Fate of metalaxyl applied in nutrient solution to gerbera (Gerbera jamesonii) grown in a ‘closed’ hydroponic system

Abstract

SummaryThe application of fungicides via the recycled nutrient solution in ‘closed’ hydroponic systems may restrict the leaching of residues into groundwater. To assess the possibility of applying selected fungicides via the recycled nutrient solution to flower crops grown in ‘closed’ hydroponic systems, the widely-used fungicide metalaxyl was applied to gerbera (Gerbera jamesonii) at two different doses (112.5 and 225 mg l–1) using this method. The experiment was conducted in a glasshouse using pumice (0–5 mm) as the substrate. The fate of the metalaxyl, which was added to the nutrient solution was traced in the plants and the environment by regularly collecting samples of irrigation solution, drainage water, pumice, roots, leaves, and flowers over a period of 121 d and analysing them by gas chromatography. Metalaxyl residues in leaves were also studied over a 50 d summer period, 7 months later. Metalaxyl concentrations were relatively high in both drainage and irrigation solutions over the first 5 d following its application, but dropped markedly on day 17. Thereafter, the metalaxyl level declined slowly to a mean value of 0.02 mg l–1 on day 121, without any significant difference between the two doses originally applied. The estimated half-life of the metalaxyl was 6.5 and 5.4 d at the low and high dose, respectively. Leaf concentrations of metalaxyl increased gradually during the first 5 d following application, and remained almost constant up to day 17. Thereafter, leaf concentrations declined progressively to 0.6 mg kg–1 (fresh weight) by day 121, without significant differences between treatments. This response, independent of application dose, implies that even lower doses of metalaxyl may be applied via the nutrient solution in ‘closed’ hydroponics. The concentration of metalaxyl in roots reached a peak earlier than in leaves. In the 50 d, summer experiment, the highest concentration of metalaxyl in leaves was found 6 d after application, followed by a rapid decrease. Theses results indicate that application of metalaxyl via the recycled nutrient solution in a ‘closed’ hydroponic system provides effective, high concentrations of the fungicide in roots and leaves, which are maintained for a long period after application.