03329naa a2200181 a 450000100080000000500110000800800410001910000110006024501150007126000090018652027050019565300190290065300130291965300250293265300150295765300140297277301610298610827922012-01-04       2011    bl uuuu     u00u1 u    #d1 aEpagri  aMultiple herbicide-resistant sagittaria montevidensis population in Santa Catarina State (Brazil) rice fields.  c2011  aSagittaria montevidensis (SAGMO) is a troublesome weed in irrigated rice areas in Santa Catarina. In the 1990?s, this weed was already disseminated in the rice fields and easily controlled with acetolactate synthase (ALS) inhibitors herbicides. The first case of SAGMO resistant to ALS herbicide was reported in 1999 and the best control option for these populations was bentazon, a photosystem II (PS II) inhibitor herbicide. In the last decade, bentazon became the main and almost the only option to control SAGMO and was used in more than 80% of the paddy rice areas. In 2008, a rice farmer from Ilhota County in the Itajai Valey, firstly reported problems to control SAGMO with bentazon, even using high rates. Field experiments were carried out in the 2008/09 and 2009/10 crop seasons at the farmer rice area to evaluate the sensitivity of the local SAGMO population to bentazon as well as to find alternatives to control these populations. Two experiments were also carried out under controlled conditions in 2009 and 2010 to confirm the multiple resistance of SAGMO populations to ALS and PS II herbicides (bentazon). In both experiments the herbicides (penoxsulam, pyrazosulfuron-ethyl, imazapic+imazethapyr and bentazon were evaluated at 0.5, 1, 2, 4 and 8 times the label rate. In the first field study (2008/09), the maximum control level with bentazon was 50% and 75%, respectively at 1 and 2X the label rate. The tank mixture of bentazon with other herbicides did not improve the efficacy. In the second field experiment (2009/10), bentazon applied at 4X rate controlled 78% (maximum) of SAGMO population, showing resistance of the population to bentazon. The maximum SAGMO control percentage using ALS inhibitors at 4X rate was 43%. The experiments carried out under controlled conditions also resulted in poor control of SAGMO populations. In 2009, SAGMO control with ALS inhibitors was lower than 50% for all herbicides and rates. Bentazon at 1, 4 or 8X rate resulted in 26%, 10% and 1% of surviving plants. In 2010, the percentage of SAGMO surviving plants was 83%, 96%, 71% and 51%, respectively for herbicides penoxsulam, pyrazosulfuron-ethyl, (imazapic+imazethapyr) and bentazon, applied at maximum rate (8X). The higher SAGMO surviving percentages in 2010 was due to the origin of the seeds. In 2009, SAGMO plants were obtained from the seed bank of the farmer area and in 2010, seedlings were established using seeds collected from surviving plants sprayed with bentazon in the year before. The data from these studies confirm the occurrence of SAGMO population cross-resistance to ALS and multiple-resistant to ALS and PS II (bentazon) in rice fields in Santa Catarina, Brazil.  aALS inhibitors  aBentazon  aHerbicide resistance  aPaddy rice  aSagitaria  tIn: WEED SCIENCE SOCIETY OF AMERICA ANNUAL MEETING, 51., 2011, Portland, United States. Proceedings... Lawrence, USA: Weed Science Society of America, 2011.