REDUCED FLURIDONE EFFICACY IN SOIL - A POSSIBLE CASE FOR REVERSIBLE MICROBIAL INACTIVATION

A reduction in efficacy of the persistent herbicide fluridone has been observed in soils with fluridone application history (FH soils). Flur idone activity, as monitored by bioassay, persisted in FH soils for ca 1 week, whereas in fluridone non-history (FNH) soils phytotoxicity co uld be observed even after 3 months of exposure. Incorporation of smal l volumes of FH soil were sufficient to confer rapid loss of herbicida l activity on larger, autoclaved FNH soil volumes. In FH soils exposed to intensive antimicrobial treatments the activity of fluridone was r estored to levels similar to those observed in FNH soils. Autoclaving and to a lesser extent, other disinfestation or antimicrobial measures (gamma irradiation, methyl bromide, several fungicides) used to treat FH soils, exposed test plants to phytotoxic compounds even though the herbicide was not re-applied to the soil. Fluridone adsorption coeffi cients (K(a)) in FH and FNH soils were similar, suggesting that physic al adsorption of the herbicide does not play a key role in the observe d inactivation of fluridone in FH soil. The possibility of reversible microbial inactivation of fluridone is discussed.