Efficacy of Chlorine on Multiple Species of Phytophthora in Recycled Nursery Irrigation Water

Recycled irrigation water is a primary source of inoculum for Phytophthora spp. and is capable of spreading propagules throughout nursery production. Chlorination commonly is used by the industry to disinfest recycled irrigation water; however, chlorine has not been fully researched as a disinfestant for this purpose. In this study, zoospores of seven species and eight isolates of Phytophthora were exposed for 2 min to free available chlorine at 0.25, 0.5, 1.0, 2.0, and 4.0 mg/liter. Zoospores, mycelial fragments, and culture plugs of P. nicotianae also were exposed to chlorine concentrations ranging from 0.25 to 8.0 mg/liter for periods ranging from 15 s to 8 min. In addition, chlorinated water was assayed monthly in 2000 and 2001 at two commercial nurseries, and quarterly in the first year at four other nurseries in Virginia, for chlorine and survival of pythiaceous species using a selective medium. No zoospores of any species tested survived endpoint free chlorine at 2 mg/liter, while limited mycelial fragments of P. nicotianae survived at 8 mg/liter, and mycelial plugs treated at the same level of chlorine were able to produce few sporangia. Phytophthora spp. were recovered only from nursery irrigation water with levels of free chlorine at 0.77 mg/liter or lower. The results of this study are essential for improving current chlorination protocols.