OCCURRENCE OF PHYTOPHTHORA SPECIES IN RECIRCULATED NURSERY IRRIGATIONEFFLUENTS

Water samples were collected from effluent holding ponds at one northe rn and two southern California nurseries that practice the capture and recirculation of irrigation runoff water. Nursery effluent samples we re collected approximately monthly over a 12-mo period and aliquots fi ltered through 0.45-mum Millipore filters. Filter residues were resusp ended and dispersed onto selective agar media in petri dishes to estim ate the numbers of viable propagules of Phytophthora spp. or total pyt hiacious fungi. Propagule numbers varied greatly from month to month a t each nursery location. Pythium propagules were consistently the most numerous, ranging from 500 to 1,500 per liter, whereas the number of Phytophthora spp. propagules ranged from 0 to 400 per liter. At the no rthern California nursery, propagule numbers were lowest during winter months and highest during warm seasons. Seasonal fluctuations in inoc ulum load were not apparent in the southern California nurseries. P. c itrophthora was the most commonly detected Phytophthora sp. Other spec ies frequently recovered included P. citricola, P. cinnamomi, and P. c ryptogea. Isolates of P. parasitica, P. megasperma, and P. syringae we re recovered less frequently. Water samples also were tested for Phyto phthora spp. using commercially available ELISA tests. The ELISA react ion intensity of filter pad extracts was correlated with the numbers o f propagules estimated to be on the filters, but the correlation was s tronger at some times than at others. This is believed to reflect temp oral differences in water sample quality or species mixtures.