P-666 - Oomycete Risk in Recirculated Irrigation Water: A Case Study from San Diego Nurseries

Reduced water availability in California has led ornamental nurseries to rely on captured irrigation runoff as a water source. However, this practice increases the risk of recirculating waterborne pathogens, particularly Phytophthora spp. and Pythium spp., into each crop cycle. In 2024, we launched a monitoring project at two bedding plant nurseries in San Diego County to identify oomycete species present in irrigation runoff ponds and associated with symptomatic plants. Ponds were baited with pears or rhododendron leaves and recovered isolates were cultured on selective media and identified in the lab. Concurrently, plants exhibiting Phytophthora and Pythium root rot symptoms were surveyed at the nurseries for further pathogen characterization. The project has two main goals: (1) to identify the most prevalent oomycete species associated with disease in ornamental crops; and (2) to assess the potential reintroduction of these pathogens into crops through recycled irrigation water. If such a connection is confirmed, it may justify the need for the adoption of costly water treatment technologies, such as chlorine or ozone injections, that are currently underutilized due to high implementation and maintenance costs and limited pathogen-specific data. Many growers use untreated recycled water and apply fungicides broadly to crops. This project aims to provide evidence-based recommendations for managing oomycete pathogens through improved water quality and irrigation practices. Preliminary results from the first year of monitoring are presented in this work.