Interactions between saprotrophic fungi, bacteria and protozoa on decomposing wheat roots in soil influenced by the fungicide fenpropimorph (Corbel®):: a field study

Litterbags containing freshly harvested wheat roots with adhering rhizosphere soil were placed in their native soil. Four levels of the fungicide fenpropimorph were applied: 0, 1, 10 and 100 mg kg(-1) soil. The experimental set-up was a randomised block design with five replicates. Litterbags were sampled six times over a 4-month period and microbial numbers determined. Bacteria (colony forming units, CFUs) and protozoa (most probable number) followed a similar pattern: a sudden decrease at the onset of the study period was followed by a fast recovery and increase. This ended at maximum mean values of approximate to 7.5 x 10(7) bacteria g(-1) root litter by day 7 and 3 x 10(6) protozoa g(-1) root litter by day 20, respectively. Fenpropimorph had no significant effect on either of these two groups, for which mean population sizes seemed to interact in a prey-predator manner. The fungi (stained by calcofluor white and fluorescein diacetate) were unaffected by the pesticide until day 20. Hereafter, fluorescein diacetate-active (FDA-active) hyphae were significantly inhibited in all fungicide treated plots, compared to the untreated plots 50 and 114 days after the application of fenpropimorph. Fungal colony forming units (fungal CFUs) on fenpropimorph-containing agar demonstrated a selection towards more fenpropimorph tolerant fungi in the 100 mg kg(-1) fenpropimorph treated plots. The delayed effect of fenpropimorph on FDA-activity could be explained as a combined effect of the pesticide and the degradation product itself, and the fluctuating water stressing field conditions of this year. Our results, thus, demonstrated long term effects of fenpropimorph on the activity of saprotrophic fungi at recommended field dose, and that high dose fenpropimorph can select for fenpropimorph tolerant fungal populations. (C) 2000 Elsevier Science Ltd. All rights reserved.