What's New

New Plant Disease first report February 2022

First report of the NA2 clonal lineage of the sudden oak death pathogen, Phytophthora ramorum, infecting tanoak in Oregon forests

E.K. Peterson, K.L. Søndreli, P. Reeser, S. Navarro, C. Nichols, R. Wiese, V.J. Fieland, N.J. Grünwald, J.M. LeBoldus

Published Online:11 Feb 2022



Phytophthora ramorum Werres, de Cock & Man in’t Veld, causal agent of sudden oak death (SOD) and ramorum leaf blight, is comprised of four clonal lineages in its invasive ranges of North America and Europe (Grünwald et al. 2012, Van Poucke et al. 2012). Of these, three – the NA1, NA2, and EU1 lineages – are found in U.S. nurseries, but only two, the NA1 and EU1 lineages, have been found infecting trees in North American forests (Grünwald et al. 2012, 2016). In the spring of 2021, tanoak (Notholithocarpus densiflorus Manos, Cannon & Oh) displaying symptoms consistent with SOD were detected north of Port Orford (Curry County, Oregon). Symptoms were canopy dieback and blackened petiole and stem lesions on tanoak sprouts. The pathogen isolated on PAR (CMA plus 200 ml/L ampicillin, 10 mg/L rifamycin, 66.7 mg/L PCNB) selective media was determined to be P. ramorum based on characteristic morphology of hyphae, sporangia, and chlamydospores (Werres et al. 2001). Positive identification as P. ramorum was obtained with a lineage-specific LAMP assay targeting an NA2 orphan gene, indicating the presence of the NA2 lineage. NA2 was confirmed by sequencing a portion of the cellulose binding elicitor lectin (CBEL) gene using CBEL5U and CBEL6L primers (Gagnon et al. 2014). Sequences (GenBank accessions MZ733981 and MZ733982) were aligned against reference sequences for all lineages (Gagnon et al. 2014) confirming the presence of NA2. Lineage determination as NA2 was further confirmed at eleven SSR loci (ILVOPrMS145, PrMS39, PrMS9C3, ILVOPrMS79, KI18, KI64, PrMS45, PrMS6, ILVOPrMS131, KI82ab, and PrMS43) using the methods of Kamvar et al. (2015). We completed Koch’s postulates using potted tanoaks, wound-inoculated at the midpoint of 1-year old stems with either hyphal plugs or non-colonized agar (n=4 per treatment). Tanoaks were maintained in a growth chamber (20°C-day / 18°C-night temperatures) with regular watering and an 18-photoperiod using F32T8 fluorescent bulbs (Phillips, Eindhoven, The Netherlands). After 7 days, brown to black lesions 1.2 to 2.9 cm in length were observed on the inoculated stems, from which P. ramorum was subsequently re-isolated; no symptoms were observed on the controls, and no pathogens were recovered when plating the wound sites in PAR. This is the first detection of the NA2 lineage causing disease in forests worldwide. The outbreak was found on private and public lands in forests typical to the SOD outbreak in Oregon (mixed conifer and tanoak), and was 33 km north of the closest known P. ramorum infestation. Follow-up ground surveys on adjacent lands have identified over 100 P. ramorum-positive tanoak trees, from which additional NA2 isolates have been recovered from bole cankers. NA2 is thought to be more aggressive than the NA1 lineage (Elliott et al. 2011), which has been present in Curry County since the mid-1990s (Goheen et al. 2017). Eradication of the NA2 lineage is being pursued to slow its further spread and prevent overlap with existing NA1 and EU1 populations. The repeated introductions of novel lineages into the western United States native plant communities highlights the vulnerability of this region to Phytophthora establishment, justifying continued monitoring for P. ramorum in nurseries and forests.


