The United States is the world’s largest producer of strawberries (Fragaria ×ananassa Duch.) with total production in 2010 valued at $2.2 billion, of which California’s contribution is $1.8 billion [Agricultural Marketing Resource Center (AMRC), 2011]. Of the 23,541 ha of land under strawberry production in the United States, 15,621 ha were in California (U.S. Department of Agriculture–National Agricultural Statistics Survey, 2011). Within California, most production occurs in the southern and central coastal areas with a growing season typically lasting 10 to 15 months (AMRC, 2011). Weeds are a threat to strawberry, particularly during the early establishment stages, when high soil moisture levels are maintained through frequent irrigations and weeds are likely to emerge (Fennimore et al., 2008). Besides competing with the crop for moisture, sunlight, nutrients, and space, weeds can attract and harbor insects and pathogens in the production area.
Methyl bromide (MB) + chloropicrin (Pic) fumigation has long been used to control weeds in strawberry with the exception of weeds with a hard seedcoat such as California burclover (Medicago polymorpha L.) and common mallow (Malva neglecta Wallr.). Where MB was used, weeding costs would traditionally range from $750 to 1750/ha (Daugovish et al., 2009). With the phase out of MB as a soil fumigant, growers have moved to using alternative fumigants including Pic, 1,3-dichloropropene (1,3-D), or metam sodium, but these alternatives do not always provide the same level of weed control as MB–Pic application (Fennimore et al., 2008; Locascio et al., 1997). Particularly difficult to control are weeds with hard seedcoats and perennial weeds like yellow nutsedge (Cyperus esculentus L.) (Daugovish et al., 2009). Hand weeding the fields increases production costs and has little effect on weeds such as nutsedge. Herbicides can help keep the weed populations down while maintaining the profitability of strawberry production. Potential use of herbicides includes sensitive border and buffer areas where the use of fumigants is restricted, new ground being used for strawberry production that has high weed pressure, and the furrow area in between raised beds that are typically non-fumigated (Fennimore et al., 2005).
Both flumioxazin and oxyfluorfen herbicides are registered for strawberry production in California, and each product was used on over 2000 ha of strawberry in 2010 (California Department of Pesticide Regulation, 2012; Daugovish et al., 2009). Flumioxazin at 0.07 and 0.11 kg·ha−1 a.i can be applied to the raised bed 30 d before strawberry transplanting, and film installation must occur before time of transplanting (Valent U.S.A. Corporation, 2011). Manning and Fennimore (2001) found that overtop application of flumioxazin at 0.07 kg·ha−1 a.i. was safe on ‘Camarosa’ and ‘Selva’ strawberries when made immediately after transplanting. When applied to the bed top at 0.07 and 0.11 kg·ha−1 a.i. rates greater than 30 d before transplanting, flumioxazin effectively controlled little mallow (Malva parviflora L.) and clover (Medicago sp.) without causing any phytotoxicity on strawberry (Fennimore et al., 2005). However, when flumioxazin was applied 2 and 14 d pre-transplant at 0.21 kg·ha−1 a.i., slight plant injury and reduced yields were observed, particularly for the 2-d pre-transplant treatment (Fennimore and Ajwa, 2003).
For oxyfluorfen, a minimum 30-d period from soil treatment to planting strawberries is recommended at rates of up to 0.6 kg·ha−1 a.i. (Dow AgroSciences LLC, 2011). Oxyfluorfen at 0.3 to 0.6 kg·ha−1 a.i. reduced weeding times by 37% to 63% compared with the untreated control, and provided satisfactory control of broadleaf weeds including California burclover, hairy nightshade [Solanum villosum (L.) Mill.], little mallow, shepherd’s purse [Capsella bursa-pastoris (L.) Medik.], and clovers (Daugovish et al., 2008, 2009). Stall et al. (1995) found pre-emergence oxyfluorfen application at 0.3 and 0.6 kg·ha−1 did not affect strawberry vigor and provided season-long control of weeds including Carolina geranium (Geranium carolinianum L.) and cutleaf evening primrose (Oenotheria laciniata Hill.). Oxyfluorfen at 0.6 kg·ha−1 a.i. + napropamide at 4.5 kg·ha−1 a.i. applied 21 d before transplanting ‘Camarosa’ strawberry increased fruit weight by 20% compared with untreated control (Gilreath and Santos, 2005). Pre-plant oxyfluorfen application at 0.6 kg·ha−1 a.i. controlled weeds similar to MB–Pic standard soil fumigation with no crop injury on ‘Albion’ strawberry (Samtani et al., 2011).
Oxyfluorfen has the potential to injure plants through codistillation or “lift off,” a process in which the herbicide moves with water vapor from soil surface to strawberry foliage (Fennimore et al., 2008). Splashing of the herbicide from the soil to strawberry crown and foliage during an irrigation event can also potentially injure the crop (Fennimore et al., 2008). These injuries through splashing and codistillation can be reduced by film installation on the bed before strawberry transplanting (Daugovish et al., 2009).
The objective of this study was to determine if tolerance to flumioxazin and oxyfluorfen herbicide varied among strawberry cultivars. Many of the cultivars evaluated in the study (‘Albion’, ‘Camarosa’, ‘Palomar’, ‘San Andreas’, ‘Ventana’) are public cultivars bred by University of California and are commonly grown in the state on over 12,141 ha (Shaw and Larson, 2008; [California Strawberry Commission (CSC), 2011]. ‘Festival’ is a short-day cultivar released by Florida Foundation Seed Producers (North Carolina Crop Improvement Association, 2012). Private and other cultivars account for over 40% of the state’s acreage (CSC, 2011). The cultivars 273M171, 211G51, and 49C129 are propriety. The Plant Sciences 5298 and 4634 cultivars are also proprietary, bred by Plant Sciences, Inc. for California and other international locations (Plant Sciences Inc., 2012).
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