Organic strawberry production has been expanding in California, but opportunities for crop rotation are limited due to lack of organically certified fields. Continuous strawberry production promotes soil-borne pathogens, such as Macrophomina phaseolina, the causal agent of charcoal rot and increases populations of weeds. These problems are exacerbated by lack of cost-effective management tools. In summer and fall production seasons at Oxnard, CA we evaluated pre-plant steam injection to raise soil temperature to 70 C or above for at least two minutes. Steam generated on-site was applied via four 25-cm long spikes to planting holes in raised beds covered with the plastic mulch. Five to ten days later, bare-root strawberry plants were placed in six steamed and six untreated plots and their performance and fruit production assessed. Additionally, we collected soil at 0-25 cm for analyses of resident Macrophomina phaseolina abundance before and after treatment and evaluated weed densities in planting holes. Steam application reduced M. phaseolina microsclerotia levels 80% in summer and 96% in fall. Steaming provided near 100% control of weeds germinated from the soil seed bank, but had no effect on germination of wind-dispersed weeds deposited to holes after steaming as was the case with fumigants. In summer season, strawberry mortality due to soil-borne pathogens was 12-18% in steamed plots and 70-75% in untreated soil. Due to loss of plants, marketable fruit yields in untreated plots were reduced 95% compared to steam treatment. No early-season plant mortality occurred in fall season but strawberry plants were 39 and 54% larger at two evaluation dates in steamed plots compared to untreated soil. We continue evaluations of fruit production and plant responses to soil pathogens. The completed work suggests that hole steaming may be very effective in suppressing soil-borne pathogens and weeds interfering with organic strawberry production in coastal California.
