California Agriculture

Methyl bromide is highly effective in reducing soil pathogens. Although its use was to be phased out completely in the United States by Jan. 1, 2005, due to environmental concerns, a 1-year critical-use exemption will allow tree fruit growers to use the fumigant through the end of the year. To explore possible replacements for methyl bromide, we compared the effects of pre- and postplant treatments and amendments on soil microbial communities and tree vigor in a replanted peach orchard. Both soil sterilization treatments and organic carbon amendments changed the composition of microbial communities in the soil. High microbial biomass is generally considered beneficial to agricultural soils; we found that it was usually highest in soils that received the organic carbon amendment and lowest in those with soil sterilization. However, tree vigor was highest with the sterilization treatments. The effects of a microbial inoculants/organic carbon combination on microbial communities and plant vigor were no different from simply adding organic carbon.