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Current California Oak Mortality Task Force Newsletter: August 2021

(Printable COMTF Report August 2021)

Monitoring and Management – Oregon

The Phytophthora ramorum NA2 wildland infestation in Oregon is larger than initially realized. Oregon Department of Forestry (ODF), U.S. Forest Service, and Oregon State University sudden oak death (SOD) crews have conducted intensive ground surveys in the Port Orford area to determine the extent of the NA2 lineage infestation. Since late May over 136 Phytophthora ramorum positive samples have been collected from tanoaks and rhododendrons surrounding the initial two tanoak trees along Highway 101 (Figures 1 & 2). Additional survey work has been completed by ODF staff including two additional stream baits and a helicopter survey. Given the significance of this infestation, a 600 ft treatment buffer has been set around all infected trees resulting in a 475 acre treatment area.

Port Orford, Oregon map and symptomatic tanoak
Figure 1. Map of the delimited area of the NA2 Phytophthora ramorum wildland infestation near Port Orford, Oregon. Figure 2. Charlie Grell, U.S. Forest Service SOD Forester, sampling a symptomatic tanoak.

The Oregon Legislature has allocated $1.7 million for detection and treatment of sudden oak death over the next two years with a further $190,000 coming in 2021 from the U.S. Forest Service under a cooperative agreement. Treating this area is estimated to cost about $1.7 million. Oregon State University Forestry and Natural Resources Extension in Coos and Curry Counties plan to provide information sessions about the disease to the public this summer in Port Orford. The programs will train local residents to identify symptoms of sudden oak death so they can report concerns on their properties. They have developed a website specifically for local residents. A new factsheet for this NA2 P. ramorum wildland infestation and press release are also available from ODF. For more information contact Sarah Navarro at

Nurseries and Managed Landscapes

USDA APHIS 2021 mid-year Phytophthora ramorum program update. USDA APHIS and APHIS accredited laboratories have confirmed 128 positive samples for P. ramorum in various establishments (regulatory incidents) thus far in 2021. Rhododendron plants continue to be the most commonly detected infected genus, comprising 61.7% of the positive samples confirmed so far this year. The next most common genera to test positive were Pieris and Loropetalum, with 13.3% and 9.4% respectively (see Table 1 and Figure 1).


For more details see the full report or contact Betsy Randall-Schadel,

California Department of Food and Agriculture (CDFA) P. ramorum program update.

Over the past few months, two trace investigations have been conducted in California. The investigations have been completed with no positive plant detections. For more information contact Carolyn Lambert at

Washington State Department of Agriculture (WSDA) P. ramorum program update.

In May, USDA APHIS and WSDA staff performed a certification survey at a wholesale shipping nursery under compliance. During the two-day survey, 413 samples were collected. All samples tested negative for P. ramorum.

In June, WSDA conducted a trace-forward investigation on plants shipped from a positive out-of-state nursery. Eight receiving locations in Washington were inspected. One sample was collected that tested negative for P. ramorum. All other inspected host plants were in good condition or had been sold. For more information contact Scott Brooks at


The California Department of Forestry and Fire Protection (Cal Fire) and California Department of Food and Agriculture (CDFA) have entered into an agreement to allow Cal Fire Forest Health Specialists to be considered “official” P. ramorum samplers. Diagnostic results for materials collected by official samplers are recognized by the State and USDA APHIS as data to inform regulations (quarantines). Without such an agreement, “official samplers” are limited to employees of agricultural regulatory agencies. All samples must be submitted and tested by a laboratory approved by APHIS, using methods approved by APHIS. For more information contact Chris Lee,

Research (Excerpts or Abbreviated Abstracts)

Hieno, A.; Li, M.; Otsubo, K.; Suga, H. and Kageyama, K. 2021. Multiplex LAMP detection of the genus Phytophthora and four Phytophthora species P. ramorum, P. lateralis, P. kernoviae, and P. nicotianae, with a plant internal control. Microbes and Environments. 36(2): ME21019.

Phytophthora species cause destructive plant diseases worldwide. All Phytophthora species, except for one, are listed as plant quarantine organisms in Japan. The exception, Phytophthora nicotianae, is considered to be a domestic species. The injurious pests Phytophthora ramorum, Phytophthora lateralis, and Phytophthora kernoviae are invasive pathogens that cause tree mortality worldwide, mainly in the United States and the United Kingdom. To effectively control Phytophthora diseases, we established detection methods that utilize the loop-mediated isothermal amplification (LAMP) of the genus Phytophthora and the four species P. ramorum, P. lateralis, P. kernoviae, and P. nicotianae. LAMP primers for P. ramorum, P. lateralis, and P. kernoviae were newly designed in the present study. Our multiplex assay includes the detection of plant DNA as an internal control. When the optimum ratio between plant and pathogen primers was used in multiplex LAMP assays, 1 pg to 100 fg of pathogen DNA was detected with similar sensitivity to that in simplex LAMP assays. The detection of plant DNA in the absence of pathogens enables us to check for and avoid undesirable negative results caused by enzyme inactivation or the contamination of amplification inhibitors from plant tissues. The total time from sample collection to results is approximately 120 min, and, thus, our multiplex LAMP assay may be used as an accurate and time-saving detection method for Phytophthora pathogens.

