Florida is the largest producer of citrus with production of more than 60% of the total citrus produced in the United States [U.S. Department of Agriculture (USDA), 2010]. In 2011, citrus was grown on more than 500,000 acres with the production of 7,427,000 tons in Florida (USDA, 2012). Weed management is an important horticultural operation in citrus production practices. Weeds are highly competitive with citrus trees under the growing conditions of central Florida because of frequent rainfall, high temperature, and humid weather (Sharma and Singh, 2007). Weed control in Florida citrus relies heavily on herbicides. Herbicides are applied as a uniform broadcast application to the grove floor or as a uniform band application directed at the base of the tree trunk (Singh and Singh, 2004).
Several preemergence and postemergence herbicides are registered for weed control in Florida citrus (Futch and Singh, 2012). Pendimethalin (Prowl H2O™; BASF Corp., Research Triangle Park, NC) and oryzalin (Surflan®; United Phosphorus, King of Prussia, PA) are commonly used soil-applied herbicides for control of many annual weeds and certain broadleaf weeds in citrus. Rimsulfuron (Solida™; Cheminova, Research Triangle Park, NC) is a sulfonylurea herbicide that was registered in 2010 for preemergence and postemergence control of certain grasses and broadleaf weeds in citrus, pome fruit (Maleae), tree nut, stone fruit (Prunus sp.), and grapes (Vitis vinifera) that have been established for at least one full growing season (Cheminova, 2010).
Rimsulfuron is also registered for weed control in several annual crops including corn (Zea mays), potato (Solanum tuberosum), and tomato (Solanum lycopersicum) (Senseman, 2007; Tonks and Eberlein, 2001). It inhibits branched-chain amino acid production in susceptible weeds by inhibition of the enzyme acetolactate synthase (ALS) or acetohydroxy acid synthase. It moves primarily in the phloem with limited mobility in the xylem. Like several sulfonylurea herbicides, rimsulfuron requires an adjuvant for effective weed control (Green and Green, 1993). The initial symptoms of rimsulfuron activity are observed in the meristematic tissues of treated plants. For broadcast application, the recommended rate of rimsulfuron is 4 oz/acre per year; however, when applied as a banded treatment (application of herbicides only covering an area of 3 to 4 ft on each side of the tree in a strip), it may be applied twice per year with a minimum 30-d interval between applications (Cheminova, 2010). Rimsulfuron degrades rapidly in the soil predominantly via chemical pathways. The half-life in laboratory studies is ≈20 d under both aerobic and anaerobic conditions in a sandy loam soil (Schneiders et al., 1993).
A study reported that rimsulfuron applied postemergence provided inconsistent weed control in potato (Eberlein et al., 1994); however, tank mixing with other herbicides such as metribuzin or pendimethalin improved weed control (Hutchinson, 2007). Hutchinson et al. (2005) reported that late-season common lambsquarters (Chenopodium album) control by rimsulfuron applied PRE alone in potatoes has been less than 80%; however, Wilson et al. (2002) reported control of broadleaf weeds including redroot pigweed (Amaranthus retroflexus) by rimsulfuron plus S-metolachlor was 99% to 100% in potato. Ivany (2002) reported that rimsulfuron tank mixed with metribuzin gave 99% control of wild radish (Raphanus raphanistrum) and 85% control of corn spurry (Spergula arvensis) in potato.
Flumioxazin (Chateau®; Valent USA Corp., Walnut Creek, CA) is an N-phenylphtalimide herbicide that is registered in several states of the United States for weed control in numerous crops and non-cropland situations (Valent USA, 2005). Flumioxazin is not registered in citrus, but currently being evaluated for weed control and crop safety. It can be absorbed by root or foliage of treated plants and it inhibits protoporphyrinogen oxidase (Dayan and Duke, 1997). Flumioxazin has both preemergence and postemergence activity; therefore, when it is applied to soil, most susceptible weed seeds and seedlings will die as they begin to emerge; whereas foliar contact to susceptible plants results in a rapid desiccation followed by necrosis (Hutchinson, 2007). A study for weed control in gladiolus (Gladiolus sp.) reported that flumioxazin provided 78% control of common ragweed, yellow nutsedge (Cyperus esculantus), and foxtail species (Setaria sp.) at 12 weeks after treatment (Richardson and Zandstra, 2006). Several studies reported that flumioxazin efficacy could be improved by tank mixing herbicides with different mode of action. For example, flumioxazin applied alone as a preemergence resulted in poor annual grass control that was improved by tank mixtures (Askew et al., 1999; Burke et al., 2002; Kelly et al., 2006).
Citrus growers use herbicide tank mixtures depending upon type and density of weed species present in the grove. By tank mixing herbicides, the weed control spectrum can be increased (Hutchinson, 2007; Jhala and Hanson, 2011). For example, saflufenacil, a contact herbicide for broadleaf weed control, was tank mixed with glyphosate and pendimethalin to provide broad-spectrum weed control in Florida citrus (Singh et al., 2011). To develop weed control programs that includes rimsulfuron, more information is required to understand compatibility of rimsulfuron with herbicides commonly used in citrus and new herbicides that are under evaluation. Therefore, the objective of this research was to compare weed control efficacy of rimsulfuron applied alone and in tank mixtures with flumioxazin, pendimethalin, or oryzalin.
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