschistus servus).1 Stink bugs are shield shaped; juveniles or nymphs do not have wings, but are similar in appearance to adults. Stink bugs damage soybeans by sucking juices from pods causing pods to drop.
Soybean Podworm, (Helicoverpa zea; also known as corn earworm). Infestations normally occur from late July through August. Small larvae are usually brown with a dark head and as the larvae grow they change color from yellowish green to black; with head turning orange. Larvae first feed on foliage. Large populations can cause severe defoliation. Once soybean pods form, larvae will feed on the pods.
Soybean Aphids (Aphis glycines). Soybean aphids are minute yellow-colored insects with distinct black cornicles. Infestations can occur from late May through August (Figure 2). Aphids damage soybean plants by sucking plant juices, causing reduced plant vigor and leaf puckering. If feeding continues through the pod filling stage, pod set may be affected resulting in fewer seed per pod.2 As much as 50% yield reductions may occur in severely infested fields. Aphids infestations during the pod filling stage can be more severe on yield potential than outbreaks at R5 or R6 growth stages.
Occurrence of insect pests are highly variable each season. Treatment thresholds and recommendations vary by state. Please contact your local agronomist for thresholds and apply insecticides as needed.
Frogeye Leaf Spot (Cercospora sojina), Sudden Death Syndrome (Fusarium virguliforme), Stem Canker (Northern states—Diaporthe phaseolorum var. caulivora , Southern states—D. phaseolorum var. meridionalis), Phytothora root rot (Phytophthora sojae), Charcoal Rot (Macrophomina phaseolina), and Pod and Stem Blight (Diaporthe phaseolorum var sojae) are fungal diseases that may affect soybeans late in the season.4
Frogeye leaf spot produce spots on leaves (Figure 3). Premature defoliation is also a concern with frogeye leaf spot. Since pod set through seed fill stages (R3 through R6) are the most critical periods for yield potential, leaf loss can significantly reduce yield potential.
Pod and stem blight can be found on stems, petioles, and pods in the late reproductive stages as parallel lines of small black dots. Infection on pods may result in fewer or smaller seeds per pod. Pod and stem blight can lead to early maturity and defoliation is an issue if the disease is severe.
Stem canker symptoms can first appear during at early reproductive stages as small, red-brown lesions near a lower leaf node and expand lengthwise as the season progresses. Lesions eventually girdle the stem, causing wilting and plant death.
To maximize yield potential, weeds must be controlled between V1 and V3 soybean stage.5 Controlling weeds early can speed up canopy closure and early development of canopies may increase flowering time and number of main-stem nodes. Late season weed emergence may have little effect on yield potential, but can interfere with harvesting and add foreign matter to grain. When in-season weed management is needed, over-the-top applications can be used to control late-germinating weeds and weeds not controlled by preplant or preemergence herbicides.6
Soybean plants require the most water during pod development (R3-R4) and seed fill (R5-R6). Moisture stress can severely reduce yield potential by producing smaller and/or fewer seeds.7
Since soybean plants produce nutrient rich, high protein seed, nitrogen (N) requirements are rather high. Peak N demand is during pod fill (R4 to R6 growth stages). During peak demand, both soil N and fixed N may be necessary for maximizing soybean yield potential, particularly under high-yield environments.8 Research shows that supplemental N during peak demand may help yield potential.
Sources: 1 Soybean insect pests. North Carolina Cooperative Extension Service. http://ipm.ncsu.edu. 2 Soybean aphid, speed scouting worksheet, Field Crops IPM, Purdue University. http://extension.entm.purdue.edu. 3 Brown spot—Septoria leaf blight. Iowa State Univ Extension. January 2006. Online at http://extension.agron.iastate.edu. 4 Late season soybean diseases. 2013. University of Missouri Extension. www.//ipm.missouri.edu. 5 Knezevic, S.Z., Evans, S.P., and Mainz, M. 2003. Yield penalty due to delayed weed control in corn and soybean. Plant Management Network. http://www.plantmanagementnetwork.org. 6 Ferrell, J.A., MacDonald, G.E., and Leon, R. 2012. Weed management in soybean, Florida Cooperative Extension Service, University of Florida, Publication number SS-AGR-05. http://edis.ifas.ufl.edu. 7 Thomas, J.G., and Blaine, A. 2010. Soybean irrigation, Mississippi State University Extension Service, Publication number 2185. http://msucares.com. 8 Wesley, T.L., Lemond, R.E., Martin, V.L., and Duncan, S.R. 1998. Effects of late-season nitrogen fertilizer on irrigated soybean yield and composition. Journal of Production Agriculture, Vol. 11: 331-336. http://www.agronomy.ksu.edu. Web sources verified 05/13/2015. 130627060107