Taylor, C.R. and Grünwald, N.J. 2021. Growth, infection and aggressiveness of Phytophthora pathogens on Rhododendron leaves. CABI Agriculture and Bioscience. 2(1): 1-15.

Background. Phytophthora species are well known as important or emerging pathogens. The genus Rhododendron is of considerable importance to plant regulatory agencies because it is host to many Phytophthora species, most notably, P. ramorum and P. kernoviae. Few studies have directly contrasted the epidemiology of different Phytophthora spp. on a given host.

Methods. We investigated aspects of the foliar epidemiology (lesion size, sporulation and temperature responses) of P. cactorumP. cambivoraP. cinnamomiP. citrophthoraP. foliorumP. kernoviaeP. lateralisP. nemorosaP. nicotianaeP. plurivoraP. ramorum and P. syringae on Rhododendron in detached leaf, whole plant chamber, and field studies.

Results. P. syringae stood out as it appeared to be a relatively weak pathogen, showing no sporulation and low levels of disease severity, except at low temperatures. P. nicotianae was consistently able to grow at higher temperatures than any of the other Phytophthora spp. and showed higher aggressiveness than any of the other species at high temperatures. P. cinnamomi and P. cactorum, typically thought of as root-infecting species, were able to cause as much foliar disease as P. syringae, a foliar pathogen. P. kernoviae was consistently among the most aggressive species with the highest sporulation.

Conclusion. These results provide novel insights into the comparative epidemiology of these important established and emerging Phytophthora species.

Related Research

Jones, C.M.; Jones, S.; Petrasova, A.; Petras, V.; Gaydos, D.; Skrip, M.M.; Takeuchi, Y.; Bigsby, K. and Meentemeyer, R.K. 2021. Iteratively forecasting biological invasions with PoPS and a little help from our friends. Frontiers in Ecology and the Environment.

O’Hanlon, R.; Destefanis, M.; Milenković, I.; Tomšovský, M.; Janoušek, J.; Bellgard, S.E.; Weir, B.S.; Kudláček, T.; Horta Jung, M. and Jung, T. 2021. Two new Nothophytophthora species from streams in Ireland and Northern Ireland: Nothophytophthora irlandica and N. lirii sp. nov. Plos one. 16(5): e0250527.

Swiecki, T.J.; Bernhardt, E.A.; Frankel, S.J.; Benner, D. and Hillman, J. 2021. An accreditation program to produce native plant nursery stock free of Phytophthora for use in habitat restoration. Plant Health Progress. (First Look).

Van Poucke, K.; Haegeman, A.; Goedefroit, T.; Focquet, F.; Leus, L.; Jung, M.H.; Nave, C.; Redondo, M.A.; Husson, C.; Kostov, K. and Lyubenova, A. 2021. Unravelling hybridization in Phytophthora using phylogenomics and genome size estimation. IMA fungus.  12(1): 1-24.


An updated map of the distribution of P. ramorum in California and Southern Oregon wildlands based on CALINVASIVES entries is now available. CALINVASIVES has also assembled a P. ramorum factsheet. CALINVASIVES is led by Matteo Garbelotto, UC Berkeley, Forest Pathology and Mycology laboratory.

Scanu, B. and Jung, T. eds. 2021. Phytophthora Infestations in Forest Ecosystems. doi: 10.3390/books978-3-0365-0801-6. 10 papers, 216 pages. (Available in print ISBN 978-3-0365-0800-9 (Hbk); or digital format ISBN 978-3-0365-0801-6 (PDF).


Congratulations, thanks and best wishes to Karen Suslow upon her retirement from the National Ornamental Research Site at Dominican University (NORSDUC). Karen has worked on addressing P. ramorum for over 20 years and served for decades on the California Oak Mortality Task Force Executive Committee. She is a leader in California horticulture having served as production manager and general manager at Hines Nurseries, Winters, CA, for a total of 18 years, followed by 9 years as leader of NORSDUC.


The 2021 annual meetings of the California Oak Mortality Task Force & Phytophthoras in Native Habitats Work Group will be held online September 21 – 23, starting at 1 pm PDT each day. On September 21, presentations will include updates on the NA2 sudden oak death infestation in Oregon, Bay Area Tree Mortality, and other current forest health concerns. On September 22, the California Oak Mortality Task Force will convene for round robin and discussion. On September 23, the Phytophthoras in Native Habitats Work Group will feature short talks on the theme of managing Phytophthoras in open space areas and nurseries. Registration is free but advanced registration is required (Go to online registration HERE). For more information,