Title: Monitoring Phytophthora ramorum distribution in streams within coastal California watersheds
Author: Murphy, S.; Lee, C.; Valachovic, Y.; Jirka, A.; Owen, D.R.; Rizzo, D.; Mark, W.;
Source: In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 304
Publication Series: General Technical Report (GTR)
One hundred eighty-seven sites were established in perennial watercourses and sampled for one to four years between 2004 and 2007 to monitor for the presence of Phytophthora ramorum throughout coastal central and northern California watersheds as well as portions of the Sierra Nevada mountain range. In 2007, 132 sites were monitored, including 65 new sites. The majority of the monitored watersheds have limited or no Pr at this time, but are near the epidemic range of P. ramorum and/or are considered high-risk for invasion by P. ramorum. Three currently infested watersheds in Sonoma, Humboldt, and Monterey Counties were included as a baseline for successful recovery of Pr. Rhododendron leaves were placed in mesh bags and secured in watercourses for one- to three-week intervals to bait for Phytophthora species. Recovered symptomatic leaves were plated on Phytophthora-selective medium (PARP-H) and monitored microscopically. Phytophthora ramorum has been detected at 37 total stream monitoring sites, including all sites with a priori knowledge of nearby forest infestation.
Phytophthora ramorum was detected at 23 streams sites without prior knowledge of adjacent forest infestation in Humboldt, Contra Costa, Mendocino, Monterey, and Santa Cruz counties. Forest infestations have thus far been confirmed at only nine of these sites; surveys are underway to identify the source(s) of inoculum for the other sites. Additionally, P. ramorum was recovered as far as 25km downstream from known forest infestations. This year (2007) was an unusually dry year in California which impacted our recovery of P. ramorum from watercourses. Phytophthora ramorum was detected in only 15 streams this year; seven of those were new sites for 2007. We recovered no P. ramorum from four streams that were positive for P. ramorum in 2006. At the most heavily infested sites, the frequency and quantity of recovery of P. ramorum was greatly reduced in 2007.
Stream monitoring has extended the southern range of Pr in Monterey County and the northern range in Humboldt County. All sites in the Sierra Nevada remain negative for Pr. With culturing and molecular sequencing we have identified several other Phytophthora species within these watersheds; P. gonapodyides is the most commonly detected species and was isolated from at least 60 sites. Streams were monitored year-round in 2004 and 2005 and revealed a distinct seasonality associated with P. ramorum recovery. Therefore, in 2006 and 2007 watersheds were monitored monthly, February through June, during the peak seasonal period.
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Murphy, S.; Lee, C.; Valachovic, Y. Mark, W.; Jirka, A.; Owen, D.R.; Rizzo, D. 2009. Monitoring Phytophthora ramorum distribution in streams within coastal California watersheds. In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 304.
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