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.