Non-Fumigant Alternatives for Managing Macrophomina phaseolina and Fusarium oxysporum In California Strawberry

Fungal pathogens Macrophomina phaseolina and Fusarium oxysporum have become increasingly troublesome in California strawberry production valued annually at $ 1.8 billion. These pathogens can be controlled with pre-plant soil fumigation; however, they proliferate in the refuge created by the non-fumigated buffer zones, causing plant decline and yield losses and can spread from infested zones to non-infested areas. In a buffer zone with the history of both pathogens in a field at Ventura, CA we have evaluated three non-fumigant combinations: mustard seed meal at 2200 kg/ha incorporated into beds and followed by solarization via standard clear polyethylene mulch, mustard meal supplemented by steaming for 10 min at 60 C to 25cm depth and steaming followed by solarization for 2 months. Average day/night soil temperature at 15 cm depth under clear mulch was 33 C in August and 27 C in September, 2011. In November, 1.5-months old ‘San Andreas’ strawberry plants were similar in size among all the tested combinations and 26% larger than untreated. Number of germinated weeds under clear mulch was 3-4 times greater for combination with mustard than with steam. Only combinations that included steam reduced Macrophomina phaseolina sclerotia colonies in soil compared to untreated check: 43% at 0-15 cm depth and 88% at 16-30 cm. However, early fruit yields (January-February) were similar for all combinations and 73% greater than in untreated check. Whole-season yields, Fusarium oxysporum survival in soil and pathogen-related plant mortality are currently under investigation. Preliminary results suggest that these non-fumigant treatments may not eliminate pathogens from strawberry rooting depths but may provide substantial yield improvements in non-fumigated areas.