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A Guide to Common Soybean Diseases in The Midwest

February 23, 2021

The potential always exists for a fungal, viral, or bacterial disease to develop in a soybean crop. Yield loss related to a disease can be minimal or dramatic depending on the disease, the environment, time of infection, and the soybean product. Knowing the disease history of a field and keeping abreast of reported disease developments during the growing season can help in making important management decisions to help protect yield potential.

Many of the diseases caused by fungi overwinter on infected crop residue. Because of climatic conditions within an area, some fungi-caused diseases are unable to overwinter; however, the spores can arrive via wind from areas where the disease can overwinter. Plants become infected when splashing rain, water movement through the soil onto roots, or wind brings the pathogen to the plant. Some fungal pathogens can also be seedborne. The fungi then enter plant tissue by 1) growing a germ tube that pierces into tissue, 2) through stomata, or 3) through wounds caused by hail, wind, insects, or mechanical means.

Fungicidal seed treatments can help protect seed and seedlings from most fungi-caused seed and seedling diseases. Foliar fungicides can help protect yield potential, depending on the fungal disease and application timing for the disease. Fungicide resistance has developed with some fungi; therefore, it is important to properly identify the causal fungus for proper fungicide selection. To help deter the development of fungicidal resistance, it is highly suggested that fungicides be rotated based on mode of action. Depending on the pathogen, improved soil drainage and soil compaction management can help reduce the potential for fungal infections. Residue destruction through tillage can help speed up the deterioration of a pathogen source.

Bacterial diseases can also overwinter on infected residue. The bacterium can contact plants through water movement in the soil, splashing rain, wind-driven rain, and mechanical means such as cultivating when plants are wet. Some bacterial pathogens also can be seedborne. Infection occurs when bacteria enter the plant through wounds or natural tissue openings such as stomates.

Most viral diseases rely on a vector such as aphids or thrips to infect a plant. As the vector feeds, infected juices are injected into plant tissue. Scouting for insects that have the potential to vector viral diseases and applying a timely insecticide may help protect plants from becoming infected via insect feeding for some viral diseases However, insecticide applications to manage vectors do not provide adequate control of many viral diseases. Caution should be taken not to spray insecticides without proper identification as many beneficial insects can also be killed.

To identify and manage common soybean diseases, click on a tab.

Bacterial Blight

Figure 1. Bacterial blight lesions on a soybean leaf.

Identification, Characteristics, and Diagnosis:

Caused by the bacterium Pseudomonas savastanoi pv. glycinea.
Pathogen overwinters on residue and can be seed-transmitted.
Pathogen spread by wind, rain, and by cultivation when plants are wet.
Lesions can occur on all aboveground plant tissues; leaves in the mid to upper canopy are favored.
Initial infection appears as small, angular, water-soaked spots that turn yellow, become brown with a yellow halo around the spot, then die (Figure 1).
Dead tissue may drop from leaves; however, leaves usually remain attached to the plant.
Favored by cool, wet weather. Hot, dry weather stops development.
Infected seedlings may be stunted or die.

Management:

Plant resistant soybean products and disease-free seed.
Rotate with non-host crops.
Avoid cultivating when plants are wet.
Manage residue through tillage.

Bacterial Pustule

Figure 2. Bacterial pustule lesions on a soybean leaf. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org.

Identification, Characteristics, and Diagnosis:

Caused by the bacterium Xanthomonas axonopodis pv. glycines.
Pathogen overwinters on infected soybean residue and seed.
Pathogen is spread by splashing or windblown water, and cultivation when plants are wet.
Initial symptoms include the development of small, yellow-green lesions with higher reddish-brown centers on upper leaf surfaces (Figure 2).
More mature lesions produce a small, slightly-raised, pale-colored pustule at the center of the lesion on the underside of the leaf.
Lesions can be small to large, irregular, and necrotic.
Leaves can become ragged in appearance as dead tissue drops from the leaf.
Premature defoliation may occur, which can reduce seed count and size.
Favors warm (86 to 92° F) and wet weather.

Management:

Plant resistant soybean products.
Utilize tillage to help decompose residue.
Avoid cultivation when plants are wet.

Sources:

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(Sources verified 6/2019)

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A Guide to Common Soybean Diseases in The Midwest

To identify and manage common soybean diseases, click on a tab.

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