St. augustinegrass is the most widely used turfgrass in commercial and home landscapes in Florida and is also a commonly used turfgrass along the coastal Gulf states. It grows throughout the year in southern Florida but usually senesces during the winter in the northern range of its distribution. St. augustinegrass is established vegetatively by planting sod, plugs, or sprigs, which under ideal conditions, rapidly expand to form a dense groundcover via extensive stoloniferous growth. Once established, st. augustinegrass should be irrigated with 0.5–0.75 inch of water two to three times per week if no rainfall occurs (Trenholm et al., 2017). Irrigation water sources vary based on site conditions and availability and may include municipal waters, private wells, and ponds located on-site. Mowing frequency and height vary among cultivars, but growth and pest resistance are optimized when st. augustinegrass is maintained at a height of 2.5–4 inches (Trenholm et al., 2014).
Topramezone is a carotenoid biosynthesis inhibitor that interferes with the production of the enzyme HPPD in the carotenoid pigment synthesis pathway (Senseman, 2007). In addition to its recent registration for aquatic weed control, topramezone is used for weed control in upland systems. For example, Armezon (336 g·L−1 topramezone; BASF, Research Triangle Park, NC) is labeled at application rates of up to 25 g·ha−1 for postemergence control of grasses and broadleaf weeds in corn (BASF, 2012) and Pylex (336 g·L−1 topramezone; BASF) is labeled for weed control at rates up to 50 g·ha−1 in cool-season turfgrasses such as golf courses, sod farms, and residential landscapes (BASF, 2015). The Pylex label specifically states that applications will control or suppress st. augustinegrass, bahiagrass (Paspalum notatum), and bermudagrass (Cynodon dactylon) (BASF, 2015). The herbicide Frequency® (336 g·L−1 topramezone; BASF) is labeled for use in pine plantations, rights of way, and other noncrop sites (BASF, 2013). Grasses listed as tolerant of topramezone include cool-season grasses such as bluestem (Andropogon sp.), fescue (Festuca sp.), bluegrass (Poa sp.), and centipedegrass (Eremochloa sp.).
Topramezone was studied for several years under an experimental use permit to evaluate its efficacy on the submersed aquatic weed hydrilla (Hydrilla verticillata). Puri et al. (2009) found that topramezone provided effective control of hydrilla at concentrations of 25–40 µg·L−1; it was highly selective at these rates and caused little damage to most native nontarget plants. It was registered for aquatic use by the U.S. Environmental Protection Agency in 2013 under the trade name Oasis (SePRO Corp., Carmel, IN). Because irrigation water is sometimes sourced from on-site ponds that may also be targeted for aquatic weed control, determining whether aquatic herbicides have phytotoxic effects on irrigated landscape plants such as st. augustinegrass is of interest. Previous greenhouse studies evaluating the effect of topramezone in irrigation water revealed that the EC10 of topramezone was 3.5 ppb on ‘Palmetto’ st. augustinegrass grown in 100% sand (Haller et al., 2017). However, these results differed from unpublished field observations; plants used in greenhouse studies are typically grown in sand to determine phytotoxicity of herbicides in the absence of soil carbon, whereas most field soils where turfgrasses are grown contain carbon, which can bind with topramezone. The soil adsorption coefficient (Koc) of topramezone is 22–172 mg·L−1, with a soil half-life of 14 d and a reported half-life of 72 d in water (Shaner, 2014). Soil half-life or degradation of a potentially phytotoxic herbicide is very relevant when the chemical is applied multiple times at sublethal doses such as may occur via herbicide-treated irrigation water. Depending on the half-life of a product, multiple irrigation events may add to the soil burden of the herbicide and eventually result in phytotoxic concentrations. The objective of these experiments was to determine the effects of irrigation with topramezone-treated water on ‘Palmetto’ st. augustinegrass grown in substrates with different carbon contents.
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