March 14, 2017

Yellow Soybean Plants

Key Points

  • Yellowing of soybean leaves can occur for multiple reasons.
  • Becoming familiar with the conditions and different causes of leaf yellowing in soybean plants can help with management decisions.

Soybean Cyst Nematode

Soybean Cyst Nematode (SCN) can cause yellow soybean leaves and stunted plants.Infested areas are typically in circular patches. Symptoms can look similar to nutrient deficiencies, compaction, or drought damage. If SCN symptoms are present above ground, the plants need to be examined below ground. The presence of female nematodes and cysts on the soybean roots is the only accurate way to diagnose SCN.1 The cysts are initially white, then yellow, and then tan to brown when mature (Figure 1). Soil samples can be collected and sent to a university plant pathology lab. Sample guidelines should be followed.


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Figure 1. Soybean cyst nematode damage can only be distinguished by the presence of cysts on the soybean roots and female nematodes.

Planting soybean varieties that are resistant to SCN is the best form of management.1Management practices used to maximize yield which can include: maintaining proper soil fertility and pH as well as managing for disease, insect, and weed pressure can also help the plant withstand SCN pressure.


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Figure 2. Soybean plant with manganese deficiency.

Nutrient Deficiencies

Iron (Fe) deficiency or iron chlorosis symptoms can include yellowing of the interveinal portions of young leaves and dead or brown leaf margins. Root rot and secondary fungal infections are also common. Iron deficiency may be more common in low areas and cool,wet soils which are typically areas with poor drainage, areas likely to flood, and areas with a pH 7.2. One way to distinguish iron chlorosis from SCN damage is that iron chlorosis occurs earlier in the season (June in the Midwest) and SCN damage is found later (July - August in the Midwest). Second, iron chlorosis appears on the veins and upper leaves and SCN damage starts on the edges of the leaves and can affect all of the leaves on the plant.

Soybean varieties with iron chlorosis tolerance should be used in field with a history of this issue. Plant when soil conditions are warm and dry to lessen the incidence of iron chlorosis. Iron applications can be used to manage iron chlorosis, but are not always feasible.

Nitrogen (N) deficiency symptoms in soybean plants consist of a yellowing or chlorosis of the lower leaves due to remobilization of N for new growth. Early on, soybean plants may be light-green to yellow in color. Once nodules are able to supply adequate N, the dark green leaf color comes back. The plants can recover if conditions are optimal. Nitrogen may become available to plants from the soil or from N fixation. Nitrogen from the soil is preferred because less energy is required for plant uptake compared to N fixed from the air. Soil N can account for 50% of the total N needed for plant growth.2


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Figure 3. Temporary yellow flash in soybean plants after a glyphosate application.

Manganese (Mn) deficiency symptoms can include yellowing of the newest interveinal trifoliates (Figure 2). Since Mn is immobile in the plant, symptoms typically appear on the younger leaves, but older trifoliates can show symptoms as well. As soil pH increases, less Mn is available to the plant; therefore, Mn deficiency is not generally found in soils with a pH 6.2. Deficiency symptoms are commonly found in poorly-drained soils, such as clay and silt loam soils on eroded hillsides with pH values that are higher than the rest of the field. Roots must reach Mn to absorb it. Conditions that limit root growth can cause Mn deficiency symptoms; examples include: wet soils, extremely dry soils, cool weather, soil compaction, root diseases, N deficiency, and herbicide damage.

Potassium (K) deficiency symptoms can include yellowing of the leaf margins on older leaves that usually begins at the leaf tip and extends down the margins toward the leaf base.4 When severe, these deficiency symptoms can include dead, brown leaf edges, and stunted plants. Low soil-test K is a common reason for K deficiency and if bad enough, can cause decreases in yield potential.5

Other Causes

Yellow flash from herbicide applications may occasionally occur with an application of a high rate of glyphosate and warm moist environmental conditions (Figure 3). The symptoms generally last for about a week and no decreases in yield have been reported.6 Yellow leaves associated with yellow flash injury can be found at the ends of fields and/or at overlap spray areas where application rates were two to three times the intended rate. Normally, leaves turn green within a week of application with little to no growth reduction. Typically, soybean plants in these fields had environmental stress (temperature or drought) when the glyphosate was applied.

A few days after glyphosate is applied, all leaves on the soybean plant remain green except the newest leaves - at the top - that were less than 0.5 inch long at the time of spraying. These leaves continue to grow and expand, but chlorophyll production is reduced, leaving a yellow color (carotenoid pigments become visible in the absence of chlorophyll) that can last up to a week. In the case of yellow flash, glyphosate does not cause green leaves to turn yellow rather, temporary yellowing is a phenomenon of leaf development when the soybean plant is under multiplestresses:

  • If the soybean plants were not properly growing due to any stress condition when glyphosate was applied, yellow leaves sometimes may not appear until 10 to 21 days later.
  • If it is dry when yellow leaves appear, these leaves may stay yellow until the crop resumes growth after rain releases the crop from stress.
  • Yellow flash is largely environmentally induced, with only a very small genotype x environmental component.
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Sources:

Sources:
1Tylka, G. January 1994. Soybean cyst nematode. Iowa State University. 2Pedersen, P. 2007. Soybean nutrient requirements. Iowa state University. Available on-line: http:// extension.agron.iastate.edu; 3Yang, X.B. July 2, 2001. Soybean iron chlorosis. Iowa State University. Integarted Crop Management IC-486 (16); 4Mallarino, A. 2005. Potassium deficiency symptoms in corn and soybean: What can we do about them? Iowa State University www.ipm.iastate.edu; 5Bohner, H. September 2007. Potassium deficiency in soybeans. OMAFRA. Available on-line: www.omafra.gov.on.ca; 6Bohner, H. June 2006. Yellow flash in soybeans. OMAFRA. Available 0n-line: www.omafra.gov.on.ca.