An investigation into the cause of Delayed Phytotoxicity Syndrome (DPS), a serious problem in the rice fields of the eastern Sacramento Valley, continued for a second year. DPS, characterized by stunted rice plants, is thought to be caused by a dechlorinated byproduct of Bolero^®, created by anaerobic soil bacteria.  Production of this byproduct, called deschlorothiobencarb, is enhanced by the addition of organic carbon, a concern for growers because of increasing amounts of incorporated rice straw and stubble.

In one part of this project, researchers studied rice plants grown from seed in a hydroponic system to compare effects of the byproduct with its parent compound, thiobencarb.  Indeed, deschlorothiobencarb proved to be 10 times more toxic to rice plants.

Scientists also compared soils known to be susceptible to DPS to those that are not to get a better sense of the physical and chemical characteristics at play.  Soils were collected from rice fields in Northern California during spring 2002 and analyzed at UC Davis.  In general, soils that are susceptible are characterized by high sand, high magnesium, low clay, low organic matter, low natural carbon (total and organic), low nitrogen (organic and inorganic) low pH, low copper and other factors. 

Toxicologists determined that in DPS-susceptible soils, organic carbon may be a limiting factor.  Thus the addition of rice straw may stimulate anaerobic bacterial action and exacerbate DPS.  Conversely, the high levels of copper found in DPS-resistant soils may inhibit bacterial action.  The action of copper on DPS should be investigated further, as it is already applied to rice fields to control tadpole shrimp and algae and might also be used to inhibit DPS.

The third objective in this project was to compare the uptake and metabolism of clomazone in rice against watergrasses.  Rice and both early and late watergrasses were exposed to different concentrations of this herbicide while still in the developmental stages.  Early watergrasses were most sensitive to clomazone, while late watergrasses were intermediate in sensitivity.  Clomazone had no significant effect on rice growth — even at the highest concentrations tested.  Nonetheless, substantial bleaching was observed and a method to measure its impact on rice plants should be investigated.  An understanding of the mechanism behind the selective toxicity of clomazone for rice versus watergrass will facilitate the development of safeners to reduce the injury caused to rice upon application in the field.