Travis Faske, University of Arkansas, Lonoke Extension Center, Lonoke, AR; John Mueller, Clemson University, Edisto REC, Blackville, SC; Kaitlyn Bissonnette, University of Missouri, Columbia, MO
Editorial support: Ethan Stoetzer, Iowa State University, Ames, IA
Nematicides are an important part of an integrated pest management system to control plant-parasitic nematodes. Over the past 15 years, seed-applied nematicides have increased in popularity, becoming one of the most commonly used nematicide-application methods in row-crop agriculture. Seed-applied nematicides can be divided into two groups: chemicals and biological agents. This article will focus on chemical agents, and illustrate the mechanisms by which seed-applied nematicides protect the expanding root system, and other factors that can contribute to their efficacy in the field.
Figure 1. Seed treatments on soybean seed.
How they work
Seed-applied nematicides are convenient to use for farmers, and work in a way that creates a “zone of protection” around the developing root system. Nematicides move from the seed coat to the surrounding soil and water to create this “zone”, which will vary in size based on several factors, these factors are described below. It is important to note that seed-applied nematicides will not provide complete or season-long protection of the expanded root system.
Factors that influence root protection by nematicides
Movement in root: Though the concentration of nematicide applied to the seed coat is sufficient to kill hundreds of nematodes, the amount that comes off the seed coat shortly after planting is only a portion of that applied on the seed coat. Seed-applied nematicides do not move through the vascular system toward the growing point of the root system, so the amount that comes off the individual seed must come directly into contact with plant-parasitic nematodes in the soil to cause paralysis which impedes root infection. Nematicides that bind to the seed coat are less likely to move into the soil water phase, thus limiting the “zone of protection” surrounding the roots.
Figure 2. Seed coat on soybean cotyledon showing seed treatment.
Movement in soil: There are three main factors that affect nematicide movement in the soil: solubility in water, soil mobility (i.e. binding to soil particles), and water infiltration (downward movement of water). Nematicides that have low water solubility (e.g. abamectin) are less likely to be distributed in the soil water phase than those with a moderate solubility (e. g. fluopyram). Nematicides that have limited movement in the soil water phase, due to low water-solubility, have a smaller “zone of protection” with less protection further away from the seed.
Once in the soil water phase, nematicides come into contact with soil particles. Nematicides bound to soil particles are ineffective at inhibiting nematode infection. As a general rule, chemical nematicides are more likely to move in coarse textured soils than fine textured soils, assuming adequate water infiltration. Thus, chemical nematicides often have a larger "zone of root protection" in coarse textured soils compared to fine textured soils (silts and clays).
Water infiltration is key to moving water in the soil water phase and downward throughout the expanding root system. Because water is quickly displaced in coarse textured soils compared to fine textured soils, the downward distribution of nematicides is often reduced in fine textured soils, which limits the "zone of protection." When using a seed-applied nematicide, water from irrigation or rainfall is needed shortly after planting to distribute the nematicide that washes off the seed coat and into the soil. This will result in a timely establishment of the “zone of protection”.
Nematode population density: Another factor that affects root protection from seed-applied nematicides is nematode population density. Seed-applied nematicides are marketed for use at low nematode population densities. When nematode densities are severe, this usually means that there are more nematodes outside the zone of root protection, but still within the zone of root expansion. In this scenario, the risk of nematode infection will remain high, as roots will expand beyond the nematicide “zone of protection”.
Other variables: Other factors such as microbial degradation and chemical half-life play a role in the longevity of nematicides present in agricultural soils, but have little impact shortly after planting on seedling root protection. Though nematicides vary in toxicity to specific species of plant-parasitic nematodes, variability in field efficacy is more often associated factors that limit nematicide movement from seed and within soils.
Applying these concepts
Seed-applied nematicides can be effective in protecting seedling root development when nematode population densities are low and in coarse textured soils where irrigation or rainfall are common. Soil samples are useful to determine nematode population densities and over time to monitor changes in nematode population densities. For season-long protection, an integrated pest management approach might be to pair a seed-treatment with at least a moderately resistant variety.
Figure 3. Soil samples are used to monitor nematode population densities.