New Plant Disease first report February 2022

First Report of Root Rot Caused by Phytophthora acerina on Metasequoia glyptostroboides in China

Duan-chong Liu, Wen-xia Zhao, Jian-ping Xia, San-shan Cai, Wenxia Huai, Ru-bin Zhang, Bin Li

Published Online: Plant Disease, 8 Feb 2022


Metasequoia glyptostroboides Hu & W. C. Cheng (Taxodiaceae), commonly called the Chinese redwood or dawn redwood, is a well-known "living fossil" and rare relict plant species endemic to China, which has been successfully cultivated throughout the world (Ma 2007). In July to September 2020, trees of Chinese redwood which were more than thirty years-old, showed symptoms of decline and death associated with branch dieback, root and collar rot (Fig. 1) in Yangtze River shelter-forests of Jiangling County in Hubei Province, China (112°15′19″E, 30°11′56″N; 40m)...


New Oregon State University Extension article - Preventing Phytophthora infestations in restoration nurseries: a key to protecting wildland plant communities.

Preventing Phytophthora infestations in restoration nurseries: a key to protecting wildland plant communities.

Kline, N.; Elliott, M.; Parke, J.; Stark, D.; Shaw, D. and Christiansen, A. 2022.  EM 9330. Oregon State University Extension Service. 38 pp. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9330.pdf.


New Plant Disease article First Look November 2021

Phytophthora spp. associated with Appalachian oak forests and waterways in Pennsylvania, with P. abietivora as a pathogen of five native woody plant species.

Devin Bily, Ekaterina V. Nikolaeva, Tracey Olson, and Seogchan Kang

Published Online: https://doi.org/10.1094/PDIS-05-21-0976-RE

To document the distribution of potentially harmful Phytophthora spp. within Pennsylvania (PA), the PA Department of Agriculture collected 89 plant, 137 soil, and 48 water samples at 64 forested sites from 2018 to 2020. In total, 231 Phytophthora strains were isolated using baiting assays and identified based on morphological characteristics and sequences of nuclear and mitochondrial loci. Twenty-one Phytophthora spp. in nine clades and one unidentified species were present. Phytophthora abietivora, a recently described clade 7a species, was recovered from diseased tissue of 10 native broadleaved plants and twice from soil from 12 locations. Phytophthora abietivora is most likely endemic to PA based on pathogenicity tests on six native plant species, intraspecific genetic diversity, wide distribution, and recoveries from Abies Mill. and Tsuga Carrière plantations dating back to 1989. Cardinal temperatures and morphological traits are provided for this species. Other taxa, in decreasing order of frequency, include P. chlamydospora, P. plurivora, P. pini, P. cinnamomi, P. xcambivora, P. irrigata, P. gonapodyides, P. cactorum, P. pseudosyringae, P. hydropathica, P. stricta, P. xstagnum, P. caryae, P. intercalaris, Phytophthora ‘bitahaiensis’, P. heveae, P. citrophthora, P. macilentosa, P. cryptogea, and P. riparia. Twelve species were associated with diseased plant tissues. This survey documented 53 new plant-Phytophthora associations and expanded the known distribution of some species.

Sudden oak death creeps toward Coos County

Reported in the Bandon Western News (May27, 2021):

A forest disease is slowly creeping towards Coos County.

Forest officials last month discovered new cases of the sudden oak death pathogen south of the county line in Curry County.

The disease isn’t new for the county — around a third of it is within a quarantine area, where some plant transport is prohibited and eradication is required. But forest experts are raising concerns about how far away the new cases are from previously known ones.

“That distance indicates to me that we have much larger spread,” said Sarah Navarro, a forest pathologist with the U.S. Forest Service. “It is concerning that it’s a new location so far away from any of the other sites farther south.”

