Overview and Challenges
In the Western United States (CA, ID, OR, MT, and WA), preplant soil fumigation by methyl bromide formulations and their alternatives were used widely to prevent diseases caused by soilborne pathogens and to reduce competition from weeds. However, soil fumigation in the forest nursery industry faces important and continuing challenges:
- The phaseout of methyl bromide (MB).
- Limited numbers of alternative fumigants. Registered fumigant alternatives to MB listed by the EPA include dazomet (Basamid), metam sodium (Vapam, Busan), chloropicrin (CP), dimethyl disulfide (DMDS) and 1,3-dichloropropene (1,3-D). Formulations of 1,3-D with CP are also available, such as Telone C35 (1,3-D:CP 63:35) and Pic-Clor 60 (1,3-D:CP 39:59). The alternative fumigants CP, DMDS, and 1,3-D are applied by shank. Metam sodium and dazomet require activation with water and soil temperatures above 50°F to achieve efficacy.
- Long-term effects of alternative fumigant chemistries on disease control and soil microbial populations remain largely unknown. For example, the efficacy of metam sodium can decrease with repeated use, apparently as a function of accelerated metam sodium degradation by soil microbes. (Triky-Dotan et al. 2010).
- Lack of alternative fumigants available for spring fumigation. The cool soil temperatures and moist conditions that predominate during spring in forest nurseries west of the Cascade Range can cause fumigants to be retained in the soil for longer periods of time, thereby increasing the risk of phytotoxicity.
- Odor issues. Some fumigants, particularly DMDS and metham sodium have an offensive odor that might prove unacceptable to surrounding neighbors at some nursery facilities.
Finally, the US EPA is reviewing the risks of all soil fumigants in the re-registration eligibility decision (RED) process, and this will likely result in updated labels with large buffer zone increases for CP and MB.