Strawberry production in the United States has increased annually since 1994, but high national strawberry demand has not translated into large tracts of land being planted to strawberries proportionately throughout the Southeast. In 2012, there were 74 farm operations in Alabama engaged in strawberry production on a combined 158 acres. By 2017, the number of farm operations increased to 123, but combined acreage was reduced to 111 (U.S. Department of Agriculture, National Agricultural Statistics Service, 2012). Growers cite difficulties in making profits due in part to a relatively small arsenal of available weed control options (Boyd and Reed, 2016).
Few physical or chemical options have the capacity to sufficiently control economically important weed species in Alabama (Alabama Cooperative Extension, 2013), such as broadleaves like morning glory (Ipomoea sp.) or yellow or purple nutsedge (Cyperus esculentus and Cyperus rotundus L.). Polyethylene mulches reduce light transmittance and create a physical barrier to weed development (Ngouajio and Ernest, 2004; Yu and Boyd, 2017). Black is often the color of choice because of its opacity and low light transmission (≈1%) reducing the chances of weed seed germination and weed growth (Brault et al., 2002; Ngouajio and Ernest, 2004). Despite this, weeds may grow in spaces left uncovered by mulch, such as in planting holes (Adcock et al., 2008). Moreover, yellow or purple nutsedge is capable of puncturing and growing through plastic mulch. Hand weeding is laborious, expensive, time-consuming, and impractical because of irregularities in availability of migrant labor (Hu et al., 2017; Taylor et al., 2012). Methyl bromide is no longer available for the majority of agricultural enterprises, compounding the weed control issue and further necessitating the need for effective weed control solutions.
Expansion in the number of chemical weed control options would provide considerable assistance to strawberry growers. Figueroa et al. (2005) stated that only 39% of the strawberry acreage in the United States received herbicides, which was primarily accommodated through the applications of only two chemistries with different modes of action, napropamide and terbacil. Postemergent (POST) herbicides are a mainstay in chemical weed control in most systems, but few are registered in strawberry, and only one, clopyralid, controls broadleaf weeds (Sharpe et al., 2018). Further, POST herbicides applied in excess of 1 or 2 months after planting will have diminished effectiveness in controlling weeds, resulting in increased competition with weeds and reduction of crop yield by 20% to 60% (Figueroa et al., 2005; Pritts and Kelly, 2001). Timely application of POST herbicides will depend on favorable climatic conditions or absence of other environmental factors. Including preemergent (PRE) herbicides would be helpful in countering some of these constraints.
Similar to POST herbicides, the list of available PRE herbicides is brief, but there is potential for expansion through 24(c) special local needs registration. The Environmental Protection Agency defines special local needs as approval of states to register a product for use on an existing or imminent pest problem for which there is not a product sufficiently available (epa.gov/pesticide-registration/guidance-fifra-24c-registrations#). This classification was sought by and awarded to some states for permission to use clopyralid, but to date, this chemical is not registered for use in strawberries in many states (S. Li, personal communication).
Currently, in the southeastern United States, napropamide and, in some states, sulfentrazone are two PRE herbicides registered as 24(c) for use in strawberries. Granting of 24(c) status to PRE herbicides for extra-label usage in strawberry would make several more chemistries available to growers. Terbacil and pendimethalin are labeled for use in matted-row strawberries and in row middles, but the matted row system has been replaced by the more productive annual hill plasticulture system for the commercial production of strawberries in most regions (Butler et al., 2002; Samtani et al., 2019). Currently, sulfentrazone has a special local needs label for Ohio, Tennessee, Michigan, Washington, and Oregon (Spartan 4F, FMC, 2011), whereas S-metolachlor (Dual Magnum, Syngenta, 2018) has a special local needs label for Michigan and Oregon. S-metolachlor is desirable because of its relatively low price and ability to control yellow nutsedge (Daugovish et al., 2007). It will be useful to determine the effects of these herbicides on growth and yield parameters of strawberries in the hill plasticulture system in Alabama.
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