Cut-flower production systems are often complex, with growers continually planting small areas with a range of species and cultivars to ensure year-round availability of the highly perishable crop while also targeting key market windows. The intensity, diversity, and high capital costs inherent in this cropping system have led to a reliance on MB fumigation for preplant control of a broad range of soilborne pathogens, weeds, and nematodes. MB was classified as a Class I stratospheric ozone-depleting substance and became subject to the provisions of the Montreal Protocol in 1993. This international treaty called for elimination of MB use in developed countries by 2005 and in developing counties by 2015 (Ristaino and Thomas, 1997); however, critical use exemptions for MB are considered when alternatives are ineffective or not economically feasible (Duniway, 2002; Martin, 2003). In California cut flowers, currently registered chemical alternatives to MB include 1,3-dichloropropene, chloropicrin (Pic), dazomet, metam potassium, and metam sodium. Because of various efficacy and regulatory concerns, it is unlikely that any of these products alone can replace all MB uses (Gerik and Hanson, 2011). Importantly, few of the currently registered MB alternatives are labeled in California for use in protected greenhouse structures, a production system that accounts for ≈11% of the cut-flower acreage in the state [U.S. Department of Agriculture (USDA), 2009].
Soil solarization was proven to effectively control plant pathogens and weeds in vegetable and cut-flower production in Turkey and Portugal during the hot summer months when greenhouses were not in production (Ozturk et al., 2002; Reis, 2002). However, fog and cooler soil temperatures make solarization a less optimal tool for pest control in coastal California, where most cut flowers are produced (Elmore et al., 2007). In addition, Stapleton and DeVay (1986) suggest that even under favorable conditions, 4 to 8 weeks is ideal for effective solarization treatment, a substantial reduction in production time in a high-value crop such as cut flowers.
Steam has been used as a soil and substrate disinfestant since the late 1800s and has been suggested as an effective MB alternative (Newhall, 1955; Pizano, 2006). In the Netherlands, ≈50% of the cut-flower acreage is steam treated for soil disinfestation, and it is also used in Australia, Colombia, Brazil, and Italy (Pizano, 2006). High temperatures can control a wide range of pests, although selectivity and efficacy depend on the temperature and exposure duration (Bollen, 1969; Pullman et al., 1981; van Loenen et al., 2003).
When steam is used in California cut-flower production, typically it is applied with the sheet steaming technique described by Runia (1984) where steam is forced under a heat-resistant film and provides pest control as the heat moves down through the soil. This has limited the use of steam disinfestation in open fields because of high energy and labor costs (Fennimore et al., 2008). Efficiency can be improved by applying steam below the soil surface such as through a permanent drain-tile system or by actively pulling steam through the soil (Lu et al., 2010; Newhall, 1955; Runia, 2000); however, these techniques have not been evaluated in coastal California.
The combined effects of solarization and, more efficient, subsurface steam application may increase the viability of steam as a nonfumigant MB alternative for preplant soil disinfestation. The objective of this research was to evaluate the efficacy of steam treatments for the control of weeds and soil borne plant pathogens using several steam delivery methods and solarization treatments as MB alternatives in California cut-flower production.
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