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  • Title: First detection of leaf blight and stem canker caused by Phytophthora ramorum on Brisbane box in the United States.
    Author: Blomquist CL, Rooney-Latham S, Soriano MC, Ochoa J, Zwart D.
    Journal: Plant Dis; 2020 Aug 04; ():. PubMed ID: 32749933.
    Abstract:
    Brisbane box,Lophostemon confertus (Myrtaceae) is a frost tender evergreen tree planted for its upright form, large ovate leaves and attractive white flowers which bloom in the spring. In June of 2017, the Plant Pest Diagnostics Center lab received a call from an arborist who described Brisbane box street trees dying in central Sausalito, Marin Co., California. Trees ranged from containing 10% to nearly 80% dead hanging leaves. Six trees along the same street were affected. Wilted brown leaves remained attached to branchlets covered in black cankers. Some healthy branchlets had leaves with angular spots which crossed the veins and were surrounded by yellow halos. Isolations were made onto CMA-PARP (Jeffers and Martin, 1986) from the canker and leaf spot margins. A Phytophthora species resemblingPhytophthora ramorum grew on CMA-PARP media with coralloid coenocytic hyphae, chlamydospores, and ellipsoidal semi-papillate sporangia. The internal transcribed spacer region (ITS) of rDNA was amplified and sequenced using primers PHY.OO.18F and PHY.OO.28SR (Rooney-Latham, et al. 2019). BLAST analysis of 770 base pairs of the sequenced amplicon (GenBank MK993541) showed 100% identity with the ITS sequence of the P. ramorum ex-type (MG865581). A portion of the cox2 gene was amplified and sequenced (Hudspeth et al. 2000) (GenBank MK994528) and 530 base pairs matched with 100% identity GU222130. Pathogenicity was confirmed by inoculating 3, initially 1.8-meter-tall trees in 18.9-liter pots. Prior to inoculations, trees were cut so they would fit into 122 cm high dew and growth chambers. For each tree, 3 lower branchlets measuring from 4 to10 mm in diameter were inoculated by wounding with a 6 mm punch, placing a colonized agar plugs in the wound, then wrapping with Parafilm. Lower branches were covered in plastic to protect them from subsequent zoospore inoculation. Branchlet inoculum was prepared by growing P. ramorum on V8 juice agar (V8) for 4 days at 22°C. Zoospores were prepared for leaf inoculation by taking 6 mm agar plugs from the margin of 6-day old cultures and flooding plugs in soil water for four days. Zoospores were released by transferring plugs to sterile distilled water at 4°C for 1.5 h. Leaves on the same three trees that were inoculated with the plugs were sprayed with 350 mL of zoospores (2 × 105 zoospores/mL), and placed in a dew chamber at 23°C for 48 h. Afterwards, they were transferred to a growth chamber (23°C, 12-h diurnal cycle) where the plastic was removed from the lower branches after leaves had dried. A single control tree was treated similarly with uncolonized V8 plugs, followed by a water spray. Leaf spots were visible 4 days later, with inoculated leaves turning necrotic and abscising after 3 weeks. Cankers from inoculated branchlets measured from 12 to 60 mm long after 60 days. Phytophthora ramorum was isolated from the margin of every inoculated canker and leaf spot. No P. ramorum was isolated from the control tree. To our knowledge, this is the first report of P. ramorum on L. confertus, in the world. Natural inoculum presumably came from infected Umbellularia californica trees located less than 800 m west of the trees in Sausalito. This detection will further limit the planting choices of arborists and landscapers in P. ramorum infected locations.
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