During Winter 2010–11, strawberries were planted in west-central Florida on ≈9900 acres, with an estimated value of $366 million [U.S. Department of Agriculture (USDA), 2011]. On an annual basis, this production accounts for a large portion of the domestically produced winter crop, and ≈10% to 15% of the total U.S. production. As of 2007, ≈90% of Florida strawberry production occurred within Hillsborough County, with the remainder of the crop scattered throughout west-central Florida (USDA, 2007). Advancements in breeding of short-day cultivars have given Florida producers a unique market advantage to provide higher volumes than any other domestic producer during the winter (Jones et al., 2003). This ability enables Florida producers to receive premium prices for strawberries, making strawberries an economically important crop to the state (Santos et al., 2007).
Weed control in annual strawberry is concentrated in two areas: the soil beneath the mulch-covered beds and the bare ground area between adjacent mulch-covered beds also referred to as the row-middle (Mossler and Nesheim, 2004). Soil fumigants in combination with herbicides are the two most common methods of weed control in commercial strawberry production (Fennimore et al., 2003). Weed pressure within the planting bed comes from weeds in the planting holes, with the exception of nutsedge (Cyperus sp.), which can puncture and grow through the plastic. Weeds growing in the planting holes typically begin to germinate within 3 to 5 weeks after planting (WAP) and compete with the crop for light, nutrients, and water (Gilreath and Santos, 2005). Although many annual weeds can be controlled with fumigation, dormant hard-seeded annual weeds such as carolina geranium (Geranium carolinianum), black medic (Medicago lupulina), and cutleaf evening-primrose (Oenothera laciniata) can survive fumigant treatment and become mid- to late-season problems (Mossler, 2010; Stall, 2008).
Clopyralid has previously been shown to be a potential candidate for postemergent (POST) broadleaf weed control in both perennial matted-row production as well as annual plasticulture production of strawberries (Clay and Andrews, 1984; Figueroa and Doohan, 2006; McMurry et al., 1996). In 2006, Figueroa and Doohan conducted trials with clopyralid application rates ranging from 25 to 400 g·ha−1 applied as a postharvest spray in a perennial strawberry production system. Common groundsel (Senecio vulgaris) control of 82% was achieved when clopyralid was applied at 200 g·ha−1 without adverse effects to strawberry plant growth. Although strawberry foliage growth was unaffected, a significant yield increase was reported at 200 g·ha−1; however, it was speculated that this increase occurred because of seedling competition within the hand-weeded plots. At the maximum application rate of 400 g·ha−1, there was evidence of a significant reduction in strawberry yield as well as crop canopy (Figueroa and Doohan, 2006).
A study conducted by McMurry et al. (1996) investigated the effects of an early season application of clopyralid on actively growing annual strawberries grown in a plasticulture cropping system. Ninety percent control of vetch (Vicia sp.) was achieved 4 weeks after treatment (WAT) with clopyralid rates from 140 to 200 g·ha−1, whereas clopyralid at 70 g·ha−1 provided only 62% control 4 WAT. Regardless of rate, 100% control of vetch was achieved 8 WAT. A rate response was observed for control of black medic with an increase in weed control as the rate of clopyralid increased. Clopyralid at 70, 140, and 280 g·ha−1 provided 49%, 63%, and 83% control of black medic 4 WAT, respectively. McMurry et al. (1996) hypothesized that the control of black medic was related directly to spray coverage. It was noted that black medic plants that survived were small and generally located under the canopy of strawberry plants, ultimately leading to less than 100% control. Regardless of season, growth stage, or application rate, crop injury was reportedly less than 6% and confined to higher clopyralid rates (McMurry et al., 1996).
As strawberry producers transition from methyl bromide to alternative fumigants, the need for alternative weed management products and practices will become important to maintain the standard of quality expected of the industry. Although strawberry has been shown to exhibit acceptable tolerance to clopyralid, differing levels of tolerance among cultivars have not been studied. Differing levels of tolerance to certain herbicides between cultivars is evident (Baker and Peeper, 1990; Lemerle and Hinkley, 1991; Monks et al., 1992). Lemerle and Hinkley (1991) showed that canola (Brassica napus and Brassica campestris) cultivars differ in their level of tolerance to clopyralid. The canola cultivar Hyola 30 was most sensitive, with the application of clopyralid resulting in a yield reduction of 16% to 19%. Greater differences were reported by Monks et al. (1992) who examined the tolerance of multiple sweet corn (Zea mays) cultivars to applications of nicosulfuron and primisulfuron. When nicosulfuron and primsulfuron were applied at 35 and 42 g·ha−1, injury was reported at 98% and 97%, respectively, for the sweet corn cultivar Merit. Other cultivars such as Incredible, Landmark, Silver Queen, and Sweetie 76 were deemed tolerant, with less than 16% injury reported (Monks et al., 1992).
The purpose of this study was to examine the extent of injury to the major strawberry cultivars grown in Florida from a POST application of clopyralid to explore the potential of this herbicide in Florida strawberry production.
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