Early Season Soybean Pests and Diseases

Early season soybean diseases and insects should be on your mind when examining seedling emergence and assessing soybean stands this spring. Soybean seedling diseases are often caused by fungal pathogens, which can slow germination and plant growth, and kill seedlings. Early season insects of soybean feed on seeds and seedlings causing plant injury, stunting, delayed emergence, or stand loss.

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Soybean Seedling Diseases

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Conditions that favor disease development include wet and poorly drained soils, as well as cool to warm soil temperatures depending on the pathogen. Once the pathogens that cause diseases are present within the environment, they must be managed to reduce potential yield loss.

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Figure 1

Figure 1. Pythium (left), characteristic reddish lesions of Rhizoctonia (center). Damping-off (right) photo courtesy of Daren Mueller, Iowa State University, Bugwood.org.

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Identifying the specific pathogen causing disease will assist with management decisions for the field in future growing seasons, including planting resistant soybean products or using fungicide treated seed. Consider soil temperatures, soil moisture (saturated, wet, or dry), and plant growth stage to help identify pathogens in each field. To help distinguish between Pythium, Phytophthora, and Rhizoctonia damping-off (Figure 1) consider these symptoms:

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Pythium. Hypocotyl will appear rotted. Leaves of infected seedlings will turn brown before dying. Stems are soft and watery. Common in wet, poorly drained, compacted soils and more common in cooler temperatures (50-60° F).

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Phytophthora. Symptoms are very similar to Pythium infections. Symptoms usually appear above the soil line. Phytophthora is more common in wet, poorly drained, compacted soils and in warm temperatures (80° F).

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Rhizoctonia. Localized reddish-brown lesions can be seen in the cortical layer of the main root or hypocotyl. Stems remain firm and dry. Symptoms usually appear below the soil line. Common in moist (but not saturated) soil conditions and warm temperatures (80° F).

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Early Season Soybean Insects

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Figure 2

Figure 2. (clockwise from top left) Bean leaf beetle, white grub, wireworm, and seedcorn maggot.

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A variety of insects are present at any given time in soybean fields. While many are not of concern, some can reduce yield potential and even result in crop failure if left unmanaged (Figure 2).

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Bean leaf beetle (BLB). Adult BLB are about 1/4 inch in length. The most distinguishing characteristic is the presence of a small black triangle located at the base of the forewings. Some beetles will have four black markings on the wings. Typically, the beetles are dark yellow in color, but can range in hues of yellow, orange, and red. Early season injury to soybean is caused by the overwintered adults feeding on cotyledons, leaves, and stems. In addition, this pest can transmit bean pod mottle virus.

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Wireworm. Wireworm can be a problem in soybean, especially when fields were planted to sod or small grains in previous years. Wireworms are hard-bodied, slender, brownish larvae that grow to about 1 inch long. They feed on seeds and seedlings.

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Seedcorn maggot. These maggots are yellowish-white, about 1/4 inch long, and lack a defined head and legs. Seedcorn maggots feed on newly planted soybean seeds and can reduce stands.

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White grub. True white grubs and masked chafer grubs can damage soybean seedlings, especially when the soybean crop follows sod or another cover crop. White grubs range in size from 1/4 to over 1 inch long and have white bodies with tan to brown heads. White grub species can be distinguished from one another by the pattern of rasters (bristle-like hairs) on the tip of the abdomen. White grubs feed on plant roots.