Pelargonic acid for weed control in onions: factors affecting selectivity

In onions (Allium cepa) few herbicides are registered. Due to their erect leaf and plant structure, onions retain less herbicide after spraying than broadleaved weeds. Further, onion plants have a thicker plant cuticle. These differences allow a selective use of pelargonic acid in onions. The main aim of this study was to determine the selective dose at different crop growth stages. Five field trials were carried out. Three pelargonic acid containing products (1 SL- and 2 EC-formulations) were applied with a logarithmic sprayer. Dose range was 0 to 56 kg active substance (a.s.) ha-1. The growth stage varied between BBCH 10 and 16. Selectivity was rated visually. The products were selective at 7.2 and 8.3 kg a.s. ha-1 (BBCH 10-11), 6.1 and 8.9 kg a.s. ha-1 (BBCH 11-12), 8.5 and 20.7 kg a.s. ha-1 (BBCH 13) and 6.3 and 9.0 kg a.s. ha-1 (BBCH 14-16) for the SL- and EC-products respectively. Selectivity depended on growth stage and weather conditions. Important for selectivity is an intact cuticle, a rain-free, light-intense period before application. Pelargonic acid could become a component for sustainable weed control in onions. Due to its foliar activity, small weeds need to be targeted and several passes are required