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

Authors: PETERSON, EBBA - Oregon State University; SONDRELI, K - Oregon State University; REESER, P - Oregon State University; NAVARRO, S - Us Fish And Wildlife Service; NICHOLS, C - Oregon Department Of Forestry; WIESE, R - Oregon Department Of Forestry; FIELAND, V - Oregon State University; Grunwald, Niklaus - Nik; LEBOLDUS, J - Oregon State University

Submitted to: Plant Disease Notes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2022
Publication Date: 8/1/2022
Citation: Peterson, E.K., Sondreli, K.L., Reeser, P., Navarro, S., Nichols, C., Wiese, R., Fieland, V.J., Grunwald, N.J., LeBoldus, J.M. 2022. First report of the NA2 clonal lineage of the sudden oak death pathogen, Phytophthora ramorum, infecting tanoak in Oregon forests. Plant Disease Notes. 106(9):2537. https://doi.org/10.1094/PDIS-10-21-2152-PDN.
DOI: https://doi.org/10.1094/PDIS-10-21-2152-PDN

Interpretive Summary: Sudden oak death (SOD) is caused by the water mold Phytophthora ramorum. Three variants, NA1, NA2, and EU1, are found in U.S. nurseries, but only two, namely NA1 and EU1, have been in North American forests. In the spring of 2021, tanoak displaying symptoms consistent with SOD were detected north of Port Orford in Curry County, Oregon. Symptoms included canopy dieback and black stem lesions on tanoak sprouts. The isolated agent was determined to be P. ramorum based on characteristic morphology, PCR, and sequencing of a portion of the cellulose binding elicitor lectin (CBEL) gene. This is the first detection of the NA2 variant causing disease in forests. The outbreak was found 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. NA2 is thought be more aggressive than the NA1 lineage present in Curry County since the mid-1990s and eradication is being pursued to limit its further spread and prevent overlap with existing NA1 and EU1 populations. The repeated introductions of novel lineages into western native plant communities highlights the vulnerability of this region to Phytophthora establishment, justifying continued monitoring for P. ramorum in nurseries and forests.

Technical Abstract: 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. Of these, three – the NA1, NA2, and EU1 lineages – are found in U.S. nurseries, but only two, the NA1 and EU1 lineages, were found infecting trees in North American forests. 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 included canopy dieback and black stem lesions on tanoak sprouts. The isolated agent was determined to be P. ramorum based on characteristic morphology of hyphae, sporangia, and chlamydospores. Positive identification as P. ramorum was obtained with a lineage-specific LAMP assay, 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. Sequences (GenBank accessions MZ733981 and MZ733982) were aligned against reference sequences for all lineages 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). 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). Tanoaks were maintained in a growth chamber ( 20°C-day / 18°C-night temperature) with regular watering and an 18-photoperiod using F32T8 fluorescent bulbs (Phillips, Eindhoven, The Netherlands). After 7 days, brown to black lesions 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 Phytophthora-semi-selective medium. This is the first detection of the NA2 lineage causing disease in forests. The outbreak was found 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. NA2 is thought be more aggressive than the NA1 lineage present in Curry County since the mid-1990s and eradication is being pursued to limit its further spread and prevent overlap with existing NA1 and EU1 populations. The repeated introductions of novel lineages into western native plant communities highlights the vulnerability of this region to Phytophthora establishment, justifying continued monitoring for P. ramorum in nurseries and forests.