See the full story at:


First Look Plant Disease article

First reports of Phytophthora ramorum clonal lineages NA1 and EU1 causing Sudden Oak Death on tanoaks in Del Norte County, California

Matteo Garbelotto, Francesco Dovana, Douglas Schmidt, Cameron Chee, Chris Lee, Valerie Fieland, Niklaus J. Grünwald, and Yana Valachovic


A year of forest health surveys has led to the first detection of Phytophthora ramorum in Del Norte County followed by the first wildland detection of the EU1 clonal lineage (Grunwald et al. 2009) of this pathogen in California. In July 2019, leaves were sampled from two tanoaks (Notholithocarpus densiflorus) and 16 California bay laurels (Umbellularia californica) in Jedediah Smith State Park in Del Norte County, the northernmost coastal County of California. Leaves displayed lesions normally associated with Sudden Oak Death (SOD) caused by P. ramorum and were discovered during the citizen science-based survey known as SOD Blitz (Meentemeyer et al. 2015). Samples were surface sterilized using 75% Ethanol and plated on PARPH-V8 agar (Jeffers and Martin 1986). After plating, DNA was extracted and amplified using two P. ramorum-specific assays (Hayden et al. 2006, Kroon et al. 2004). Leaves from two tanoaks exhibiting twig die-back had typical SOD lesions along the midvein, gave positive PCR results and yielded cultures with colony morphology, sporangia and chlamydospores typical of the NA1 lineage of P. ramorum originally isolated in California from tanoaks and coast live oaks (Quercus agrifolia) (Rizzo et al. 2002). The ITS locus and a portion of the Cox-1 locus were sequenced from DNA extracts of each culture using primers DC6-ITS4 (Bonants et al. 2004) and COXF4N-COXR4N (Kroon et al. 2004), respectively. ITS sequences (GB MN540639-40) were typical of P. ramorum and Cox-1 sequences (GB MN540142-3) perfectly matched the Cox-1 sequence of the NA1 lineage (GB DQ832718) (Kroon et al. 2004). Microsatellite alleles were generated as described in Croucher et al. (2013) for the two Del Norte cultures and for eight P. ramorum cultures, representative of the four main multilocus genotypes (MLGs) present in California, namely c1 (Santa Cruz/Commercial Nurseries), c3 (San Francisco Bay Area), c2 (Monterey County), and c4 (Humboldt County) (Croucher et al. 2013). The two Del Norte MLGs were identical to one another and most similar to MLG c1, with a single repeat difference at a single locus. SSR results suggest the inoculum source may not be from Humboldt County, neighboring to the South, but from a yet unidentified outbreak, possibly associated with ornamental plants. Jedediah Smith State Park was surveyed for 12 months following the initial detection, however the pathogen has yet to be re-isolated in that location. In July 2020, SOD symptomatic leaves from two tanoak trees exhibiting twig cankers were collected 8 Km north of Jedediah Smith State Park, where three additional tanoak trees displayed rapidly browned dead canopies consistent with late stage SOD. Leaves were processed as above. Colonies from these samples produced chlamydospores and sporangia typical of P. ramorum on PARPH-V8 agar, but displayed a growth rate faster than that of NA1 genotypes and were characterized by aerial hyphae, overall resembling the morphology of EU1 lineage colonies (Brasier 2003). The EU1 lineage was confirmed by the perfect match of the sequence of a portion of the Cox-1 gene (GB MW349116-7) with the Cox-1 sequence of EU1 genotypes (GB EU124926). The EU1 clonal lineage has been previously isolated from tanoaks in Oregon forests, approximately 55 Km to the North (Grünwald et al. 2016), but this is the first report for California wildlands and will require containment and government regulations. It is unknown whether the EU1 strains in Del Norte County originated from Oregon forests or elsewhere.

New Forests article November 2020

Phytophthora Introductions in Restoration Areas: Responding to Protect California Native Flora from Human-Assisted Pathogen Spread.

Frankel, S.J.; Conforti, C.; Hillman, J.; Ingolia, M.; Shor, A.; Benner, D.; Alexander, J.M.; Bernhardt, E.; Swiecki, T.J.

