Creeping bentgrass is used widely on golf-putting greens and fairways in temperate regions. The strong tolerance to close mowing and low temperatures makes creeping bentgrass very competitive in its adapted region compared with other species (Adams, 1981). However, creeping bentgrass becomes a weed when escaped into other turfgrasses because it reduces uniformity due to differences in textures and growth habit (Turgeon, 1981). Selective removal of creeping bentgrass may be necessary in such circumstances. Bhowmik and Drohen (2001) reported selective control of creeping bentgrass using isoxaflutole (Balance® Flexx; Bayer CropScience, Research Triangle Park, NC), with minor injury to other cool-season grasses. Askew et al. (2003) reported that both isoxaflutole and mesotrione (Tenacity™ 4SC; Syngenta Crop Protection, Greensboro, NC) could provide selective bentgrass removal in kentucky bluegrass (Poa pratensis) and perennial ryegrass (Lolium perenne). Mesotrione is a systemic herbicide and works by inhibiting p-hydroxyphenylpyruvate dioxygenase (HPPD), an enzyme essential for the biosynthesis of carotenoids. Without carotenoids, excessive light energy destroys chlorophyll and causes new growth to appear white before necrosis and death (Giese et al., 2005). Selective removal of creeping bentgrass from kentucky bluegrass was achieved by using mesotrione at three sequential applications of 420 g·ha−1 in a study by Branham et al. (2005). Beam et al. (2006) reported 58% to 100% creeping bentgrass control by repeated treatments of the grass with mesotrione at a minimum rate of 280 g·ha−1. Volunteer creeping bentgrass often recovers from the injury caused by the herbicide, and thus, more applications are needed for eradication (Branham et al., 2005).
Mixing a herbicide with an adjuvant is a common practice to improve herbicide's efficacy (Nalewaja et al., 1995). Giese et al. (2005) reported that mesotrione (140 to 420 g·ha−1) provided better control of creeping bentgrass with NIS than with methylated seed oil. Fertilizer salts, such as UAN and ammonium sulfate, often are added to spray solutions with NIS to help increase the activity of foliar-applied herbicides (Pearson et al., 2008). The exact mechanism of action of ammonium salts is not clear, although increased absorption into plants has been reported (Bruce et al., 1993; Dodds et al., 2007; Fandrich et al., 2001). Li and Howatt (2006) reported that creeping bentgrass control was improved when UAN was added to mesotrione at a rate of 17.5 g·ha−1 plus NIS in a greenhouse experiment.
Cultural practices have been recommended alone or in combination with chemical methods for weed control (Calhoun et al., 2005). The efficacy of chemical weed control in turfgrass can be improved by using relatively taller mowing heights, which keep the turfgrass healthier in many cases (Dernoeden, 2001; Jagschitz and Ebdon, 1981). At a mowing height that is suitable for perennial ryegrass, little of the creeping bentgrass will be removed. Constant low mowing will remove the dead tissues of the creeping bentgrass and will increase the exposure of leaves in the lower canopy to sunlight. This may increase the efficacy of mesotrione. McCurdy et al. (2009) reported that mesotrione control of large crabgrass (Digitaria sanguinalis) was unaffected by various levels of temperature and irradiance. Limited information is available on the effects of light on creeping bentgrass control using mesotrione.
We hypothesize that increased exposure to sunlight by removing the dead tissues may result in more injury to the lower leaves by mesotrione. The level of phytotoxicity could then be detected by monitoring the integrity and activity of photosynthesis systems that are affected by the mode of action of mesotrione. Chlorophyll content and quantum yield of photosynthesis system II (PSII)–mediated electron transport are often measured for this purpose in physiological studies (Ounis et al., 2001). The objective of this study was to evaluate the effect on selective creeping bentgrass control from adding UAN to NIS plus mesotrione and from removing dead tissues of creeping bentgrass before the application of herbicides.
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