First report of Phytophthora ramorum causing leaf spots on Cornus capitata (evergreen dogwood) in the United States

Authors: BELISLE, W - California Department Of Food And Agriculture; ROONEY-LATHAM, S - California Department Of Food And Agriculture; SORIANO, M - California Department Of Food And Agriculture; Grunwald, Niklaus - Nik; BLOMQUIST, C - California Department Of Food And Agriculture

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2024
Publication Date: 2/12/2024
Citation: Belisle, W.H., Rooney-Latham, S., Soriano, M.C., Grunwald, N.J., Blomquist, C.L. 2024. First report of Phytophthora ramorum causing leaf spots on Cornus capitata (evergreen dogwood) in the United States. Plant Disease. 108(4):1119. https://doi.org/10.1094/PDIS-12-23-2638-PDN.
DOI: https://doi.org/10.1094/PDIS-12-23-2638-PDN

Interpretive Summary: Evergreen dogwood (Cornus capitata ) is a bushy evergreen tree or shrub native to East Asia grown for its showy creamy bracts in late spring. In February 2023, a sample of foliage with leaf spots and tip dieback from C. capitata was submitted from a Humboldt Co. nursery as part of a CDFA inspection program for diagnosis. We conducted detailed research to establish that the symptoms were caused by the sudden oak death pathogen Phytophthora ramorum. To our knowledge, this is the first report of P. ramorum infecting C. capitata in the United States. We also for the first time inoculated an reisolated the pathogen from dogwood thus completing Koch’s postulates on any Cornus spp. and demonstrating that this pathogen is the causal agent of disease on dogwood. This work has implications for managing sudden oak death in California nurseries.

Technical Abstract: Cornus capitata Wall. ex Roxb. (evergreen dogwood) is a bushy evergreen tree or shrub native to East Asia grown for its showy creamy bracts in late spring followed by attractive red fruit. In February 2023, a sample of foliage with leaf spots and tip dieback from C. capitata ‘Mountain Moon’ was submitted from a Humboldt Co. nursery as part of a CDFA inspection program for Phytophthora ramorum. The leaf spots were medium to dark brown, irregularly shaped, and ranged from 5 to 8 mm in diameter. They were located primarily along the leaf midrib and covered up to 1/4 of the leaf surface. Six 6-mm-diameter leaf discs taken from margins of brown lesions and tip dieback were plated on Phytophthora selective CMA-PARP (PARP) media (Jeffers and Martin 1986). After 6 to 10 days, colonies resembling P. ramorum, with coralloid coenocytic hyphae, chlamydospores, ellipsoidal semi-papillate and caducous sporangia, and a relatively slow growth rate were recovered. Abundant sporangia formed singly or in clusters on sympodially branched sporangiophores (n = 50), varying in size from 35 to 60 µm × 20 to 30 µm (mean 45.6 × 24.8 µm) with a length/breadth ratio ranging from 1.3 to 2.3 (mean 1.8). Chlamydospores (n = 50) ranged from 35 to 62 µm in diameter (mean 51.9 µm) on 14-day-old PARP cultures. The internal transcribed spacer region (ITS) using primers ITS5/ITS4 (White et al. 1990; accession no. OR636225) and cytochrome oxidase subunit 1 region (cox1) using primers OomCox1Levup/Fm85mod (Robideau et al. 2011; OR635665) of one isolate (0254-32A) were amplified and sequenced. BLAST analysis showed 100% identity of both regions to P. ramorum ex-type strain (MG865581 and MH136973). Microsatellite loci placed the P. ramorum isolate in the NA2 clonal lineage (Goss et al, 2011). Pathogenicity of P. ramorum isolate 0254-32A was tested using 5 C. capitata plants (2.5-year-old, 28-cm-tall, 3.78-liter pot). Zoospore inoculum was produced as described in Blomquist et al. (2021). Above ground parts of each plant were sprayed with inoculum (15 ml, 1.3 × 105 zoospores/ml). Inoculated plants were incubated in a dew chamber in the dark at 23°C for 72 h and then placed in a 23±1°C growth chamber with a 12-h photoperiod. Five control plants were treated as above but with sterile water instead of the zoospore suspension. Two days after inoculation, brown spots were visible on leaves on all inoculated plants, initiating from where the drops of inoculum had persisted. After 3 days, brown lesions, from water drop- to majority of entire leaf-sized, were observed on approximately 75% of inoculated leaves. After 6 days, lesions expanded to the edges of leaves, causing leaf curling and defoliation. Lesions stopped expanding after 3 weeks, and by 4 weeks, most infected leaves had abscised, with no new infections observed. Phytophthora ramorum was consistently isolated from foliar lesions of inoculated plants on PARP. P. ramorum was not isolated from leaf or stem tissues of control plants, which remained asymptomatic during the 4-week experiment period. Phytophthora ramorum was detected on C. capitata in the UK in 2015 (DEFRA 2015). To our knowledge, this is the first report of P. ramorum infecting C. capitata in the United States and the completion of Koch’s postulates on any Cornus spp. Incidence on C. capitata in the California nursery was low. However, their proximity to other infected foliar hosts suggests Cornus spp. may present a potential risk for the spread of P. ramorum.