Involvement of microorganisms other than pseudomonad's on the degradation of the non-fumigant organophosphate nematicide fenamiphos.

Fenamiphos is a broad spectrum, non-volatile, systemic, organophosphorus nematicide extensively used throughout the world to control plant-parasitic nematodes. The efficacy of this nematicide can decrease in soils where microorganisms accumulate that are capable of rapidly degrading the active ingredient. Among the documented microorganisms to degrade organophosphate compounds, Pseudomonas spp. was frequently identified. However, it still not clear whether or not this bacterial genus is the major responsible one in the biodegradation process. Our objective was to study the roll of Pseudomonas spp. and other soil bacteria on the degradation of fenamiphos in soils with different nematicide application history. In some of these soils fenamiphos metabolizing microorganisms were found, whereas in others not. For example, a soil with 42 fenamiphos applications during 16 years neither contained Pseudomonas spp. nor biodegrading microorganisms. In two different soils, both with 25 fenamiphos applications in 12 years and containing metabolizing microorganisms, only one of them contained Pseudomonas spp., demonstrating that the nematicide was rapidly metabolized by microorganisms other than Pseudomonas spp. Conversely, a control soil, with no previous nematicide application history, contained the highest number of Pseudomonas spp. of all soils analyzed. The number of bacteria of this genus could be increased when compost was added, although this did not alter the lack of the fenamiphos degradation process, even after six weeks and three consecutive nematicide treatments. The Pseudomonas diversity of the non-degrading control soil was composed of P. putida (50%), P. fluorescens (31%), P. syringae (13%) and P. chlororaphis (6%) according to gas chromatography identification. Individual analysis of the Pseudomonas spp. bacteria showed that none were capable of metabolizing fenamiphos in vitro. In conclusion, we demonstrated that Pseudomonas spp. are not intrinsically capable of fenamiphos metabolization. We also found that Pseudomonas spp. may not always accumulate upon fenamiphos treatment and that there are other microorganisms able to metabolize this nematicide.