04 Sep Soybean Cyst Nematodes: What You Should Know About the Devils
As soybean harvest approaches, some growers may be noticing a low yielding, yellow portion of their crop. Several diseases or health issues may be the culprit, but there is one that can make or break your soybean crops: Soybean Cyst Nematodes (SCN). In this week’s edition of Growing Possibilities, we will discuss the significant impact these nematodes can have your soil, and how you can mitigate the damages they cause.
The SCN are present in both Ontario and Manitoba here in Canada, and have been present all across the Midwest in the USA for several decades (1). The SCN colonize and parasitize plant roots, effecting plant nutrient uptake, water uptake, root development and nodule formation (2). If plants are stressed and not receiving the nutrients they need due poor root health, they will not spend the energy forming nodules with bacteria and nitrogen (N) will suffer significantly. The SCN eggs can survive for several years in soil and nematode populations can persist in infested fields even in years with no soybean crop (3).
The SCN has few above ground symptoms (4), the main ones being stunted growth and yellowing of foliage during the season. These symptoms are vague and are often only present after significant infection has taken place, leaving plants with low levels of infection appearing perfectly healthy. By the time this is noticed though, the damage has already been done, and up to 30% yield losses can be expected (3).
Scouting, both in-season and by soil sampling after harvest, are the best ways to diagnose a SCN problem (4). Removing plants from the ground and inspecting roots for small, white, lemon-shaped cysts is suggested. Nematode cysts are much smaller than root nodules, and it should not be difficult to differentiate the two (fig.1). These cysts are egg-filled growths present after the reproductive stage of SCN and are the method for pathogen spread. Because SCN live in the soil and spread their eggs within it, transfer of eggs between fields on tillers and other contaminated equipment can lead to new infestations elsewhere. Be sure to properly clean equipment because of this.
Scouting from July onward is recommended (3). Late season and post-harvest scouting is a must before any tillage or soil disturbances to avoid potentially contaminating equipment. Field entrances, low spots, and any areas showing low yields or below average plant health are the most at-risk areas and should be your main targets when scouting. Scouting for SCN in the fall, after harvest, is optimal, as most growers are already taking soil samples for fertilizer considerations for the following season. Soil sampling after harvest also allows you to see the effect that any SCN populations have had over the course of the entire growing season, which is most valuable for comparing SCN infestations year-over-year.
There are many SCN-resistant seed varieties on the market, but recent studies are showing that nematodes are slowly beginning to overcome the resistant traits of these varieties (4). Seed treatment nematocides are also available for combatting SCN, and using one of these in combination with a rhizobial inoculant or biological may help limit nematode presence while encouraging plant health and root nodulation, giving your plants the best chance at achieving their potential highest yields.
1) Tylka, G. L., & Marett, C. C. (2017). Known distribution of the soybean cyst nematode, Heterodera glycines, in the United States and Canada, 1954 to 2017. Plant Health Progress, 18(3), 167–168. https://doi.org/10.1094/PHP-05-17-0031-BR
2) Masonbrink, R., Maier, T. R., Muppirala, U., Seetharam, A. S., Lord, E., Juvale, P. S., … Baum, T. J. (2019). The genome of the soybean cyst nematode (Heterodera glycines) reveals complex patterns of duplications involved in the evolution of parasitism genes. BMC Genomics, 20(1), 1–14. https://doi.org/10.1186/s12864-019-5485-8