Effects of Water Quality on Survival of Zoosporic Oomycetes and Implications

Zoosporic oomycetes including Phytophthora and Pythium attack hundreds of agriculturally and economically important plant species worldwide.  The destructiveness of these pathogens is partly attributed to their multiple dispersal strategies.  Some are soil-borne plant pathogens while others are air-borne if producing caducous sporangia.  They also can be water borne by motile zoospores.  Numerous species have been recovered from irrigation systems and natural waterways which are one of the most efficient means for the pathogen dispersal in nature.  However, survival of zoosporic oomycetes in water is poorly understood which hampered development of effective strategies to control disease spread.  We recently investigated effects of electrical conductivity (EC), dissolved oxygen (DO), pH and dissolved CO₂ on zoospore survival.  We found that all species tested thrived at a wide range of EC from 0.11 – 3.58 dS m^-1 although their survival reduced when EC was lower than 0.21 dS m^-1.  They also tolerated a wide range of DO but many species were sensitive to DO that is higher than 11 ppm or lower than 2 ppm.  Many species survived at a wide range of pH and preferred basic pH.  Some species including P. ramorum were tolerant of extreme pH 3 and/or 11.  Regular dissolved CO₂ in irrigation water little affected zoospore survival but high levels of dissolved CO₂ at 210 ppm or higher which may be achieved by bubbling significantly reduced zoospore survival while reducing DO and pH.   Phytophthora spread in irrigation system may be mitigated through water retention, extension of turnover time and pump house location.