Linda Benedict, Sanders, Dearl E., Groth, Donald E. | 10/7/2009 7:16:54 PM
Donald E. Groth, Dearl E. Sanders and Greg Rich
Rice plants showing herbicide damage where no herbicides had been applied for several weeks were first found in 1991 in southwest Louisiana. Symptoms included stunting, excessive tillering, curvature or “fishhooking” of tillers, dark green color, stem brittleness and plant death. Areas with damaged plants were irregularly shaped and unevenly distributed within fields, which suggested a biological cause. Symptoms were similar to those caused by high rates of the herbicide rice molinate (Ordram), but soil analysis for molinate showed none.
Affected acreage increased to nearly 20,000 acres in 1993. A similar problem had been reported in Japan during the mid-1970s and was associated with dechlorination of herbicides containing a chlorinated benzene ring by an unidentified facultative anaerobic bacterium.
In the United States, the problem has been termed Delayed Phytotoxicity Syndrome (DPS) and has been associated with the several rice herbicides. These herbicides have halogenated aromatic compounds as their active ingredient and include thiobencarb (Bolero), quinclorac (Facet), triclopyr (Grandstand), propanil and 2,4-D, both available commercially in various formulations.
Fungi cause the problem
To determine if dechlorination of halogenated herbicides and DPS are directly related, a greenhouse experiment using the white mold fungus Phanerochaete chrysosporium, which is known to dechlorinate aromatic rings, was conducted. The procedure involved incorporating the fungus into sterile soil in the presence and absence of thiobencarb, transplanting four-leaf rice seedlings into the soil and flooding the pots. Separate experiments were conducted with quinclorac and triclopyr. Plants were then monitored for DPS symptoms.
At the same time, rice plants were transplanted into a conducive soil from a field exhibiting DPS that was treated with thiobencarb. When plants began showing symptoms, a soil sample was collected from under the plant, and various microorganisms were isolated. These microorganisms were purified and then tested for their ability to dechlorinate thiobencarb using a bioassay in sterile soil. Soil from the white mold/ thiobencarb experiment was tested in an analytical testing lab for dechlorinated thiobencarb.
It was shown that the white mold fungus was able to dechlorinate thiobencarb based on symptoms and the presence of dechlorinated thiobencarb in the soil. Quinclorac and triclopyr also were dechlorinated, based on symptom development, but symptoms were not as severe. Two fungi and one bacterium were isolated from the conducive soil and identified as having the ability to dechlorinate thiobencarb.
Fungicides offer cure
Since fungi were involved, experiments were conducted to determine if fungicides could be used to control the problem. Soil from a field exhibiting thiobencarb-induced DPS was placed in pots in the greenhouse. Thiobencarb was applied to the soil, and then fungicides were applied. Pots were then flooded, and four-leaf rice seedlings were transplanted into the soil. Treatments included an untreated check, thiobencarb at 4 pounds active ingredient per acre, benomyl (Benlate) and iprodione (Rovral) fungicides used at 2 pounds active ingredient per acre, and a herbicide and fungicide combination. Plants were monitored for DPS symptoms.
The experiment was repeated in the field in the conducive soil. Treatments included the fungicides impregnated separately onto the 10 G formulation of thiobencarb and onto blank granules. In field experiments, the soil was prepared for water seeding, and galvanized steel rings were placed in the field to stop herbicide movement in the soil or water. A preplant surface application of thiobencarb was made, and presprouted seeds were water-planted 48 hours after herbicide application. Water was temporarily drained for 24 hours after seeding to allow seedling attachment to the soil. Plots were monitored for DPS development. The experiment was repeated using similar methods but using the liquid formulations of thiobencarb and sequential applications of the fungicides after the herbicide.
Severe injury developed 30 days after treatment on greenhouse plants where thiobencarb was applied alone. No injury was apparent in the untreated check, fungicides only and thiobencarb plus fungicide treatments. Iprodione and benomyl were the most effective, and iprodione had more residual activity.
Seven days after herbicide application in the field, there was a 95 percent stand loss with thiobencarb alone. Granular thiobencarb plus iprodione resulted in a 2 percent stand loss. Similar results were noted 30 days later. In comparison, thiobencarb plus benomyl plots had 28 percent stand loss after seven days and a 91 percent stand loss after 30 days. The untreated checks and the fungicide alone showed no injury. Weed control was not affected by the addition of the fungicide to the herbicide. A close association of the herbicide and fungicide was necessary since sequential sprays with the liquid herbicide and fungicides were less effective than tank mixes.
Control limited to draining fields
Currently, the only control for DPS is to drain rice fields and permit the soil to dry. This allows the soil to become oxygenated and causes the dechlorinated herbicide, which is apparently unstable in aerobic conditions, to break down. Draining causes other problems, however, including nitrogen loss and more susceptibility to weeds and diseases.
Th DPS problem appeared right after the U.S. Environmental Protection Agency banned use of fungicide-treated seed in 1990. Although use of fungicides is preventive, it is not legal at this time. Further work is being done to obtain a label for the use of fungicides against DPS and to determine the most cost-effective fungicide.
The authors would like to express their appreciation to Valent U.S.A. Corp. and the Louisiana Rice Research Board for providing funds to support this research.
Donald E. Groth, Professor, Rice Research Station, Crowley, La.; Dearl E. Sanders, Extension Weed Specialist, LSU Agricultural Center, Baton Rouge, La.; and Greg Rich, Valent U.S.A. Corp., Germantown, Tenn.
(This article was published in the winter 1999 issue of Louisiana Agriculture.)