Identifying Rates of Meadowfoam (Limnanthes alba) Seed Meal Needed for Suppression of Meloidogyne hapla and Pythium irregulare in Soil

Abstract

Meadowfoam (Limnanthes alba) is a commercial oilseed annual crop grown in Oregon. After extracting oil from seed, the remaining seed meal is rich in the secondary plant metabolite glucolimnanthin, which can be converted into pesticidal compounds such as 3-methoxybenzyl isothiocyanate (ITC) and 3-methoxyphenylacetonitrile (nitrile) in the presence of the enzyme myrosinase. In previous studies, we demonstrated that ITC and nitrile, produced by mixing freshly ground meadowfoam seed with meadowfoam seed meal, are toxic to the plant-parasitic nematode Meloidogyne hapla and the plant pathogen Pythium irregulare. In this study, we evaluated factors that might influence the implementation of meadowfoam seed meal into agricultural production systems for soilborne pathogen and nematode control. Rate-finding experiments demonstrated that a minimum 1.0% seed/seed meal formulation (wt/wt) was necessary to achieve nematode and pathogen suppression; seed meal alone was insufficient for complete control of M. hapla and stimulated the growth of P. irregulare. When this 1.0% seed/seed meal formulation was used, a greater soil amendment rate was required to cause 100% mortality of P. irregulare (1.0% wt/wt) than for M. hapla (0.5% wt/wt). In phytotoxicity experiments, soil amended with the 1.0% seed/seed meal formulation was consistently phytotoxic to wheat, cucumber, and tomato. However, phytotoxic effects were mitigated by a delayed planting into the amended soil. A final assay to monitor concentrations of ITC and nitrile in conjunction with assessing M. hapla and P. irregulare mortality was conducted over a 6-day period in soils amended at 0.5 and 1.0% (wt/wt) with the 1.0% seed/seed meal formulation. The response was rapid, with 100% mortality of both organisms within 2 h after exposure to amended soil. Concentrations of nitrile remained relatively constant over the 6-day period (approximately 0.017 and 0.032 mg/ml at 0.5 and 1.0% amendment rates, respectively), whereas ITC production increased rapidly and peaked 12 to 24 h after amendment (0.083 and 0.171 mg/ml at 0.5 and 1.0% amendment rates, respectively) before returning to near undetectable levels.