The population of oomycetes in a recycled irrigation water system at a horticultural nursery in southern California.

Recapture and recycling of irrigation water is often required to meet enormous water demands at horticultural nurseries. We tested four water types associated with a recycled irrigation system at a commercial container nursery in southern California for presence of oomycete plant pathogens from July 2015 to December 2017. These water types included: the main source of water originating from a reservoir, retention water from an on-site collection pond, irrigation water received by different growing areas within the nursery, and irrigation runoff captured in polyethylene sheet-lined runoff channels. The genera Phytophthora, Pythium, and Phytopythium together contributed more than 85% of the total oomycete population detected in the recycled irrigation system. The Phytophthora and Pythium genera were represented by member species from nine (1-4, 6-10) and eight (A, B, D-F, H-J) different sub-generic clades, respectively. Incoming water sourced from the reservoir was found to harbor known plant pathogens such as Phytophthora citricola-complex, P. capsici-cluster, P. tropicalis,P citrophthora-cluster, P. nemorosa-cluster, P. riparia, P. cryptogea-complex, P. parsiana-cluster, P. sp. nov. aff. kernoviae, Pythium dissotocum-complex, Py. oligandrum-cluster, Py. irregulare, and Phytopythium litorale. Runoff water showed the highest oomycete species richness and frequency of detection with both filtration and leaf baiting methods. In addition to plant pathogens, oomycete fish pathogens such as Aphanomyces laevis, Pythium chondricola-complex, Pythium flevoense-complex, and Saprolegnia diclina-complex were also detected in greater abundance in the recycled irrigation water. The oomycete species richness in the runoff water was correlated with several environmental parameters such as soil temperature. Greater oomycete richness in incoming water was associated with higher soil temperatures, whereas richness in runoff declines with increasing soil temperature, likely suggesting connections to weather-dependent nursery operations.