Preparing for Plant 21 (PFP21): Soybean Seed Treatments 

Prepared by Adrienne Gorny, Dominic Reisig, and Rachel Vann

Fungicidal Seed Treatments: 

Soybeans in North Carolina are susceptible to several early-season pathogens. These pathogens are most effectively managed using a diversity of strategies including varietal resistance, cultural tactics, and chemical control options. As growers prepare to plant soybeans, the question arises about when fungicidal seed treatments are needed to protect soybean stand and ultimately yield. Research from 2014-2018 across 15 North Carolina environments showed there was no impact of a fungicidal seed treatment on soybean yield when soybeans were planted after mid-May. These results were substantiated by research conducted by County Extension Agents across North Carolina in 2020 showing no yield benefit from the use of fungicide seed treatments at early May or later planting dates, despite an occasional protection in stand at these locations. However, more recent small-plot research in North Carolina indicates that a fungicidal seed treatment can have value for protecting yield at planting dates earlier than mid-May in environments where disease pressure and varieties used are conducive for disease development. When looking at data combined over four environments in 2019 and 2020, the use of a fungicidal seed treatment protected yield by 5.2, 2, and 0.8 bu/A at late March to early April, mid-April, and mid-May planting dates, respectively. County Extension agents will continue with on-farm testing of fungicidal seed treatments at earlier planting dates in 2021 to expand upon these results. We encourage scouting and subsequent diagnosis of soybean seedling diseases using the NC State Plant Disease and Insect Clinic as this can aid in the future selection of effective seed treatments for the various seedling diseases in that field and can inform variety selection for varieties with resistance to the identified seedling diseases in the future.

Insecticidal Seed Treatments: 

Soybean insecticide seed treatments do not provide a yield benefit in North Carolina, or most of the area where soybean is grown. Out of 26 replicated Extension tests across North Carolina, we have never measured a yield advantage to using insecticide seed treatments. While insecticide treatments do kill soybean pests, such as thrips, the pests are not at levels that are high enough to cause yield loss. Fingers are commonly pointed at thrips, which can pucker and silver leaves at high densities. In two 2012 trials, we had thrips numbers in untreated plots that averaged over 50 and 38 per plant, respectively, in untreated plots compared with various insecticides, including Temik (aldicarb). Even at these densities (which could kill small cotton seedlings), we did not see differences in yield. Bean leaf beetle is another common target. However, numbers are too low when soybeans are seedlings to cause enough defoliation to lead to yield loss. Not only are soybean insecticide seed treatments an unnecessary expense, their overuse can drive resistance in other crops and can kill predatory ground beetles, releasing slugs that clip plants and reduce yields (Douglas et al. 2015). Unfortunately we don’t have any remedial management methods to kill slugs. Therefore, skip insecticide on soybean seed when possible.

Nematicidal Seed Treatments: 

Plant-parasitic nematodes are microscopic roundworms that feed on the roots of the crop plant, causing damage and reduced growth. Management of nematodes (such as soybean cyst nematode (Heterodera glycines) and root-knot nematode (Meloidogyne spp.), among others) requires a multi-pronged plan. This plan should include monitoring of nematode populations, use of soybean varieties with genetic resistance to the nematodes of concern where available, and chemical options including nematicide seed treatments where appropriate.

What are nematicide seed treatments?  Nematicide seed treatments are chemical or biological compounds with nematode-killing properties applied to the seed before planting. These seed treatments are designed to provide early season protection, guarding young plants during an early growth period. Nematicide see treatments are most beneficial under low to moderate nematode pressure. Fields with high or heavy levels of nematode pressure are unlikely to be successfully managed through seed treatment alone.

What nematicide seed treatment options are available?  Several products are available including biological-based and conventional chemistry-based (Table 1). Additional information of seed treatments for soybean and other crops is also available in the 2021 North Carolina Agricultural Chemicals Manual. Please refer to the current chemical labels in all cases for directions on safe use and information on application rates, timing, and harvest intervals.