Persistence of fenhexamid in the nutrient solution of a closed cropping system

There are concerns about emissions of plant protection products (PPP) from protected cultivations, including high-tech production systems. Modern high-tech greenhouse horticulture is performed in cropping systems with closed water and nutrient loops, so PPP residues are not leaked to the external environment, but are captured and may accumulate within the recycled greenhouse solution. In this pilot study, the botryticide fenhexamid (FEX; Teldor WG50®) was used as a model PPP and incubated (0.01 times the recommended concentration spray solution) in the colonized nutrient solution from a closed tomato production system in darkness at 20°C. Samples were taken after 0h, 24h, 48h, 96h, 192h and 504h (0, 1, 2, 4, 8 and 21 days) of incubation to monitor FEX concentration, dissolved organic carbon content (DOC), electrical conductivity, pH and microbial dynamics. Culture-dependent (viable counts for general bacterial, fungal flora and fluorescent pseudomonads on stationary agar, with/without supplement of FEX) and independent (denatured gradient gel electrophoresis, PCR-DGGE) methods were used for microbial analyses.DOC and FEX concentration did not change during incubation, whereas pH and electrical conductivity increased significantly. Density of culturable fungi decreased from the start of incubation, while density of culturable bacteria declined in late incubation. Fluorescent pseudomonads declined in early incubation but increased significantly at later stages. Under nutrient-deprived conditions, density of culturable bacteria decreased during the course of incubation. Microbial community analysis by DGGE supported these findings. Differences between the density of culturable bacteria assessed on semi-selective media with and without FEX supplementation were low. However, addition of FEX to diluted malt extract agar negatively affected the density of culturable fungi.