Characterization of Phytophthora Species in Recycled Irrigation Water at a Container Nursery in Southwestern Virginia

The potential of increasing disease problems through the use of recycled irrigation water in horticultural operations is a serious concern, yet basic research on waterborne plant pathogens in Virginia is lacking. In this work seasonal fluctuations and locations of Pythiaceae in a recycled water irrigation system at a container nursery were determined. Pythium spp. were recovered more frequently and in greater numbers than Phytophthora spp. Species of Phytophthora recovered in filtering assays were identified as P. capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, and P. nicotianae. P. cryptogea and P. drechsleri were the only Phytophthora spp. recovered from baits placed on the surface of the irrigation reservoir, whereas a greater diversity of species was recovered from baits placed at depths. Hymexazol-amended medium was found to have limitations in recovery of Phytophthora spp. In pathogenicity tests, P. cactorum, P. capsici, P. citrophthora, and P. nicotianae caused significant mortality of Salvia officinalis and P. cactorum showed limited pathogenicity on Gerbera jamesonii. Asymptomatic (aboveground) plants were found to harbor inoculum long after Phytophthora-inoculation. Fresh weight analyses of roots and shoots of asymptomatic plants demonstrated that Phytophthora inoculation may either reduce or stimulate plant shoot growth, but little effect is apparent on roots. Irrigation with naturally infested irrigation water reduced plant growth. This research provides data for prioritizing development of detection technology and management practices for plant pathogens in irrigation water. The results may also lead to improvements in conventional water assay protocols for plant pathogens.