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First Report of Leaf Rust on Southern Highbush Blueberry caused by Thekopsora minima in California

Leaf rust symptoms have been intermittently observed on southern highbush blueberry (Vaccinium corymbosum L. × V. darrowii Camp) in California. Rust symptoms were first seen in 2010 near Carpinteria, CA (M. Kong, personal communication) and then in July 2016 on leaves of three blueberry cultivars Emerald, Jewel, and Snow Chaser in a field trial in Watsonville. In early 2017, leaf rust was observed again in Watsonville on the same cultivars and on various 3-year-old plants. Symptoms were found primarily on older leaves. Purplish-brown necrotic spots were present on the adaxial leaf surface, while the abaxial leaf side exhibited orange to yellow colored uredinia. Upon closer inspection using a light microscope, the uredinia were clustered on the leaf surface, ovoid in shape, and approximately 100 to 400 μm in diameter. Certain rust species possess ostiolar cells projections near the ostiole of the uredinium, which aid in taxonomic identification; however, ostiolar cells were not observed. Each uredinium contained numerous ellipsoid urediniospores with spines that emanated from the spore walls. When measured, urediniospores were 17.5 to 30 × 17.5 to 20 μm (average 24 × 19 μm, n = 25). Leaf samples with rust signs and symptoms were taken and stored at 4°C for further taxonomic examination. Potential alternate hosts (i.e., Tsuga spp.) for blueberry leaf rust were not present. After morphological evaluation, the rust pathogen was identified as Thekopsora minima P. Syd. & Syd. and two specimens from different plants were sent to the U.S. National Fungus Collection (BPI 910346 and BPI 910349) (Sato et al. 1993; Schilder and Miles 2011). DNA was extracted from six samples and two loci amplified from each sample. A 1,410-bp fragment spanning 5.8S-ITS2-28S rDNA was amplified with Rust2inv and LR6 primers (fragment 1) and a 1,697-bp fragment spanning the 18S rDNA was amplified with Rust18S-R and NS1 primers (fragment 2) (Aime 2006). Fragment 1 shared 100% identity (1,007 of 1,007 bp) with a T. minima isolate (BRIP57654) that infected V. corymbosum from Australia; fragment 2 shared >99% identity (1,207 of 1,210 bp) with the same isolate (accession nos. KC763340 and KT199391, respectively). Sequences for fragment 1 and fragment 2 from isolate (001) were deposited in GenBank (KY991374 and KY991375, respectively). Although another rust, Naohidemyces vaccinii, has been reported on V. membranaceum Douglas ex Torr. in CA, recent reports of T. minima in Australia, China, Mexico, South Africa, and in several states in the U.S. (OR, MI, DE, NY) suggest that T. minima is the most commonly encountered pathogen causing leaf rust on northern and southern highbush blueberries (Schilder and Miles 2011; Wiseman et al. 2016). To our knowledge, this is the first report of T. minima on any plant species within CA. In 2017, another rust species was found in Rubus spp. that was unobserved in CA previously, which could be related to significant winter precipitation (Shands et al. 2018). Since the alternate host Tsuga spp. is outside the central coast region of CA, further research is necessary to understand the epidemiology of T. minima in CA.