Efficacy of Novel Nematicide Seed Treatments for the Control of Heterodera glycines in Soybean Production

Abstract

Heterodera glycines Ichinohe, soybean cyst nematode (SCN), is the most damaging pathogen of soybean [Glycine max (L.) Merr.] in the United States. SCN juveniles are small thread-like, nonsegmented, microscopic worms that penetrate soybean root systems to establish a parasitic relationship that diverts essential plant nutrition to the feeding site. Adult females become pyriform while the males are vermiform. SCN suppresses growth and yield, but because rotation may be unattractive to growers, resistant cultivars may be unavailable, and effective safe alternatives are currently lacking, new control tactics for SCN management are needed. Currently the only non-fumigant nematicide registered for soybean is aldicarb. Aldicarb is highly toxic and is under review by the EPA for groundwater contamination and may not be available in the future. Two chemical seed treatments for management of plant-parasitic nematodes are Avicta (Syngenta Crop Protection) and Aeris (Bayer Crop Sciences). Avicta is a fermentation product (abamectin) derived from an actinomycete and Aeris is a mixture of the neonicotinoid insecticide imidacloprid and thiodicarb, a carbamate insecticide/nematicide. Both are currently registered for use as a seed treatment on cotton. Objectives for the current project were to evaluate the efficacy of both Avicta and Aeris as seed treatments on soybean for management of the soybean cyst nematode. In 2007 and 2008 field trials and microplot experiments were initiated to evaluate the efficacy of these seed treatments against SCN. Three rates of each seed treatment were applied, 0.10, 0.15, and 0.20 mg a.i/seed abamectin, and 0.20, 0.28, 0.36 mg a.i./seed imidicloprid+thiodicarb and compared to an untreated control and an in-furrow rate, 1.17 kg a.i./ha, of aldicarb. SCN populations were not reduced at the end of the growing season and that there were only limited yield benefits seen with either the Avicta or Aeris product at the three different rates. Population densities of the soybean cyst nematode in field studies lowest (275 eggs/500cm3 soil) at the Chowan County location to highest (4663 eggs/500cm3 soil) at the Scotland County location. Highest yields were seen at the Scotland County location which was more than likely due to soil nutrients and environmental conditions at that location since the seed treatments did not lower infection rates or increase yield on the susceptible cultivars. Pm(28 DAP) were not lowered with any of the rates of abamectin or imidacloprid+thiodicarb on any of the cultivars with the exception of the cv. NK S76-L9 at the Scotland County location, suggesting that the seed treatments were either not effective against the SCN or did not move with the root system in order to give adequate protection. At the Lenoir County location increasing rates of abamectin and imidacloprid+thiodicarb gave increasing yields on the cv. Fowler, and abamectin on the cv. Hutcheson. Other locations where similar results were seen were in Scotland County for both abamectin and imidacloprid+thiodicarb on the cv. Hutcheson, Scotland County for imidacloprid+thiodicarb on both the cv. NC Raleigh and NK S76-L9 and abamectin on cv. NC Raleigh. However, the only significant difference was seen at the Scotland County location with the abamectin treatment on the cv. NC Raleigh where increasing rates also gave increasing Pm(28 DAP) values which leads to possible host tolerance being responsible for maintaining yield since the low and mid rates were not significantly different from the untreated control with respect to yield or Pm(28 DAP) values. Our current research suggests that host status is the most influential effect on the SCN and that the seed treatments are either short lived in the soil or are not moving with the root system. Rapid germination seen in soybean can push the seed coat out of the soil in as little as 72 hours after planting as was seen in greenhouse and microplot studies. A majority of the seed treatment was left on the seed coat where the chemicals could have been degraded by ultra-violet rays from the sun.