Nathan Kleczewski, Extension Plant Pathologist, University of Delware
Many growers in the region have started using, or are interested in using seed treatments for disease and/or nematode suppression in soybean production. The use and interest in seed treatments has increased for several reasons. First, because most seed treatments are applied by the seed supplier or are custom treated, growers do not need to worry about self treating and the various issues associated with this practice. Second, there are many different chemicals that can be added to modern seed treatments, allowing for a broader spectrum of activity. Safety, both to the user and the environment is increased due to low use rates as well as reduced exposure. The big question is- what can you expect with seed treatments?
The first thing that needs to be realized is that the amount of active ingredients included in seed treatments is limited. Yes, you can often request a range of concentrations, but the overall amount is limited to the amount of area on the seed. Safety to the germinating seedling also needs to be taken into consideration. For this reason efficacy of seed treatments may be somewhat limited, and most effective in situations where soil borne diseases are present, but not at high levels. In addition, the zone of effectiveness for these products is limited. Figure 1 illustrates the general zone of effectiveness and movement of active ingredients in the soil and developing seedling .
Initially, the seed is completely coated with a seed treatment (A in Figure 1). As water enters the soil and seed starts to swell, the seed treatment may move into a limited area of the soil. Movement is limited by organic matter in the soil, soil texture, water, and other factors (B). Pathogens that are affected by active ingredients and fall within this zone can be suppressed. As the seedling starts to grow, roots come in contact with the active ingredient and may be translocated into the developing seedling. The amount of active ingredient moved is limited to what is available in the soil and will be diluted by the development and size of seedling over time. Roots eventually grow outside of the zone of protection and can come in contact with soil-borne pathogens (C). As a rule of thumb, you can expect roughly 2-3 weeks of protection from seed treatments.
How effective are seed treatments? There has been a significant amount of comprehensive work from the Midwest on this topic, and it falls in line with work we have conducted in Delaware and Maryland. Basically, there is no consistent effect on plant health. Some years and locations they may work, other years they may have no effect. We are currently collaborating with UMD to assess three “new” seed treatments on soybean establishment, health, and yield in different locations in DE and MD. Thusfar, the biggest thing we have noticed is that early season germination can be affected by some seed treatments. This season, one of the seed treatments we are testing appears to slow germination and even reduce germination. However, because soybeans compensate so well, we do not know if this will impact end season yield. Only time will tell. In sum, when using a seed treatment, do not expect miracles to happen. They are easy to include when ordering seed, but the overall impact, particularly to a plant like soybean, is not clear. A link to some seed treatment data from Iowa can be found here: http://crops.extension.iastate.edu/cropnews/2016/04/2015-evaluation-commercial-seed-treatments-soybean-three-locations-iowa