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Soybean seedling diseases are often caused by fungal and fungi-like pathogens, which can slow germination and plant growth, and kill seedlings. 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.
Figure 1. Pythium (left), characteristic reddish lesions of Rhizoctonia (center). Damping-off (right) photo courtesy of Daren Mueller, Iowa State University, Bugwood.org.
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:
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).
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).
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).
To help manage soybean seedling diseases, avoid planting in cool (temperatures below 68° F), wet, and/or compacted soils. Plant high quality seed that is disease free; use fungicide seed treatments; plant Phytophthora resistant or tolerant soybeans; rotate with a grass crop (corn or sorghum) for several years to help reduce disease inoculum in the soil. Fungal pathogens can survive by overwintering in plant debris and in the soil and may be more problematic in fields with high amounts of plant residue. Use production practices to help reduce plant stress.
1 Esker, P. Common soybean seedling diseases. University of Wisconsin- Madison. 2 Yang, X.B. 2009. Scouting soybean seedling diseases. Iowa State University Extension. Integrated Crop Management News. 160406112758