OCCURRENCE OF PHYTOPHTHORA SPECIES IN RECIRCULATED NURSERY IRRIGATION EFFLUENTS

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-mum 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 to 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. citrophthora 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 ELISA tests. The ELISA 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.