Mugwort or false chrysanthemum is an adaptable, nonnative perennial plant that is a management challenge to commercial agronomic and ornamental crop production worldwide (Barney and DiTommasso, 2003; Henderson and Weller, 1985; Holm et al., 1997; Uva et al., 1997). In the eastern United States, mugwort has expanded beyond ornamental nurseries into turf, pastures, and cropping systems including corn (Zea mays), cotton (Gossypium hirsutum), and soybean (Glycine max) (Bradley and Hagood, 2002a, 2002b; Yelverton et al., 2012).
Mugwort reproduction and dispersal is achieved primarily by transport of rhizome pieces on contaminated cultivation equipment and within soil and soilless substrates around the roots of crop plants (Barney and DiTommasso, 2003; Bradley and Hagood, 2002a, 2002b; Holm et al., 1997; Rogerson and Bingham, 1964; Uva et al., 1997). In nursery fields, rhizomes are abundant in the upper 4 inches of soil (Pridham, 1963; Rogerson and Bingham, 1964). Tillage will exacerbate localized mugwort infestation because rhizome sections 2-cm long reproduce well when grown in pine bark, sand, and soil substrates (Guncan, 1982; Klingeman et al., 2004; Rogerson et al., 1972).
Postemergence (POST) control of weeds in nursery operations remains a particularly attractive option for both field-grown and container production systems due, in part, to need to reduce labor costs incurred for hand-weeding in addition to the current practice of making about three annual herbicide applications per nursery (Gilliam et al., 1990; Mathers, 1999). Yet, control options for mugwort are limited due in part to challenges presented by herbicide costs, herbicide phytosafety across desirable crop commodities (including turfgrass and ornamentals), and limited efficacy available herbicides may have on other key weed species. For example, ethyl N, N-di-n-propylthiolcarbamate application at 15 lb/acre has controlled mugwort in field nurseries, yet this rate is too high to safely manage mugwort in stands of field corn (Bing and Pridham, 1963, 1964). Glyphosate is effective on young mugwort plants but is less effective at controlling large mugwort plants in later stages of growth (Ahrens, 1976; Bradley and Hagood, 2002a, 2002b). Plant growth regulating herbicides that mimic indole-3-acetic acid, like 2,4-dichlorophenoxyacetic acid and clopyralid, have generally provided poor mugwort control, even at high application rates (Ahrens, 1976; Bingham, 1965; Bradley and Hagood, 2002a, 2002b; Day et al., 1997). Clopyralid can control mugwort and reduce regrowth the following year, yet clopyralid is relatively expensive, has limited efficacy against other commonly encountered weed species, and is challenged by limited crop safety across a diverse assemblage of ornamental plant species (Bradley and Hagood, 2002a, 2002b; Bradley et al., 2000; Day et al., 1997; Koepke-Hill et al., 2011). Still, the broad utility of clopyralid (Stinger® and Transline®; Dow AgroSciences, Indianapolis, IN) across corn, sugarbeet (Beta vulgaris), pasture and rangeland, Christmas tree, ornamental plant, and fruit and vegetable crop production systems makes this herbicide a practical choice for selective control of mugwort.
Mesotrione and topramezone impede carotenoid biosynthesis by inhibiting the enzyme p-HPPD (Norris et al., 1998). Mesotrione is registered for preemergence (PRE) and POST applications in corn and turfgrass (BASF, 2013; Mitchell et al., 2001; Syngenta Crop Protection, 2011, 2012) and provides effective POST control against many annual broadleaf weeds and certain grass species (Armel et al., 2003, 2009; Beckett and Taylor, 2000; Sutton et al., 1999). Topramezone is also registered for POST weed control in corn and turfgrass (e.g., Amvac, 2007; BASF, 2013). Apparent as foliar photobleaching, topramezone reduces total chlorophyll, β-carotene, lutein, and other xanthophyll cycle pigments in susceptible grassy and broadleaf weeds, including common bermudagrass (Cynodon dactylon), hybrid bermudagrass (C. dactylon × C. transvaalensis), and annual bluegrass (Poa annua) (BASF, 2013; Brosnan et al., 2011; Elmore et al., 2011, 2013).
In limited tests with ornamental plants, some HPPD herbicides have provided acceptable ornamental tolerance at rates that provide some level of weed control (Armel et al., 2009; Elmore et al., 2013; Little et al., 2004; Senesac and Tsontakis-Bradley, 2007; Stamps and Chandler, 2009). Increased POST control of some perennial weed species was achieved when HPPD-inhibiting herbicides, like mesotrione, were combined with other herbicides, especially those that inhibit PSII inhibitors (Bradley et al., 2000; Kaastra et al., 2008). For example, POST mixtures of reduced rates of the PSII inhibitor atrazine plus mesotrione has enhanced management of larger and more difficult to control weed species, including nutsedge (Cyperus sp.) and canada thistle (Cirsium arvense) (Armel et al., 2005, 2008, 2009; Beckett and Taylor, 2000; Johnson and Young, 1999, 2000, 2002; Johnson et al., 2002; Kaastra et al., 2008; Mueller, 2000). Efforts to assess reduced rates of mesotrione and topramezone in mixtures with the PSII inhibitor bentazon, which is one of the few PSII-inhibiting herbicides registered for use in turfgrass and ornamental production systems, revealed some weed control efficacy yet induced moderate phytotoxic injury to foliage and terminals of flowering dogwood (Cornus florida) and Knock Out™ rose (Rosa sp. ‘Radrazz’). Still, trials revealed acceptable plant safety for treatments applied to ornamental cultivars of woody japanese holly (Ilex crenata), burning bush (Euonymus alatus), weigela (Weigela florida), spiraea (Spiraea japonica), arborvitae (Thuja plicata), herbaceous daylily (Hemerocallis), hosta (Hosta), autumn fern (Dryopteris erythrosora), and pachysandra (Pachysandra terminalis) (Cutulle et al., 2013b).
Few selective weed control options are available for mugwort control, and it generally is not adequately controlled with single applications or reduced rates of POST herbicides (Bradley and Hagood 2002a, 2002b). It follows that synergistic mixtures of reduced rates of HPPD and PSII inhibitors could optimize weed control and increase herbicide tolerance to turfgrass and ornamental crops (Cutulle et al., 2013b). Therefore, objectives of this research were to evaluate low rates of the HPPD herbicide mesotrione and to assess control efficacy of mesotrione combined with known synergist like the PSII inhibitor atrazine for enhanced activity on mugwort compared against the standard clopyralid. The second objective was to quantify the ability of reduced rates of multiple HPPD-inhibiting herbicides, either alone or paired with the PSII-inhibiting herbicides atrazine or bentazon, to provide commercially acceptable mugwort control for ornamental plant producers, whether growing in container or field systems.
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