In the United States, 21,576 ha of strawberries were harvested for fruit in 2006 at a value of $1.5 billion. California leads the nation in strawberry production, where strawberry growers harvested 14,494 ha for fruit in 2006 with a value of $1.2 billion, accounting for 79% of the total U.S. gross sales (National Agricultural Statistics Service, 2007). California nurseries produce more than a billion strawberry runner plants annually with a total value of ≈$60 million (California Strawberry Commission, 1999). The diversity of climates in California along with the use of methyl bromide (MB) fumigation permits the production of high-quality runner plants. Strawberry runner plant production begins in virus-free rearing facilities, i.e., screenhouses. Plants are then vegetatively propagated in the field for two or three seasons. One or two 8-month-long propagation seasons at a low-elevation (less than 150 m) nursery (LEN) are followed by a 5-month-long propagation at a high-elevation (greater than 1000 m) nursery (HEN). Favorable warm climatic conditions at LEN allow rapid plant propagation, and cool late-summer HEN conditions ensure strawberry plants that are ready to transplant into fruit fields by early fall (Kabir et al., 2005; Larson, 1994; Larson and Shaw, 2000; Strand, 1994; Voth, 1989; Voth and Bringhurst, 1990).
Soil fumigation is used to manage soilborne pests, including weeds, and to ensure pest-free planting stock. Beginning approximately 1960, MB became the foundation of soilborne disease and weed control in California strawberries (Wilhelm, 1966). Methyl bromide in combination with chloropicrin (Pic) (Tri-Cal, Hollister, CA) controls weeds, soilborne pathogens, and nematodes (Wilhelm and Paulus, 1980). Virtually all runner plant nurseries fumigate soil with a mixture of MB and Pic to ensure pest-free planting stock. However, MB has been classified as a Class I stratospheric ozone-depleting chemical, and use of MB was phased out 1 Jan. 2005 under the Montreal Protocol (Anbar et al., 1996; U.S. Environmental Protection Agency, 1993) with exemptions for critical use [U.S. Department of State (USDS), 2007]. Approximately 4690 kg of MB was requested for preplant fumigation of California strawberry nurseries in 2009 under the critical use exemption (USDS, 2007).
This is the second article that describes research with MB alternative fumigants in California strawberry nurseries. The first paper, by Kabir et al. (2005), describes strawberry nursery runner plant yield results and this article describes weed control results in strawberry nurseries.
Some fumigant alternatives to MB currently under investigation are Pic (trichloronitromethane; Niklor Chemical Co., Long Beach, CA), iodomethane (IM, methyl iodide, Midas; Arysta LifeScience, Cary, NC), 1,3-dichloropropene (1,3-D, Telone; Dow AgroSciences, Redeck, NC), and dazomet (tetrahydro-3, 5-dimethyl-2H-1, 3, 5-thiadiazine-2-thione; Certis USA, Columbia, MD). Chloropicrin has been applied in combination with MB since the late 1950s for control of weeds and pests (Wilhelm, 1966). Application of Pic alone controls some weeds (Haar et al., 2003; Wilhelm and Paulus, 1980). However, because of the predominant use of Pic in combination with MB, little information is available regarding the effectiveness of Pic without MB for weed control in strawberry runner plant fields.
Iodomethane has an ozone depletion potential value of 0.016 (Ohr et al., 1996) compared with an ozone depletion potential of 0.4 for MB. Light rapidly degrades IM, and the stratospheric ozone-depleting potential of this fumigant is very low (National Oceanic and Atmospheric Administration, 2002; Solomon et al., 1994). Iodomethane alone is 1.5 times more effective than MB in controlling weeds such as purple nutsedge (Cyperus rotundus L.; Zhang et al., 1997) and is as effective as MB for control of common chickweed (Stellaria media L.), common knotweed (Polygonum arenastrum L.), and common purslane (Portulaca oleracea L.) seed in strawberry (Fennimore et al., 2002). In Florida, researchers found that control of nutsedge (Cyperus spp.) and winter annual weeds with 100% IM at 336 kg·ha−1 was comparable to MBPic 98:2 (98% MB and 2% Pic) at 560 kg·ha−1; however, control of Ipomea spp. weeds with IM was less than MBPic (Unruh et al., 2002). Iodomethane can be used in combination with other fumigants such as Pic to increase its spectrum of activity against soil pests such as yellow nutsedge (C. esculentus L.; Hutchinson et al., 2003).
