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Soybeans grown under duress

I got back to my office this morning to find a care package of soybean plants in a plastic bag and a sticky note listing a name and phone number on my desk (Figure 1).  With the heat and periodic rains over the past several weeks, so many fields of soybeans in northwest Minnesota have closed, thick canopies with nodes loaded with full-sized trifoliates, flowers and pods.
Figure 1. Bag filled with soybeans.

Above-ground issues

Overall the bagged plants were stunted and developmentally behind other Red River Valley soybeans with shorter internodes, smaller leaflets, few if any flowers and no pods.  Several of the plants were suffering from iron deficiency chlorosis and some leaves were also exhibiting signs of injury from a contact herbicide (Figure 2).   The seed salesperson that brought in the bag of soybeans confirmed that the field had been sprayed post-emergence with the PPO inhibitor Flexstar®.
Figure 2. Herbicide injury symptoms consistent with PPO inhibitor herbicides.

Below-ground issues

The plants were a mess below-ground, suffering from poor nodulation and an evident soybean cyst nematode (SCN) infestation (Figure 3).  Unfortunately, neither the salesperson nor the farmer were aware that the field was infested with SCN and so may have not been monitoring or managing SCN population densities.   According to ISU Extension, SCN infection can reduce the number of nodules that develop.
Figure 3. Cysts of the soybean cyst nematode (white arrows) and root nodules (yellow arrows).
The farmer had opted to have Bradyrhizobium japonicum inoculant applied to his seed in an effort to ensure good nodulation.  When asked, the salesperson indicated that the crop had been planted into salty soil with very poor structure.  The inoculant bacteria that live inside nodules and convert atmospheric nitrogen into a form that soybeans can use require oxygen in order to survive.  Soils with poor structure have fewer pore spaces that allow both water and air to penetrate.  A cursory literature search failed to find research looking at how salinity may affect soybean nodulation, but research in another legume (chickpea) has shown that nodulation is reduced in salty soils (Rao et al. 2002).

Reddish, sunken lesions characteristic of Rhizoctonia root rot were also evident (Figure 4). Rhizoctonia root rot is caused by a soil-borne fungus (Rhizoctonia solani) capable of infecting many of the other crops grown in the Red River Valley including dry edible beans, corn and sugarbeet (Markell and Malvick, 2018).  Soybean infection is favored by plant stress (Malvick, 2018).  The poor nodulation, iron deficiency chlorosis, SCN infection and herbicide injury that these plants have endured in 2019 would have created the stress-filled environment that led to the widespread Rhizoctonia root rot reported.

The farmer decided against a seed treatment fungicide this year.  There are several seed treatment fungicides that can provide very good to excellent early season protection against R. solani (Smith, 2019) and one must assess a field’s history of disease and their tolerance in deciding whether or not to plant treated seed (Wise et al. 2018).

Figure 4. Reddish, sunken lesions characteristic of Rhizoctonia root rot.
Disclaimer: Product names are included for educational purposes. No endorsement is implied of products mentioned nor is non-endorsement implied of products not mentioned.

References and additional resources

Malvick, D. 2018. Rhizoctonia root and stem rot on soybean. University of Minnesota Extension.

Markell, S. and Malvick, D. Soybean Disease Diagnostic Series. NDSU Extension Service and UMN Extension. PP1867.

Mueller, N., Roger, E., Daugherty, R.B. and Shapiro, C. 2015. Soybean inoculation: When, where and why. University of Nebraska-Lincoln Extension.

Rao, D.L.N., Giller, K.E., Yeo, A.R and Flowers, T.J. 2002. The effects of salinity and sodicity upon nodulation and nitrogen fixation in chickpea (Cicer arietinum). Annals of Botany. 89:563-570.

Sarangi, D. and Jhala, A.J. 2015. Tips for identifying postemergence herbicide injury symptoms in soybean. University of Nebraska-Lincoln Extension. Institute of Agriculture and Natural Resources. EC497.

Smith, K.L. (ed.) 2019. Fungicide efficacy for control of soybean seedling diseases. Crop Protection Network. CPN-1020-W.

Soybean Cyst Nematode. Iowa State University Extension Encyclopedia Article.

Wise, K., Bradley, C., Chilvers, M., Giesler, L., Johnson, B., Legleiter, T., Licht, M., Mueller, D., Freije, A., Sisson, A., Smith, D., Tenuta, A. and Young-Kelly, H. 2015. Soybean Seedling Diseases. Crop Protection Network. CPN-1008.

Wise, K., Bradley, C., Chilvers, M., Conley, S., Faske, T., Geisler, L., Mueller, D., Sikora, E., Smith, D., Tenuta, A. and Tilmon, K. 2018. Factors to consider before using a soybean seed treatment. Crop Protection Network. CPN-4003-W.
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