Abstract. In plant pathosystems in which some hosts are transmissive and some are dead-ends, infection is mediated by multiple factors including the susceptibility of both hosts, the sporulation potential of transmissive hosts, the mobility of infectious propagules, the presence of environmental factors conducive to infection, and the variability in distribution of both host types. The factors above were studied for the California forest disease sudden oak death caused by the pathogen Phytophthora ramorum. This pathogen is exotic to California, and while it sporulates at significant levels on the leaves of California bay laurels, four susceptible oak species appear to be non-infectious dead-end hosts. Here, we report, for the first time, on inoculum levels necessary to successfully infect adult oaks and on the distribution of such inoculum levels through time and space thanks to a seven-year-long monitoring effort across a network of 128 monitoring points. Through a series of geostatistical and statistical analyses, we show that the presence of high inoculum loads is positively correlated with close proximity to bay laurels, with high rainfall levels, and with warmer temperatures. Data are consistent with splash dispersal of the pathogen and show that increased presence of tanoak corresponds to a reduced presence of bay laurels and to a lower frequency of high inoculum events. Removal of bay laurels resulted in a substantial decrease of number of events in which spore loads were high enough to infect oaks. This effect was significant when bays were removed 10 m around sampling points, thus indicating that removal of bays 10 m around oaks is a valid approach to reduce infections of oaks.

Aims:  To determine the effects of heat and composting treatments on the viability of the plant pathogen Phytophthora ramorum grown on both artificial and various natural substrates. Methods and Results:  Phytophthora ramorum was grown on V8 agar, inoculated on bay laurel leaves (Umbellularia californica) and on woody tissues of coast live oak (Quercus agrifolia). Effects on growth, viability and survival were measured as a result of treatment in ovens and compost piles. Direct plating onto PARP medium and pear‐baiting techniques were used to determine post‐treatment viability. No P. ramorum was recovered at the end of the composting process, regardless of the isolation technique used. By using a PCR assay designed to detect the DNA of P. ramorum, we were able to conclude the pathogen was absent from mature compost and not merely suppressed or dormant. Conclusions:  Some heat and composting treatments eliminate P. ramorum to lower than detectable levels on all substrates tested. Significance and Impact of the Study:  Composting is an effective treatment option for sanitization of P. ramorum‐infected plant material. Assaying for pathogen viability in compost requires a direct test capable of differentiating between pathogen suppression and pathogen elimination.

A.M. de Cock was isolated from leaflets of wood rose (Rosa gymnocarpa Nutt.), a native, low shrub of the Rosaceae family, at the Jack London State Park in Sonoma County, California. Affected leaflets had cream-tobrown lesions or spots, sometimes delimited by a chlorotic zone. Lesions coalesced with time and spread into the petiole and rachis. Lesions on the stems were not observed. Isolates were typical of P. ramorum with large chlamydospores and caduceus, semipapillate sporangia, and the sequence (GenBank Accession No. AY526571) of the internal transcribed spacer (ITS) region of the rDNA matched those published previously (4). The site was a mixed forest containing some confirmed P. ramorum-infected trees of coast redwood (Sequoia sempervirens), bay laurel (Umbellularia californica), and tanoak (Lithocarpus densiflorus) (3,4). These sites also contained California rose (R. californica Cham. & Schldl.); however, no symptoms were observed on this species. A terminal leaflet of asymptomatic, pesticidefree, potted-plants of California rose and wood rose (four plants each) was inoculated with zoospores of a P. ramorum isolate (American Type Culture Collection, Manassas, VA, ATCC MYA-3281; Centraal Bureau voor Schimmelcultures, Baarn, the Netherlands, CBS 114390) obtained from infected wood rose (2). A control leaflet of each plant was dipped in sterile deionized water. Branches containing the inoculated and control leaflets were placed in moist plastic bags, and plants were maintained at 21 to 22°C in the laboratory for 6 days. The inoculation experiment was repeated. In both inoculations, brown lesions (extending up to 8 mm from the leaflet tip) were observed on leaflets of both species 2 days after inoculation with P. ramorum. At 6 days after inoculation, lesions starting from the leaflet tip averaged 12.2 mm in length (range 10 to 16 mm) for wood rose and 9.6 mm (range 3 to 20 mm) for California rose. Some lesions extended into the petiole in both rose species. Sporangia were observed in washings of the lesions from four plants of California rose and one plant of wood rose, and P. ramorum was reisolated on Phytophthora-selective agar medium modified with 25 mg of pentachloronitrobenzene (PCNB) (4) from all lesions. Control leaflets had no lesions, and P. ramorum was not reisolated. To our knowledge, this is the first report of a species of Rosa as a natural host for P. ramorum, although R. sempervirens was identified as being susceptible in artificial inoculations of detached leaves (E. Moralejo and L. Hernández, personal communication). Toyon (Heteromeles arbutifolia) in California and salmon berry (Rubus spectabilis) in Oregon are the other known hosts from the family Rosaceae (1). Wood rose is popular in the horticultural industry and is readily available from native plant nurseries in California, Oregon, Washington, and British Columbia, Canada. California rose is also popular, primarily in California. The pathogen could be disseminated on these plants, especially since sporangia were obtained from inoculate...

Composted municipal green waste is a potential vehicle for the transmission of Phytophtora ramorum, the pathogen responsible for the disease known as sudden oak death. To assess the survival rate of the pathogen in compost, we introduced zoospores -a type of infectious propagule -into six composts of varying provenance and maturity. The compost samples represented three production facilities, two production techniques (turned windrow and forced air static pile) and two levels of maturity (fresh, defined as aged for less than 1 week; and mature, aged for more than 4 weeks). Positive re-isolations -indicating survival of the pathogen -were obtained from all composts. The re-isolation rate from the compost from one of the three production facilities was greater than that obtained from an inert substrate (filter paper) inoculated with the pathogen (P < 0.01), while re-isolation rates from the other two sources were statistically indistinguishable from those obtained from the inert substrate (P > 0.01). There was no significant difference in re-isolation rate between composts produced by the turned windrow method and composts produced by the forced air static pile technique. Re-isolation rates were greater from mature composts than from fresh composts (P < 0.01). The results show that P. ramorum may be present and infectious if introduced into finished compost, and that variations in compost characteristics appear to influence survival rates.