Cool conditions and heavy rains after planting can favor seedling diseases, especially those caused by Pythium species, which require saturated soils for their swimming spores. Phytophthora sojae is also favored by wet soils; however, it prefers warm temperatures of greater than 60 degrees Fahrenheit and wet conditions. Pythium species can cause both pre- and post-emergence damping off. Diseased plants can occur as individuals or in small patches, particularly in low lying areas. Typical symptoms of Pythium damping off will include soft rotted seedlings (see photo).
Fields should be scouted and stand counts taken to determine if replant is necessary. Information on assessing soybean emergence is available online at the Michigan State University Extension website. Other pathogens such as Fusarium and Rhizoctonia can also cause seedling disease, however Pythium is likely to be a prime suspect given the cool, wet conditions that we’ve experienced. As it can be difficult to differentiate the various possible causes of seedling disease, it is best to submit a sample to a diagnostic clinic for confirmation, such as MSU Diagnostic Services.
To reduce the likelihood of seedling disease by Pythium species, planting into cold, wet soils should be avoided. Seed treatments including mefenoxam/metalaxyl (Apron XL, Allegiance) and the newly registered ethaboxam (Intego) can help reduce stand loss due to oomycetes including Pythium and Phytophthora. Seed treatments can help protect seedlings from seedling diseases; however, under extreme conditions they can fail. It should also be noted that other factors such as PPO inhibitor herbicides can result in seedling damage and may exacerbate seedling disease.