The fumigant 1,3-D controls nematodes but not weeds (Noling and Becker, 1994; Roby and Melichar, 1997). To improve control of soilborne diseases and weeds, 1,3-D is often combined with 17% or 35% Pic, Telone C17, or Telone C35, respectively [Crop Data Management System (CDMS), 2007]. Telone C35 (TC35) at 355 L·ha−1 was as effective as fumigation with MBPic 67:33 at 364 kg·ha−1 for strawberry production in California (Duniway et al., 1998). Weed control with TC35 at 374 L·ha−1 controlled California burclover (Medicago polymorpha L.) and common chickweed comparable to MBPic 67:33 at 425 kg·ha−1 (Fennimore et al., 2003), and Unruh et al. (2002) found that 1,3-D at 140 L·ha−1 plus metam sodium at 748 L·ha−1 controlled carpetweed (Mollugo verticillata L.), winter annuals, and redroot pigweed (Amaranthus retroflexus L.) as well as MBPic 98:2 at 560 kg·ha−1. Nutsedge control with Telone C17 at 327 L·ha−1 was not acceptable compared with MBPic 98:2 at 450 kg·ha−1 (Gilreath and Santos, 2004; Locascio et al., 1997). Dazomet (DZ) is a microgranular fumigant that controls soilborne fungi, bacteria, nematodes, and weeds (Fritsch and Huber, 1995; Harris, 1991; Mappes, 1995). When incorporated into moist soil, DZ breaks down into methylisothiocyanate, which kills many soilborne organisms and weeds. Dazomet is registered for use in strawberry nursery and fruiting fields (CDMS, 2006). Csinos et al. (1997) found that control of purple cudweed (Gnaphalium purpureum L. var. purpureum) with DZ at 387 kg·ha−1 alone was comparable to MBPic 98:2 at 489 kg·ha−1. Unruh et al. (2002) found that Pic at 168 kg·ha−1 plus DZ at 392 kg·ha−1 controlled carpetweed and winter annual weeds comparable to MBPic 98:2 at 560 kg·ha−1, but DZ at 440 kg·ha−1 did not control nutsedge either alone or in combination with 1,3-D at 140 L·ha−1 or Pic at 168 kg·ha−1. Similarly, DZ at 440 kg·ha−1 resulted in poor control of nutsedge compared with MBPic 98:2 at 450 kg·ha−1 (Locascio et al., 1997).
Weed control in strawberry nurseries is more difficult than in fruiting fields. Differences between fruiting and nursery strawberries are: 1) nurseries require rapid growth of the mother plants and rooting of daughter runner plants and preclude use of herbicides such as napropamide that interfere with rooting of daughter plants (Weed Science Society of America, 2002); 2) dark plastic mulches used in fruiting fields for a variety of reasons, including weed control, cannot be used in nursery fields because they would prevent daughter plant rooting; 3) in nurseries, the row middles fill in with daughter plants limiting cultivation to the early season, whereas in fruiting fields, row middles are kept open and can be cultivated all season; 4) tolerance to risk of disease infection from weeds is near zero in nurseries as a result of potential for disease to pass from weed to strawberry runner plant and then carryover to infect fruiting fields. Fumigants are one of the most important weed control tools available for strawberry nursery fields, more critical than in fruiting fields where other weed control tools are available.
Soil fumigation is the cornerstone of the current strawberry production system; finding alternatives that effectively control a wide range of soilborne pests, including weeds, is necessary to maintain current levels of productivity and profitability. Because soil fumigation is used in several phases of the strawberry production system, MB alternative fumigants were evaluated at each production cycle. The objectives of the research described here were to evaluate the efficacy of alternative fumigants for weed control in LEN and HEN strawberry nurseries and to measure weed control costs when alternative fumigants are used.
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