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  • Author or Editor: Steven A. Fennimore x
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Steam injected into the soil, raising soil temperatures to >70 °C for 15 to 20 minutes, will control weed seed and soilborne pathogens. The effect of this reduction in the weed seedbank viability results in weed control in the treated zone that can persist for several weeks or months. The effect of steam pasteurization of soil on weed seeds produces results similar to a preemergence herbicide. In our study, steam was applied to the soil to control weed seed and propagules of Sclerotinia minor and Pythium spp. Replicated field trials in carrot, lettuce, and spinach were conducted using two types of band steam applicators in 2020 and 2021. Data collected were soil temperatures after steam application, weed control, hand weeding times, diseased plant counts, pathogen populations in the soil, and crop yields. Post-steam soil temperature intervals >70 °C in the top 10 cm of the soil ranged from 67 to 176 minutes. Steam reduced weed densities by 64% to 100% and lowered hand weeding times by 23% to 91%. The reduction of S. minor sclerotia propagules after steaming was 69% to 95% compared with the no steam control. The percentage of lettuce plants infected with lettuce drop was reduced by 60% to 70% and the reduction of Pythium spp. propagules in the soil was reduced by 50% to 100% compared with the no steam control, respectively. Lettuce head diameters in steamed soils were 10% to 24% larger compared with the no steam control. Carrots grown in the steam-treated soil had a 10% greater root diameter than the no steam control. Steam increased lettuce yields in two of three trials 22% to 28% compared with the no steam control. Gross revenues for the steam-treated lettuce were $3231/ha higher than in the no steam control. The data suggest that band steam is a viable soil pest control treatment for vegetable crops.

Open Access

Fumigants are used to control soilborne pests before planting high-value crops such as strawberry. The use of specialized tarps during fumigation can reduce fumigant emissions and mitigate the need for large buffer zone requirements mandated by regulators. Increased fumigant retention by use of barrier films during fumigant application may increase fumigant retention and allow use of lower fumigant rates to control soil pests than would be needed with permeable film. The objective of this study was to determine the minimum effective rates of the alternative fumigants, 1,3-dichloropropene (1,3-D) + chloropicrin (Pic), and Pic required under virtually impermeable film (VIF) and a high-density polyethylene (HDPE) tarp to provide weed control equivalent to methyl bromide:chloropicrin (67/33% v/v MBPic) standard soil fumigation at 392 kg·ha−1 under HDPE. 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. In 2002–2003 and 2003–2004 growing seasons, the fumigants 1,3-D + Pic and Pic were tested at 0, 56, 112, 224, 336, and 448 kg·ha−1 under HDPE and VIF tarps at Oxnard and Watsonville, CA. An untreated control and a MBPic standard at 392 kg·ha−1 were also included in the study. Weed control was assessed using weed propagule viability bioassays for four common weeds, time required for hand weeding, and weed fresh biomass. The fumigant rate that would be needed for a 90% reduction in viability (GR90) for all weeds was 21% to 84% less for 1,3-D + Pic under VIF compared with the HDPE tarp. For Pic, the GR90 values were 5% to 64% less under VIF compared with the HDPE tarp. Hand weeding times and weed biomass decreased with increasing fumigant rates. With the exception of Pic in 2002–2003 at Oxnard, VIF reduced the rate required for weed control compared with the HDPE tarp for both fumigants and at both locations. Economic benefits of VIF relative to the HDPE tarp were not consistent and additional work is needed to quantify these relationships and the production conditions under which VIF will be beneficial.

Free access

The phase-out of methyl bromide as a soil fumigant for strawberry (Fragaria ×ananassa, Duch.) and increasingly strict regulations of all fumigants suggest that non-fumigant methods of soil disinfestation are needed. In warm climates, solarization controls soilborne pests, but fog and lower summer soil temperatures in coastal California render it unsuitable for pest control relative to chemical fumigation. The first objective of this study was to test the efficacy of steam in controlling soil pests in strawberry production. The second objective was to determine if combining solarization with steam in coastal California would achieve greater pest control and higher yields compared with steam or solarization used alone. The final objective was to determine the economic feasibility of steam and solarization treatments relative to MBPic fumigation. Field studies were conducted at Salinas, CA, in 2007–2008 and in 2008–2009 growing seasons. Treatments included MBPic 67/33% v/v at 392 kg·ha−1, untreated control, solarization, steam, and steam + solarization. For steam + solarization plots, beds were solarized for 2 weeks before and 2 weeks after steam application. Before application of a clear film for solarization, beds were irrigated so the soil moisture was optimal for solarization. Steam was injected into the beds to reach soil temperatures to 70°C or higher up to a depth of 25 cm for 20 min. Soil temperatures during steam and solarization treatments were monitored. Control of soil pests was measured using pathogen and weed propagule bioassays in all treatments. After the 4-week treatment period, ‘Albion’ strawberry was transplanted in all plots. After transplanting, weed density, weed fresh biomass, and hand weeding time were recorded periodically in each treatment over the cropping season. Weed seed viability in steam and steam + solarization-treated plots was the same or lower than MBPic standard fumigation. Compared with MBPic fumigation, solarization alone was less effective in controlling weeds or reducing the hand-weeding time. Steam and steam + solarization treatments resulted in weed control similar to MBPic fumigation. Only certain steam treatments reduced the number of Verticillium dahliae Kleb. microsclerotia similar to the MBPic fumigation at 15-cm depth with no reductions at greater depths. There were no significant differences among treatments in 2007–2008 with regard to yield, but in 2008–2009, yields from steam treatments were comparable to the MBPic-treated plots. Economic analysis performed for the 2008–2009 season showed that net returns from steam or solarization treatments were less than MBPic treatment.

