Considerations for a Successful Soybean Crop in 2019​

A couple of events occurred in 2018 that may impact soybean growers in 2019. Weather conditions favored an increasingly common collection of fungal diseases known as the Diaporthe complex, and a new soybean insect known as the gall midge was identified in the Midwest.

The Diaporthe complex is a group of four closely related fungal pathogens that can cause diseases that were seen widely across the Midwest in 2018: northern canker (Diaporthe caulivora), southern stem canker (Diaporthe aspalathi), pod and stem blight (PSB) (Diaporthe sojae), and zone lines and seed decay (ZLSD) (Diaporthe longicolla). These diseases likely occurred in higher levels in 2018 due to the unusually high amounts of rainfall and inoculum buildup in no-till systems.

Figure 1. Soybean stem showing linear lesions typical of pod and stem blight. Photo courtesy of and permission granted for use by Daren Mueller, Iowa State University.
Figure 2. Dark lines in stem tissue from zone lines.
Figure 3. Diaporthe damage to soybean seed. Photo courtesy of Adam Sisson. Iowa State University. Bugwood.org.

All four fungi in the Diaporthe complex can infect plants at any time during the growing season. Symptoms of canker (tip dieback, wilting and cankers) can appear at any time but are often seen as plants reach R3 growth stage. For PSB, pycnidia (fruiting bodies of the fungus) can be seen on the pods as random black dots while on the stem, and these dots appear in parallel rows (Figure 1). For ZLSD, you may observe dark lines in root and/or stem tissue (Figure 2), which are compact fungal structures, while seeds can be cracked or shriveled and white in color (Figure 3). The quality of seeds impacted by Diaporthe species can suffer and may result in lower germination rates if used as seed the following year. To help protect seed in 2019, seed treatments should be considered.

Figure 4. Map showing counties with identified sightings of gall midge.1 Map courtesy of and permission to use granted by the University of Nebraska.

On a different front, a new soybean insect pest was identified in the Midwest. Gall midge appeared in areas of South Dakota, Iowa and Nebraska in June and began spreading east within those states (Figure 4). The midges enter the field as adult flies (Figure 5) and lay eggs in the soybean stem.

Figure 5. Adult soybean gall midge. Adult soybean gall midge with characteristic black and white banding on legs.1 Photo courtesy of and permission granted for use by Justin McMechan, University of Nebraska.
Figure 6. Soybean gall midge damage advancing into the field from the field border.1 Photo courtesy of and permission to use granted by the University of Nebraska.

Over the course of the summer, the gall midge advanced very slowly into the field from the edge of the field. However, even in fields affected the most, damage was rarely found more than 400 feet from the field edge.

Figure 7. Soybean stem breakage caused by gall midge larvae feeding.

Near field edges where plants were impacted the greatest, yield was reduced by 100% by total plant death. Moving to about 100 to 400 feet from the field edge, damage occurred to a lesser amount.

Currently, good management considerations are not available or known. No differences were found among soybean products or seed treatments. Insecticides were not found to be very effective because the larvae are shielded within the stem and insecticides would only be effective on the adult (fly) stage. Additionally, growers would need to know when the fly was entering the field to allow for proper application timing.

In 2019, keep an eye out for this new pest and follow university extension updates to see if the pest has occurred in your area.

 

Sources:

1McMechan, J., Hunt, T., and Wright, R. 2018. Soybean gall midge: Adult stage identified. CROPWATCH. University of Nebraska – Lincoln. https://cropwatch.unl.edu

McMechan, J., Wright, R., Hunt, T., and Nygren, A. 2018. Orange gall midge in soybeans. CROPWATCH. University of Nebraska – Lincoln. https://cropwatch.unl.edu

Hodgson, E. 2018. New soybean pest in Iowa: Soybean gall midge. Integrated Crop Management. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu

Mueller, D. Pod and stem blight and Phomopsis seed decay. Integrated Crop Management. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu

Stoetzer, E. and Mueller, D. 2018. Should you use a fungicidal seed treatment on low-quality soybean seed? Integrated Crop Management. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/cropnews/2018/12/should-you-use-fungicidal-seed-treatment-low-quality-soybean-seed

 

Web sites verified 12/31/18.

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