First Report of Gnomoniopsis idaeicola Causing Cane Wilt and Canker in Commercial Blackberry Fields in Oregon

Oregon is the leading producer of blackberries in the United States (USDA 2021). In November 2020, we visited a seven-year-old, 5.6 HA field of thornless trailing cultivar ‘Columbia Star’ in Marion County, Oregon in response to grower reports of rapid plant death that they called ‘Blackberry Collapse.’ The pattern of disease in the field was semi-circular patches of dead plants. We estimate that ca. 17% of the plants were killed. Symptoms included fewer and stunted primocanes, floricane reddening, premature floricane senescence, or elliptical or irregular brown to purple cane lesions often near the base of necrotic petioles clasping the cane. Twenty-two cane lesions were collected, surface-disinfested, and placed on potato dextrose agar with streptomycin. In addition to common Rubus cane pathogens (Kalmusia coniothyrium, Botrytis spp., Diaporthe spp., and Seimatosporium lichenicola), Gnomoniopsis idaeicola was isolated from 23% of the samples. After isolation of G. idaeicola from ‘Columbia Star’, we also isolated the pathogen from symptomatic ‘Black Diamond’ blackberry in fields in Washington and Linn Counties in Oregon. G. idaeicola grew as white- to cream-colored circular colonies on the agar surface with sparse to dense aerial hyphae. Some isolates produced cream-colored exudate from irregular conidiomata. Conidia were single-celled, hyaline, oval to ellipsoid, and ranged from 4.9 to 6.9 µm long and 1.2 to 2.5 µm wide (n = 100). Perithecia and ascospores were not seen on canes or in culture; molecular methods were used to identify the fungus. Genomic DNA was extracted from nine isolates and β-tubulin, ITS region, and tef-1α were amplified using the primers and conditions described by Walker et al. (2010). Amplicons were Sanger sequenced in both directions by the Core Facilities of the Center for Quantitative Life Sciences of Oregon State University, Corvallis, OR. Sequences were deposited in GenBank (β-tubulin OK539758-OK539766; ITS OK348854-OK348862; tef-1α OK539767-OK539775). β-tubulin sequences (737 bp) were 100% identical to sequence of G. idaeicola from Rubus procerus, Oregon (GU320783). ITS sequences (518 bp) were 99.2 to 100% identical to G. idaeicola CBS125674T (GU320796). tef-1α sequences (860 bp) were 99.9 to 100% identical with reference sequences of G. idaeicola. Phylogenetic analyses of concatenated sequences using Tamura-Nei neighbor-joining (Tamura et al. 2004) confirmed identity. Pathogenicity of G. idaeicola isolates was tested in repeated experiments on four to five detached ‘Black Diamond’ primocanes and potted ‘Columbia Star’ plants with 4 mm hyphal plugs placed on fresh wounds and wrapped in parafilm; controls were treated with sterile agar plugs (Ellis et al. 1984; Stevanović et al. 2019). By 14 days, in each experiment, expanding necrotic lesions were observed on canes inoculated with G. idaeicola, but not with sterile agar plugs. ITS-sequence of fungi isolated from lesions confirmed identity as G. idaeicola, fulfilling Koch’s postulate. G. idaeicola was described as a new species by Walker et al. (2010) from perithecia collected from wild Rubus spp. in the Pacific Northwest. Despite its presence on native Rubus, G. idaeicola has not been reported in commercial blackberry fields in OR. Stevanović et al. (2019) identified G. idaeicola as an important pathogen causing canker, wilt, and death of blackberry in Serbia. Surveys of Oregon blackberry fields for G. idaeicola are ongoing, along with research on the epidemiology and management of this emerging pathogen.