In late autumn of 2015, Oregon grown ‘Peach Sorbet’ blueberry (Vaccinium corymbosum L.) leaves were submitted to the Oregon State University Plant Clinic with concern of leaf rust. Shortly after, additional symptomatic material from cultivars Jelly Bean and Blue Crop were collected from a different location in Oregon. Adaxial symptoms initially appeared as subcircular to angular chlorotic spots, which then developed necrotic reddish to brown centers, often with reddish purple margins. In severe infections, lesions coalesced and premature defoliation occurred. Uredinia were hypophyllus, dome shaped, solitary to grouped, sometimes coalescing, erumpent, lacking conspicuous ostiolar cells, and approximately 150 to 300 µm in diameter. Urediniospores were yellowish orange, broadly obovate or subellipsoidal, and measured 19 to 32 × 14 to 22 µm (average: 22 × 18 µm, n = 30). Cell walls were minutely echinulate, hyaline, and measured 1 to 1.5 µm. No telia or teliospores were observed. The pathogen was identified morphologically as Thekospora minima (Arthur) P. Syd. & Syd. Voucher specimens were deposited into the Oregon State University Herbarium (OSC Accessions: OSC153683, OSC153684, and OSC153685). Genomic DNA was extracted from urediniospores of three isolates collected from two different locations in Oregon. Using the primer pair Rust2inv/LR6, approximately 1,400 bp of each isolate was amplified, spanning parts of the 5.8S, ITS2, and 28S large subunit regions (Aime 2006). GenBank BLAST results of the isolates from the three cultivars (GenBank Accessions: KU136371, KU136372, and KU136373) resulted in 99 to 100% identity with T. minima (GenBank Accession: KC763341.1; 100%). This fungus has been reported in the United States from Michigan (Schilder and Miles 2011), Delaware, and New York (Sato et al. 1993). To our knowledge, this is the first report of T. minima on blueberries in Oregon. Two native hemlocks in Oregon, Tsuga mertensiana and T. heterophylla, can serve as alternate hosts for T. minima and are necessary for the completion of the rust’s lifecycle. To date, the fungus has not been recovered from Tsuga. In western Oregon, T. minima is mainly problematic for nurseries and greenhouse production, as symptoms typically do not recur in field plantings after premature defoliation. T. minima is a quarantine status pest in the European Union and elsewhere, hence this find will place limitations on nursery plant and fruit growers hoping to export to countries where the quarantine is in effect. © 2016, American Phytopathological Society. All rights reserved.
