An outbreak of Septoria canker caused by Sphaerulina musciva on Populus trichocarpa in eastern Oregon
Kelsey L. Søndreli, Susanna Keriö, Kenneth Frost, Wellington Muchero, Jin-Gui Chen, Kkathy Haiby, Carlos Gantz, Gerald Tuskan, and Jared M. LeBoldus
20-Oct-2020, Plant Disease 104(12); https://doi.org/10.1094/PDIS-03-20-0494-PDN
Branch and stem cankers were observed in a Populus trichocarpa L. plantation in late summer of 2018 near Boardman, Oregon, U.S.A. Stem cankers were flat faced with swollen margins, and the bark in the center of the cankers was black and depressed. Two hundred fifty-four out of 1,054 trees (25%) observed had cankers. Symptomatic stems and branches from several trees were brought to Oregon State University (OSU) for identification. Cankers were surface disinfested in 5% NaOCl for 2 min and rinsed twice in deionized H2O for 2 min. Bark was removed from cankers, and pieces of wood at the margin between healthy and necrotic tissue were plated on KV8 medium amended with streptomycin sulfate at 100 mg/liter and chloramphenicol (Amresco) at 240 mg/liter. Eight sporulating colonies typical of Sphaerulina musiva (Peck) Quaedvlieg, Verkley, and Crous (syn. = Septoria musiva Peck) developed within 7 days of plating. S. musiva was identified by morphological characteristics (conidium size = 28 to 54 × 3.5 to 4 µm) and confirmed by comparing the sequence of the ITS region (accession nos. MN275180 to MN275187) to JX901814 with 99% identity. We tested 13 additional cankers collected from the plantation using S. musiva-specific primers. Eight of the 13 samples were positive for S. musiva DNA. Dormant cuttings from three genotypes of P. trichocarpa (BESC-184, BESC-259, and BESC-388) were collected in a field planting near OSU. These were planted in cone-tainers in the greenhouse at OSU and grown to approximately 30 cm in height. Four replicates of each of the three genotypes were inoculated in the greenhouse with one Oregon isolate using a suspension of 1 × 106 conidia/ml. The entire tree was sprayed until runoff using a spray bottle. The experimental design was a completely randomized design with four replicates of each genotype. Sterile deionized H2O was sprayed on one control tree of each genotype. The mean number of resulting cankers were BESC-184 = 11, BESC-259 = 1, and BESC-388 = 13.75. The controls did not develop any cankers. The pathogen was successfully reisolated from one canker from each genotype. S. musiva can cause severely damaging leaf spot and stem canker diseases in the eastern and central United States and Canada. Susceptible Populus species and hybrids develop cankers that can girdle and weaken stems, leading to breakage or tree death. A previous survey of the region did not detect S. musiva. This outbreak of Septoria canker underscores the potential threat of this pathogen to P. trichocarpa in the Pacific Northwest, where it is a keystone species in riparian ecosystems.