California strawberries account for over 85% of the total U.S. production, and in 2009, the California strawberry industry was valued at $1.7 billion for the 16,107 harvestable hectares under production (Agricultural Marketing Resource Center, 2010; Economic Research Service–U.S. Department of Agriculture, 2010). The long strawberry growing period in California fields means that the crop must be protected from weed infestations for as long as 15 months. In addition to competing with strawberries, weeds can harbor pathogens and insects that could affect the crop. All but a few fumigant-tolerant weed seed species such as California burclover (Medicago polymorpha L.) and common mallow (Malva neglecta Wallr.) are controlled by MB and Pic mixtures. Pre-plant mixtures of MBPic have been widely used since the 1960s in California strawberries to control soilborne diseases, nematodes, and weeds (Wilhelm and Paulus, 1980). However, MB has been classified as a Class I stratospheric ozone-depleting chemical and was phased out starting in 2005 under the Montreal Protocol (U.S. Environmental Protection Agency, 1993). Critical use exemptions for MB in strawberry and some other uses are still permitted under the Montreal Protocol terms (Duniway, 2002).
Currently approximately two-thirds of the strawberry acreage use alternatives to MB, but finding a complete replacement for MB has proven difficult for the strawberry industry as a result of numerous reasons (U.S. Environmental Protection Agency, 2009). The lack of MB may result in revenue decline of 25% for California strawberry growers (Goodhue et al., 2005). Chemical alternatives to MB include 1,3-D, Pic, and metam sodium (Duniway, 2002; Medina et al., 2006). The fumigant 1,3-D alone controls nematodes and some soilborne insects but has limited activity against soilborne plant pathogens and weeds (Noling and Becker, 1994). Pic controls pathogens but is less effective in controlling weeds and nematodes than MB (Noling and Becker, 1994; Wilhelm, 1999). The weed control effectiveness of 1,3-D + Pic is better than that of Pic alone (Fennimore et al., 2003), yet strawberry yields from the alternatives, 1,3-D + Pic and Pic have been comparable to MB treatments (Medina et al., 2006). Total annual applications of 1,3-D are however restricted in an area of 93.2 km2 (defined as township) in California as a result of air quality concerns (California Department of Pesticide Regulation, 2009). Chloropicrin being volatile and toxic can also contribute to air pollution (Gan et al., 2000). Given these considerations, it is essential that fumigants be applied in a way that minimizes their impact on human health and the environment (Papiernik and Yates, 2002).
Under California's fumigant use regulations, the use of drip irrigation to apply fumigants and the use of specialized tarps such as VIF during fumigation reduce the required buffer zones (Ajwa et al., 2002). VIF differs from a HDPE tarp because VIF has additional gas-impermeable layers (such as ethylene vinyl alcohol, nylon, or polyaminides) between polyethylene layers (Wang et al., 1997). The use of VIF can minimize fumigant emissions, increase fumigant retention over time, and reduce the rate needed for effective pest control (Gamliel et al., 1998; Minuto et al., 1999; Nelson et al., 2001).
Higher fumigant concentrations of 1,3-D and Pic were measured under VIF compared with a low-density polyethylene (LDPE) tarp 1 to 4 d after drip fumigation (Desaeger and Csinos, 2005). Improved retention of fumigants in soil under VIF also provides more opportunity for soil degradation of fumigants rather than release into the atmosphere (Wang and Yates, 1998). Use of VIF as a tarp can reduce 1,3-D + Pic needed for effective soil disinfestations by 50% (De Cal et al., 2004; Medina et al., 2006). Santos et al. (2005, 2007) found that reducing MBPic rates by one-half under VIF or metalized mulches provided nutsedge control similar or superior to application made at 392 kg·ha−1 under LDPE or HDPE tarps. Desaeger et al. (2004) studied the movement of 1,3-D and Pic under an LDPE tarp in raised sandy beds and found that fumigant concentrations were higher in the bed center than at the edge of the bed.
Weeds that escape on the edges of the beds can be problematic. The challenge is to drive fumigants to the edge of the planting bed in a concentration sufficiently high to kill the weed propagules there. Our study addressed the question whether 1,3-D + Pic and pure Pic applied under VIF has the potential to improve weed control in bedded strawberry compared with similar treatments applied under an HDPE tarp. The objectives of this study were to determine 1) the minimum effective rates of the alternative fumigants, 1,3-D + Pic and Pic, under VIF and an HDPE tarp required to control weeds equivalent to 67/33% v/v MBPic standard soil fumigation at 392 kg·ha−1 under an HDPE tarp. A second objective was to determine fumigant rates under VIF and HDPE tarps needed to provide weed control and the economic costs of using VIF and reduced rates of the alternative fumigants.
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