Adjuvants Affect Duckweed (Lemna minor) Control with Pelargonic Acid

Duckweeds (Lemna spp.) are small, free floating aquatic plants that flourish on stagnant or slow moving water surfaces throughout the world. Members of the genus are among the smallest flowering plants, providing food for fish and fowl, but their aggressive growth and invasive tendencies make them formidable aquatic weeds, which when uncontrolled can result in oxygen depletion, fish kills, and death of submerged aquatic plants. Pelargonic acid is a fatty acid naturally occurring in many plants and animals, and present in many foods. AXXE (65% pelargonic acid, BioSafe Systems LLC) is a potential organic herbicide. Research was conducted to determine the impact of spray adjuvants on duckweed control with pelargonic acid. Duckweed was sprayed with 7 pelargonic acid concentrations (0, 1, 2, 4, 6, 8, and 10% v/v) combined with 1 of 3 adjuvant treatments [control, BioLink (30% garlic extracts, 10% yucca extracts, and 60% water) at 0.5% v:v, and orange oil (90% d’limonene and 10% inert ingredients) at 0.5% v:v)]. Visual ratings, measuring percentage duckweed control (percentage dead), were collected at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11 days after treatment (DAT). The experiment was conductive twice using 5 replications. Pelargonic acid phytotoxicity increased as spray concentration increased whether an adjuvant was used or not. The 6% pelargonic acid concentration resulted in 90% or greater duckweed control for all adjuvant treatments (control, BioLink, and Orange Oil) across all evaluation dates (1 – 11 DAT). The addition of either adjuvant applied at the 6% pelargonic acid concentration produced consistently greater duckweed control across all evaluation dates (1 – 11 DAT) compared to the control (no adjuvant). The 8% pelargonic acid rate produced excellent (97% or greater) duckweed control for all adjuvant treatments, with consistently better control with the BioLink adjuvant and typically with the orange oil, compared to the control. At the 10% rate all treatments provided outstanding (99% or greater) control at all evaluation dates, with no significant differences among adjuvant treatments. The research demonstrated the effectiveness of pelargonic acid in controlling duckweed when applied over-the-top. The addition of the BioLink adjuvant often increased the duckweed control compared to the control (no adjuvant). The authors suggest that further research should investigate whether higher rates of these adjuvants or the use other adjuvants would provide satisfactory duckweed control at lower pelargonic acid application rates and the economic implications of the changes.