The Microbial Degradation
of Pesticides Important to
Rice Culture-2000



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Project Leader and Principal UC Investigators

Ronald S. Tjeerdema, professor, Department of Environmental Toxicology, University of California, Davis

Donald G. Crosby, professor emeritus, Department of Environmental Toxicology, University of California, Davis


A new effort has been undertaken to understand the cause of delayed phytotoxicity syndrome (DPS) in rice. The syndrome, characterized by yellowing, stunted growth and eventually plant death, is a serious problem in some parts of the Central Valley.

27MicroWater2.jpg (17377 bytes)A wide array of organisms - from bacteria to vertebrates - have a tremendous capacity to break down chemicals in the environment. In a flooded rice field most bacterial activity tends to be concentrated in the underlying soils. It is here where toxicologists believe the activity of anaerobic bacteria on the rice herbicide thiobencarb (Bolero) is creating a dechlorinated byproduct that is the underlying cause of DPS.

Previous studies have suggested that organic matter, such as rice straw incorporated into the soil, is an important factor in the microbial breakdown of 27MicroWater.jpg (352521 bytes)thiobencarb. In environmentally controlled culture chambers, toxicologists studied San Joaquin grade soil known to be prone to DPS. The soil was then covered with well water to model flooded rice field conditions. Anaerobic soils with varying amounts of rice straw incorporation showed significant production of the byproduct within 10 to 20 days of herbicide application. The highest amounts occurred in soil with the highest straw content. Toxicologists emphasize, however, that these are preliminary results.

Nonetheless, this dechlorinated thiobencarb byproduct is known to be highly toxic to rice. It is also apparent that it is stimulated by an anaerobic process in the presence of soil-incorporated organic matter. Thus, conditions that either reduce the viability of anaerobic soil bacteria in rice fields or enhance the formation of aerobic conditions will potentially reduce the production of the toxic byproduct and the potential for DPS. Research into this problem is continuing.

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