First Report of Phytophthora ramorum Causing Leaf Spot on Arbutus × reyorum 'Marina' in the United States.

Abstract

The Marina strawberry tree (Arbutus × reyorum Demoly. 'Marina') is a popular ornamental tree species, prized for its glossy evergreen foliage, display of pink and white bell-shaped blooms, and strawberry-like ornamental fruits. In April 2024, a foliar sample from a Humboldt County, California nursery, where P. ramorum had been detected earlier in the year, was submitted to the CDFA Plant Pest Diagnostic Laboratory exhibiting symptoms of marginal leaf necrosis (Fig. S1). A limited number of symptomatic strawberry trees were located near infected Cornus capitata plants. Six 6-mm-diameter disks were excised from the margins of diseased leaf tissues and cultured on semi-selective CMA-PARP media (Jeffers and Martin 1986). After approximately 7 days, white, coralloid, and coenocytic hyphae interspersed with globose chlamydospores (22.5 to 52.3 µm in diameter, n = 30), and ellipsoidal, semi-papillate sporangia (32.5 to 75 × 20 to 22.5 µm, n = 30) grew from the disks. This morphology is consistent with that reported for P. ramorum. The pathogen was genetically identified by sequencing the internal transcribed spacer region (ITS) and cytochrome oxidase subunit 1 region (cox1) using the primers ITS5/ITS4 (White et al. 1990; accession no. PQ431562) and OomCox1Levup/Fm85mod (Robideau et al. 2011; accession no. PQ438384), respectively. A BLAST search of both amplicons revealed 100% identity with the P. ramorum ex-type strain CPHST BL 55G (MG865581 and MH136973). Based on microsatellite loci, the isolate was placed within the NA2 clonal lineage (Goss et al. 2011). Koch's postulates were performed to confirm pathogenicity using 4-year-old Arbutus × reyorum 'Marina' trees (58 to 75 cm tall) grown in 3.78-liter pots. The foliage of three plants was inoculated with 15 ml of a zoospore suspension of 1 × 104 zoospores/ml following the methods of Blomquist et al. (2021). Two control plants were sprayed with 15 ml of sterile water. All plants were placed in a dew chamber at 23°C. After three days, plants were moved to a growth chamber at 23±1°C with a 12-h photoperiod. During this time, black discoloration was noted on the youngest leaves of the inoculated plants. After approximately 7 days, symptoms characteristic of Phytophthora infection were observed including drooping leaves, dieback, and dark foliar lesions extending from the petiole along the midrib into the leaf. By 13 days, the discoloration extended into the flower panicles (Fig. S2). These symptoms differed from those in the original nursery samples, which only displayed lesions along the leaf margins. Both the marginal necrosis in the submitted samples and the symptoms from the pathogenicity tests are consistent with ramorum leaf blight on many hosts under varying environmental conditions. P. ramorum was consistently isolated from symptomatic foliage of the inoculated plants, while no symptoms were observed in the control group and no Phytophthora was isolated. Although Arbutus unedo and A. menziesii are known hosts for P. ramorum (Farr and Rossman 2024), to our knowledge, this represents the first report of P. ramorum infecting Arbutus × reyorum 'Marina'. P. ramorum is a devastating pathogen affecting numerous plants across diverse environments. The identification of this popular landscape plant as a new host will increase the already extensive time required to inspect for P. ramorum in nurseries and may limit its usage in the wildland-urban interface, particularly in areas where P. ramorum is present in nearby forests.