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    Author(s): Neelam Redekar; Joyce Eberhart; Jennifer Parke
    Date: 2020
    Source: Proceedings of the seventh sudden oak death science and management symposium: healthy plants in a world with <em>Phytophthora</em>. Gen. Tech. Rep. PSW-GTR-268
    Publication Series: General Technical Report (GTR)
    Station: Pacific Southwest Research Station
    PDF: Download Publication  (915.0 KB)


    Lake Mathews is a 182,000 acre-feet capacity reservoir located in Riverside County, California. It is the western terminus of the Colorado River Aqueduct that serves as the main source of irrigation water for several horticultural nurseries in southern California. Some of these nurseries do not disinfest Lake Mathews water before using it for irrigation. This could increase disease pressure at these nurseries and facilitate survival, propagation and spread of important waterborne plant pathogens such as Phytophthora spp. on infested nursery stock.

    We examined the diversity of Phytophthora species detected in this water over the span of 30 months to determine what risks it poses to the container nurseries that use this water source for irrigation. Metabarcoding approach: We periodically collected two 1-L water samples from Lake Mathews as supplied by the Western Municipal Water District to a large container nursery in southern California. One set of samples was filtered through 5μm Millipore nylon membranes to physically capture all Phytophthora species present in the water. Another set of samples was baited using rhododendron leaves to detect viable Phytophthora species in the water. In both cases, the ITS1 region was first amplified from the filter and leaf bait DNA using ITS6 and ITS7 primers (Cooke and others 2000), and then sequenced with high-throughput Illumina MiSeq 250PE sequencing for metabarcoding. The paired-end Illumina sequences were first cleaned, quality filtered and queried against a custom oomycete reference ITS database (Redekar and others 2019) using a nucleotide megablast search. Operational taxonomic units (OTUs) or taxa were identified based on percent sequence similarity to the reference sequence of known Phytophthora species.

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    Redekar, Neelam; Eberhart, Joyce; Parke, Jennifer. 2020. Phytophthora diversity in Lake Mathews, the irrigation water source for a southern California nursery. In: Frankel, Susan J.; Alexander, Janice M., tech. cords. Proceedings of the seventh sudden oak death science and management symposium: healthy plants in a world with Phytophthora. Gen. Tech. Rep. PSW-GTR-268. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 98-100.

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