Evaluation of wetland plants for susceptibility to species of Phytophthora present in runoff water at ornamental plant nurseries

Competition between agricultural producers and urban communities for high quality water is increasing; therefore, growers need to adopt water conservation and reuse practices to ensure an adequate supply of irrigation water. One concern with recycled irrigation water is the potential to recirculate propagules of oomycete pathogens. Constructed wetlands are a biologically-based treatment option for nutrient and chemical remediation that potentially also could filter pathogen propagules. The goal of this study was to assess the susceptibility of plants routinely used in constructed wetlands to infection by species of Phytophthora commonly found at ornamental plant nurseries. In laboratory and greenhouse experiments, an effective and relatively consistent inoculum delivery method for aqueous systems was developed using zoospores of five species of Phytophthora (P. cinnamomi, P. citrophthora, P. cryptogea, P. nicotianae, and P. palmivora ) that were released from agar plugs placed in aqueous solution over 29- and 13-day test periods. In a separate greenhouse experiment, four wetland plant species (Canna flaccida, Juncus effusus, Sagittaria latifolia, and Typha latifolia) were evaluated for potential infection by the five species of Phytophthora. Zoospore presence and viability were monitored throughout each experiment by a baiting bioassay and filtration. Susceptibility of each plant species was determined by visual observation and isolation on medium selective for Phytophthora spp. Two of the four wetland plant species screened in this study were not susceptible to species of Phytophthora; however roots of Canna flaccida and Juncus effusus were infected by P. cinnamomi and P. cryptogea respectively. Zoospore production by all species of Phytophthora, with the exception of P. cinnamomi, was suppressed by Canna flaccida. During all plant trials, a reduction in zoospore activity for the five species of Phytophthora in the plant treatments was documented when compared to control containers with no plants. On samples collected in the field over an 18-month period, 14 species of plants growing in established constructed wetlands at a production nursery in Georgia were sampled and roots were assayed for species of Phytophthora. Wetland plant species exhibited only limited association with naturally-occurring species of Phytophthora; pathogens were found infesting roots of only 22 of the 350 plants sampled. Phytophthora spp. were not found infesting roots of Canna flaccida, Juncus effusus, and Lemna valdiviana. Further testing of the susceptibility of Canna flaccida, Hydrocotyle umbellata, Pontederia cordata, Sagittaria latifolia, and Typha latifolia are needed because my results suggest that these species do not support growth and reproduction of the five species of Phytophthora used in this study. By establishing constructed wetlands with plant species known to be not susceptible, propagules of Phytophthora will be less likely to establish themselves in roots and reproduce within the constructed wetland system. This should allow the safe re-use of runoff for irrigation at the nursery and may limit or prevent escape of Phytophthora spp. in runoff from nurseries into the environment.