Free access

Steam-disinfestation of soil as an alternative to chemical fumigation was investigated in both research and commercial strawberry (Fragaria ×ananassa Duch.) production field trials at four sites over 2 years (2011–13) using new prototype commercial application equipment: a tractor-drawn device that physically mixed the steam with the soil as it passed through the shaped planting beds. Results included significant suppression of weeds and soilborne pathogens equal to commercial chemigation of chloropicrin with 1,3-dichloropropene (Pic-Clor 60). Also, the combination of steam treatment with soil amendments of mustard seed meal (MSM; two of four trials included treatment), a fertilizer and source of additional organic matter, showed very favorable strawberry production in terms of yield as well as weed and pathogen control. Soil nitrogen-containing ions were monitored at two of the sites and the MSM treatment significantly elevated available soil nitrates by the time of transplanting as did the steam treatment alone, but only significantly at one of the sites.

Free access

For years, strawberry (Fragaria ×ananassa L.) runner plant nurseries have relied on methyl bromide (MB) fumigation of soil to produce healthy transplants. Methyl bromide, however, has been phased out due to its environmental risks. The potential for alternative fumigants to replace MB was evaluated at low and high elevation strawberry nurseries in California. The alternative fumigant iodomethane plus chloropicrin (IMPic) and a nonfumigated control (NF) were compared to methyl bromide plus chloropicrin (MBPic) at a low elevation nursery (LEN) and at a high elevation nursery (HEN) near Susanville, Calif. At a HEN near Macdoel, Calif., MBPic was compared to alternative fumigants IMPic, 1,3-dichloropropene plus chloropicrin mixture (Telone C35) followed by dazomet, chloropicrin (Pic) followed by dazomet and NF. Plants produced at the LEN were transplanted at the Macdoel HEN to measure the effects of soil fumigant history on plant health and runner plant production. Plants produced at both high elevation nurseries were evaluated for fruit yield and quality at two commercial fruit production sites in soils previously fumigated with MBPic or Pic. Runner plant production at the nurseries was similar in plots fumigated with either MBPic or alternative fumigants. All fumigation treatments had higher runner plant production than plants produced for two production cycles on NF soils. Generally, fruit yields from nursery plants produced on soils fumigated with IMPic, Pic followed by dazomet, or Telone C35 followed by dazomet, were similar to fruit yields from plants produced on MBPic fumigated soils. Overall, our results indicate that preplant soil treatments with IMPic, Pic followed by dazomet, and Telone C35 followed by dazomet, are potential alternatives to MBPic fumigation for strawberry runner plant nurseries. Fruit yields by plants in MBPic and Pic fumigated soils were comparable; however, they were more variable in Pic fumigated soils. Chemical names used: 1,3-dichloropropene (1,3-D), methyl bromide, methyl iodide (iodomethane), trichloronitromethane (chloropicrin), tetrahydro-3, 5-dimethyl-2 H-1,3,5-thiadiazine-2-thione (dazomet).

Free access

The phase out of methyl bromide (MB) requires effective alternatives for soil disinfestation, particularly in high-elevation strawberry (Fragaria × ananassa Duch.) nurseries. Methyl bromide alternative fumigants were evaluated over a 3-year period for weed control and runner plant yields at strawberry nurseries in Spain. Two types of field trials were carried out: replicated experiments and commercial-scale field demonstrations. In the replicated experiments, eight fumigant treatments were evaluated each year, including the nonfumigated control and commercial standard methyl bromide plus chloropicrin mixture (MB : Pic) (50 : 50 w/w). Among the treatments evaluated were dazomet, chloropicrin (Pic) alone, metam sodium plus chloropicrin (MS + Pic), 1,3-dichloropropene:chloropicrin (1,3-D : Pic) (61 : 35 w/w), DMDS plus chloropicrin (DMDS + Pic), and propylene oxide. The best alternative fumigant treatments from the replicated experiments were carried forward to the demonstration phase of the project. Treatments such as 1,3-D : Pic (300 kg·ha−1), the combination of metam sodium plus chloropicrin (Pic) (400 to 500 + 150 to 250 kg·ha−1), Pic alone (300 kg·ha−1) as well as dazomet (400 kg·ha−1) controlled weeds at the level of MB : Pic (400 kg·ha−1). Runner plant yields, in soils previously fumigated with alternative fumigants varied, among years, locations, and trial scale, i.e., commercial scale, or small plot. By comparison, runner plant yields in MB : Pic-fumigated soils were consistently high among years, location, and trial scale. Chemical names used are: 1,3-D, 1,3-dichloropropene; MB, methyl bromide; Pic, trichloronitromethane; MS, sodium N-methyldithiocarbamate; DMDS, dimethyl disulphide; dazomet, tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione; PO, propylene oxide

Free access