Forests 2020, 11, 1291. https://doi.org/10.3390/f11121291


Over the past several years, plantings of California native plant nursery stock in restoration areas have become recognized as a pathway for invasive species introductions, in particular Phytophthora pathogens, including first in the U.S. detections (Phytophthora tentaculata, Phytophthora quercina), new taxa, new hybrid species, and dozens of other soilborne species. Restoration plantings may be conducted in high-value and limited habitats to sustain or re-establish rare plant populations. Once established, Phytophthora pathogens infest the site and are very difficult to eradicate or manage—they degrade the natural resources the plantings were intended to enhance. To respond to unintended Phytophthora introductions, vegetation ecologists took a variety of measures to prevent pathogen introduction and spread, including treating infested areas by solarization, suspending plantings, switching to direct seeding, applying stringent phytosanitation requirements on contracted nursery stock, and building their own nursery for clean plant production. These individual or collective actions, loosely coordinated by the Phytophthoras in Native Habitats Work Group ensued as demands intensified for protection from the inadvertent purchase of infected plants from commercial native plant nurseries. Regulation and management of the dozens of Phytophthora species and scores of plant hosts present a challenge to the state, county, and federal agriculture officials and to the ornamental and restoration nursery industries. To rebuild confidence in the health of restoration nursery stock and prevent further Phytophthora introductions, a voluntary, statewide accreditation pilot project is underway which, upon completion of validation, is planned for statewide implementation.

Early view Forest Pathology article November 2020

The influence of time, soil moisture and exogenous factors on the survival potential of oospores and chlamydospores of Phytophthora cinnamomi. 

Jamba Gyeltshen, William A. Dunstan, Andrew H. Grigg, Treena I. Burgess, and Giles E. St. J. Hardy



The mode of persistence of Phytophthora cinnamomi, a highly aggressive soil‐ and water‐borne pathogen, remains unclear. This study investigated the survival of viable oospores and chlamydospores of P. cinnamomi when present as free propagules in untreated soil, or in soil subject to four exogenous treatments: smoke water, fish emulsion and two fungicides (ridomil and furalaxyl). The exogenous treatments were applied under moist and dry soil conditions. Spore viability was determined by the thiazolyl blue tetrazolium bromide (MTT) staining technique, with a qPCR assay used to compare general patterns of decline. Over 96% of oospores lost viability over a period of 48 weeks irrespective of soil moisture conditions. The mean percentage viability for oospores decreased from 91% at time zero to 72, 35, 20 and 1% after 6, 12, 24 and 48 weeks, respectively. Reduction in viability of chlamydospores was more rapid than oospores, with viability declining from 92% to zero after 12 weeks. There was no significant difference between untreated soil and the exogenous treatments. The RNA‐based qPCR assay indicated a strong presence of viable oospores of P. cinnamomi up to week 12 for moist soil and week 3 for dry soil, but thereafter failed to detect RNA even though viable oospores could be detected by MTT staining. Based on the MTT staining, this study indicated that viability of P. cinnamomi oospores may be entirely lost within 1 year and that of chlamydospores within 3 months for the soil type tested. Therefore, oospores and chlamydospores when existing as free propagules in soil appear unlikely to be involved in the long‐term survival of P. cinnamomi.

New "Forests" article January 2020

A Survey in Natural Forest Ecosystems of Vietnam Reveals High Diversity of both New and Described Phytophthora Taxa including P. ramorum.

Jung, T.; Scanu, B.; Brasier, C.M.; Webber, J.; Milenković, I.; Corcobado, T.; Tomšovský, M.; Pánek, M.; Bakonyi, J.; Maia, C.; Bačová, A.; Raco, M.; Rees, H.; Pérez-Sierra, A.; Horta Jung, M.

