August 9, 2017

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 yield losses.

By this time of the season, many decisions have already been made about your soybean crop that will affect 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 yield loss 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. This Channel Advice provides items 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) 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. Pyrethroids are not very effective on TSSM, so organophosphates may be a management option.
  • 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.
Figure 1

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) are fungal diseases that may affect soybeans late in the season.2

Figure 2

Frogeye leaf spot produce spots on the leaves, while interveinal yellowing may form with sudden death syndrome (SDS) and stem canker. Premature defoliation is also a concern frogeye leaf spot, phytophthora root rot and stem canker. Since pod set through seed fill stages (R3 through R6) are the most critical period for yield potential, leaf loss can significantly reduce yield potential. Healthy, green plant material is necessary for photosynthesis and converting sugars for pod fill; therefore, it is important to protect the leaf tissue.

Pod and stem blight can be found on stems, petioles, and pods in the early reproductive stages as irregularly-shaped brown blotches. Infection on pods may result in fewer or smaller seeds per pod. Leaf defoliation is also an issue if infestation is severe.

Symptoms of stem canker first appear during the early reproductive stages as small, red-brown lesions. Initial lesions are usually found near a lower leaf node and expand lengthwise as the season progresses. Lesions eventually girdle the stem, causing wilting and plant death.

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 late-season 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. Spray applications of fungicides after R6 is generally not recommended.

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 the 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.

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 and or smaller seeds. 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 demands the most nitrogen and water during pod fill, particularly after the R3 growth stage. High-yielding soybeans, 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

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Sources:

1Wesley, 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. 2 Late season soybean diseases. 2013. University of Missouri Extension. www.//ipm.missouri.edu. (verified 6/23/14) 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 6/18/2014). Kranz, W.L. and J.E. Specht. 2012. Irrigating soybean. NebGuide. University of Nebraska-Lincoln Extension. G1367. http://extension.unl.edu/publications (verified 6/18/14). Pedersen, P. 2006. Brown spot—Septoria leaf blight. Iowa State Univ. Extension. http://extension.agron.iastate.edu (verified 6/18/14).