2018 ASHS Annual Conference

Slow sand filters (SSF) are an old technology used to produce drinking quality water, but there are few systems installed in the U.S. for horticultural purposes. Operational costs, other than pumping the water to the SSF, are low since chemical or energy inputs are not needed. Filters consist of a sand bed that serves as a substrate for a biofilm of microorganisms to develop. As water flows through the sand bed, these microorganisms degrade pathogens and other pollutants. We were interested in the capacity of these filters to remove different kinds of plant pathogens from captured irrigation run off and this report reviews our work on the capacity of these filters to remove Phytophthora ssp., Fusarium oxysporum, and Tobacco mosaic virus.

To determine if SSFs established against Phytophthora capsici were capable of removing other Phytophthora species, one set of SSFs was set up at University of California, Davis and exposed only to P. capsici. At the same time, another set of SSFs was set up in Santa Cruz County and provided water from Lompico Creek which was known to contain P. ramorum, the pathogen that causes Sudden Oak Death, in addition to other species of Phytophthora. After 30 days, all of the SSFs removed Phytophthora and the filters at UCD were moved to Santa Cruz Co. and provided creek water. Phytophthora was not recovered from water that flowed through the SSFs from UCD after relocation.

To see if SSFs can remove a newly introduced pathogen, a set of SSFs was exposed to P. capsici and a second set was exposed to F. oxysporum. After 7 weeks, the SSFs removed P. capsici, but the other set of SSFs did not remove F. oxysporum. Then, the SSFs exposed to P. capsici were now provided F. oxysporum and the SSFs previously provided F. oxysporum were supplied with P. capsici. P. capsici was immediately removed from the SSFs, but F. oxysporum was not removed from the other set of SSFs, which was expected. To simulate a pump failure, water was not provided to the filters for 7 days. After water flow was restarted, P. capsici removal resumed immediately, but there continued to be no removal of F. oxysporum. Another class of pathogens of concern are plant pathogenic viruses. In our tests, the SSFs were able to remove Tobacco mosaic virus from inoculated runoff water after 6 to 9 weeks of exposure.