Since around 1960, methyl bromide (MB) has been the foundation for soilborne disease and weed control in California strawberries. MB, classified as a Class I stratospheric ozone-depleting chemical, has been phased out since 2005, but is still being used in strawberry production under a critical-use exemption.
Strawberry production is important to California's economy; the state leads the U.S. in strawberry production. In 2006, the fruit harvest yielded a value of $1.2 billion, and accounted for 79% of the total U.S. gross sales.
The diversity of climates in California along with the use of this fumigant, permits the production of high-quality runner plants, which are propagated in virus-free growing facilities called "screenhouses". Plants are reared first in a low-elevation facility during the warm early season, and then moved to higher-elevation facilities for cool, late-summer conditions. This process ensures that strawberry plants will be ready to transplant into fruit fields by early fall.
A research study published in the American Society for Horticultural Science journal HortScience evaluated the effectiveness and cost efficiency for weed control in lower- and higher-elevation nurseries with MB-alternative fumigants.
Researchers Steven A. Fennimore, Milton J. Haar, Rachael E. Goodhue, and Christopher Q.Winterbottom noted that weed control in strawberry nurseries is more difficult than in fruiting fields. "Because weed control methods such as mulches used in fruit fields can't be used in nurseries, fumigants are one of the most important weed control tools available for strawberry nursery fields", the team explained.
Assessing weed control in the study consisted of three methods: weed seed viability, weed density counts, and timing of hand-weeding inputs by crews. Strawberry and weed seed samples were treated with the alternative fumigants to determine the potential for strawberry and weed seed to survive in the nursery fields. Fumigant effectiveness was studied in fields treated with the combination of MB plus Pic, iodomethane (IM) plus Pic, and control fields. Additional treatments tested were 1,3-dicloropropane (1,3-D) plus Pic followed by dazomet, and Pic followed by dazomet. Overall, there were few to no differences in weed control between IMPic, 1,3-D Pic followed by dazomet, Pic followed by dazomet, and MBPic. Relative to the control fields, hand-weeding times were reduced in all fields that had been fumigated.
The main difference between fumigating with the different methods was material cost. The researchers explained that, because current prices were used to calculate hand weeding and treatment costs, these prices will change over time and may become more equitable given different circumstances.
The study results showed that that fumigating with MB is currently much cheaper than using IM. On the basis of weed control, all of the alternative fumigant treatments were acceptable replacements for MB.
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