As planting season nears, many growers who plant soybean early are concerned about seedling diseases and seed treatment. As we all know, the use of treated seed is the most effective way to manage seedling diseases. Before you treat seed or purchase treated seed, there are two aspects

Your risk of experiencing seedling disease may be highest when you:

1. plant in fields that have had seedling disease within the past few years, or have a history of replanting not caused by hail or insects. Previous seedling disease experience or replant history indicates the presence of pathogenic fungi in a field. Such risks increase if you plant early because soybean seeds take longer to emerge in low soil temperatures. The longer that seeds stay in the soil, the greater their chance of being attacked by a fungus.
2. plant fields early with low-quality seed. Research at ISU showed consistent benefits for fungicide treatment if low quality soybean seeds were planted at soil temperatures equal to lower than 50 F. Planting low quality seeds in cooler soil increases the risk. In recent years, plant pathologists have observed an increase in seedling disease in no-till fields. No-till fields normally have lower soil temperatures and higher soil moisture, both favorable conditions for pathogenic fungi. Be aware that use of no-till does not mean you will see seedling diseases because pathogenic fungi also must be present in soil. Use weather as your guide for making your decision. Wet and cool weather in early spring increases the risk of seedling diseases.

You also must select seed treated with the right chemicals to control seedling diseases. Several fungi can cause seedling diseases and different fungicides are effective for different fungi. According to an extensive two-year extensive by the ISU Department of Plant Pathology, major fungal pathogens in Iowa soybean fields are Rhizoctonia solani (which causes 23-27 percent of seedling disease), Fusarium spp. (10-13 percent), Pythium and Phytophthora (cumulative 55-63 percent).

Two or three fungi often can be found in the same field. Usually a field contains only one dominant pathogen, so select chemicals that target the dominant fungus. If you do not know which fungus is causing stand reduction in your fields, target Rhizoctonia, Pythium, and Phytophthora, which account for 90 percent of seedling diseases in Iowa. Some chemicals contain several compounds that are effective on all three fungi. Pythium and Phytophthora are closely related, and most chemicals have the same efficacy for the two fungi.

Pathologists in Ohio have conducted seed treatment experiments for many years because of seedling disease problems in that region. The following table shows the relative efficacy of various chemicals in controlling seedling disease pathogens in Ohio. These recommendations are applicable to Iowa because they are based on experimental results generally consistent with many other states, including Iowa.

Relative efficacy of seed treatments for control of soybean seedling diseases.

4=Excellent, 3=Good, 2=Fair, 1=Poor, N=no activity. Source: Ohio State University.