Pathogen Filtration in Recycled Irrigation Water to Control Plant Disease Outbreaks in Greenhouse.

Water mold pathogens including Phytophthora spp. and Pythium spp. are infective to a wide range of vegetable and floriculture crops, and they are primarily harbored in soils and disseminated through water flow. These pathogens are challenging to control because they quickly develop resistance to fungicides. Moreover, irrigation water recycling in greenhouses is becoming increasingly popular due to environmental and economic benefits, but this practice can exacerbate the spread of water mold pathogens. This study investigated physical removal of water mold pathogens in recirculating irrigation water using small-scale ebb and flow irrigation and filtration systems packed with varying filter media (i.e. granular sand, iron oxide coated ceramic porous media, and activated carbon) in mitigating disease outbreaks of Phytophthora and Pythium for greenhouse-grown squash and poinsettia, respectively. An additional fungicide treatment was also included for comparison. For squash, fresh foliar weight under iron oxide coated media and sand treatments was not different from that of non-inoculated control in a two-week trial (P < 0.05). In the three-week trial, however, fresh foliar weight of squash under filtration or fungicide application were significantly lower than that of non-inoculated control, and the iron oxide coated medium performed significantly better than the sand and fungicide application. For poinsettia, the sand filter, fungicide, and non-inoculated control treatments all had significantly greater (P < 0.05) fresh foliar weight and plant height than the inoculated control and activated carbon filter treatments (P < 0.01). Non-inoculated control, activated carbon, and sand filter treatments showed 0% infection rate by Pythium, but inoculated control and fungicide treatments showed 93 and 50 % infection rate, respectively. The sand filtration performed better in controlling the Pythium infection than fungicide application, and nutrient limitation in crops was observed under filtration by activated carbon. Overall, our results suggests that filtration of irrigation water can be effective in reducing crop disease outbreaks, while decreasing the use of fungicides and thus promoting the crop and environmental health.