Phytophthora species associated with plants in constructed wetlands and vegetated channels at a commercial plant nursery over time.
Constructed wetlands have been used for decades in agricultural settings to remediate nutrients and other agrichemicals from irrigation runoff and drainage; however, little is known about the presence and distribution of Phytophthora species within irrigation runoff water being treated in constructed wetlands. Therefore, we collected plant samples from within vegetated runoff collection channels and treatment stages of two constructed wetland systems receiving irrigation runoff at a commercial plant nursery in Cairo, GA, to determine if roots of wetland plants were infested by species of Phytophthora. Samples were collected 12 times, at 1- to 2-month intervals, over a 19-month period, from Mar. 2011 through Sept. 2012. The sample period covered all four seasons of the year, so we could determine if the association of Phytophthora species with roots of specific plant species varied with season. Approximately 340 samples from 14 wetland plant species were collected, and 22 isolates of Phytophthora species were recovered. Phytophthora species were typically isolated from plants in channels receiving runoff water directly from plant production areas; Phytophthora species were not detected on plants where water leaves the nursery. No seasonal patterns were observed in plant infestation or presence of species of Phytophthora. In fact, Phytophthora species were rarely found to be associated with the roots of the wetland plants collected; species of Phytophthora were found infesting roots of only 6.5% of the 336 plants sampled. Species of Phytophthora were not found to be associated with the roots of golden canna (Canna flaccida), lamp rush (Juncus effusus var. solutus), duckweed (Lemna valdiviana), or sedges (Carex sp.) during the study period. The exotic invasive plant species marsh dayflower [Murdannia keisak (33% of samples infested)] and alligatorweed [Alternanthera philoxeroides (15% of samples infested)] were found to have the first and third highest, respectively, incidences of infestation, with smooth beggartick (Bidens laevis) having the second highest incidence of samples infested (22%). Management of invasive species in drainage canals and constructed wetland systems may be critical because of their potential propensity toward infestation by Phytophthora species. Plant species recommended for further investigation for use in constructed wetlands to remediate irrigation runoff include golden canna, marsh pennywort (Hydrocotyle umbellata), pickerelweed (Pontederia cordata), and broadleaf cattail (Typha latifolia). The results from this study provide an important first look at the associations between species of Phytophthora and wetland plants in constructed wetland systems treating irrigation runoff and will serve to further optimize the design of constructed wetlands and other vegetation-based treatment technologies for the removal of plant pathogens from irrigation runoff.