Forests 2020, 11, 93. https://www.mdpi.com/1999-4907/11/1/93

In 2016 and 2017, surveys of Phytophthora diversity were performed in 25 natural and semi-natural forest stands and 16 rivers in temperate and subtropical montane and tropical lowland regions of Vietnam. Using baiting assays from soil samples and rivers and direct isolations from naturally fallen leaves, 13 described species, five informally designated taxa and 21 previously unknown taxa of Phytophthora were isolated from 58 of the 91 soil samples (63.7%) taken from the rhizosphere of 52 of the 64 woody plant species sampled (81.3%) in 20 forest stands (83.7%), and from all rivers: P. capensis, P. citricola VII, VIII, IX, X and XI, P. sp. botryosa-like 2, P. sp. meadii-like 1 and 2, P. sp. tropicalis-like 2 and P. sp. multivesiculata-like 1 from Phytophthora major phylogenetic Clade 2; P. castaneae and P. heveae from Clade 5; P. chlamydospora, P. gregata, P. sp. bitahaiensis-like and P. sp. sylvatica-like 1, 2 and 3 from Clade 6; P. cinnamomi (Pc), P. parvispora, P. attenuata, P. sp. attenuata-like 1, 2 and 3 and P. ×heterohybrida from Clade 7; P. drechsleri, P. pseudocryptogea, P. ramorum (Pr) and P. sp. kelmania from Clade 8, P. macrochlamydospora, P. sp. ×insolita-like, P. sp. ×kunnunara-like, P. sp. ×virginiana-like s.l. and three new taxa, P. sp. quininea-like, P. sp. ×Grenada 3-like and P. sp. ×Peru 4-like, from Clade 9; and P. sp. gallica-like 1 and 2 from Clade 10. The A1 and A2 mating types of both Pc and Pr co-occurred. The A2 mating type of Pc was associated with severe dieback of montane forests in northern Vietnam. Most other Phytophthora species, including Pr, were not associated with obvious disease symptoms. It is concluded that (1) Vietnam is within the center of origin of most Phytophthora taxa found including Pc and Pr, and (2) Phytophthora clades 2, 5, 6, 7, 8, 9, and 10 are native to Indochina.


New Plant Pathology article Jan 2020

Phytophthora agathidicida: research progress, cultural perspectives and knowledge gaps in the control and management of kauri dieback in New Zealand.

Bradshaw, RE, Bellgard, SE, Black, A, Burns, BR, Gerth, ML, McDougal, RL, Scott, PM, Waipara, NW, Weir, BS, Williams, NM, Winkworth, RC, Ashcroft, T, Bradley, EL, Dijkwel, PP, Guo, Y, Lacey, RF, Mesarich, CH, Panda, P, Horner, IJ.

Plant Pathology (2020) 69, 3–16, https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1111/ppa.13104

Kauri (Agathis australis), which is one of the world's largest and longest‐living conifer species, is under threat from a root and collar dieback disease caused by the oomycete pathogen Phytophthora agathidicida. The noted incidence of kauri dieback has increased in the past decade, and even trees >1000 years old are not immune. This disease has profound effects on both forest ecosystems and human society, particularly indigenous Māori, for whom kauri is a taonga or treasure of immense significance. This review brings together existing scientific knowledge about the pathogen and the devastating disease it causes, as well as highlighting important knowledge gaps and potential approaches for disease management. The life cycle of P. agathidicida is similar to those of other soilborne Phytophthora pathogens, with roles for vegetative hyphae, zoospores and oospores in the disease. However, there is comparatively little known about many aspects of the biology of P. agathidicida, such as its host range and disease latency, or about the impact on the disease of abiotic and biotic factors such as soil health and co‐occurring Phytophthora species. This review discusses current and emerging tools and strategies for surveillance, diagnostics and management, including a consideration of genomic resources, and the role these play in understanding the pathogen and how it causes this deadly disease. Key aspects of indigenous Māori knowledge, which include rich ecological and historical knowledge of kauri forests and a holistic approach to forest health, are highlighted.