Occurrence of Phytophthora Species in Recirculated Nursery Irrigation Effluents. J. D. MacDONALD, Professor, Department of Plant Pathology, University of California, Davis 95616. M. S. ALI-SHTAYEH, Associate Professor, Department of Biological Sciences, An-Najah National University, Nablus, West Bank, via Israel; J. KABASHIMA, Farm Advisor, University of California Cooperative Extension, South Coast Field Station, 7601 Irvine Blvd., Irvine 92718; and J. STITES, Staff Research Associate, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 78:607-611. Accepted lor publication 1 March 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0607.

Water samples were collected from effluent holding ponds at one northern and two southern California nurseries that practice the capture and recirculation of irrigation runoff water. Nursery effluent samples were collected approximately monthly over a 12-mo period and aliquots filtered through 0.45-µm Millipore filters. Filter residues were resuspended and dispersed onto selective agar media in petri dishes to estimate the numbers of viable propagules of Phytophthora spp. or total pythiacious fungi. Propagule numbers varied greatly from month to month at each nursery location. Pythium propagules were consistently the most numerous, ranging from 500 lo 1,500 per liter, whereas the number of Phytophthora spp. propagules ranged from 0 to 400 per liter. At the northern California nursery, propagule numbers were lowest during winter months and highest during warm seasons. Seasonal fluctuations in inoculum load were not apparent in the southern California nurseries. P. ciirophthora was the most commonly detected Phytophthora sp. Other species frequently recovered included P. citricola, P. cinnamomi, and P. cryptogea. Isolates of P. parasitica, P. megasperma, and P. syringae were recovered less frequently. Water samples also were tested for Phytophthora spp. using commercially available ELIS A tests. The ELIS A reaction intensity of filter pad extracts was correlated with the numbers of propagules estimated to be on the filters, but the correlation was stronger at some times than at others. This is believed to reflect temporal differences in water sample quality or species mixtures.