First Report of Leaf Rust on California Blackberry (Rubus ursinus) Caused by Kuehneola uredinis in California

In 2017, several wild California blackberry plants (Rubus ursinus Cham. & Schldl.) were observed with symptoms of leaf rust and severe defoliation within Monterey County, CA (GPS coordinates N 36°35′33.025″ W 121°49′58.67″). Representative samples (n = 5) were collected and stored at 4°C. Symptoms appeared primarily on older leaf surfaces and leaf edges. The adaxial leaf surface had circular to angular spots surrounded by brown necrotic tissue, and the abaxial surface had bright yellow-orange rust pustules (uredinia). Uredinia were erumpent and were 100 to 400 µm in diameter. Urediniospores were broadly obovoid, pale yellow-orange in color, and measured 28 to 40 × 24 to 32 µm (average 33 × 27 µm, n = 25) with echinulate spore walls 1.0 to 1.5 µm thick. Based on morphology and host information, the pathogen was identified as Kuehneola uredinis (Link) Arth., and a sample was deposited in the U.S. National Fungus Collection (BPI 910348) (Arthur 1906). To confirm this identification, DNA was extracted from urediniospores of one representative sample, and two separate PCR reactions were performed. A 1,755-bp fragment spanning 5.8S-ITS2-28S gene region of rDNA was amplified with Rust2inv and LR6 primers (fragment 1), and a 2,684-bp fragment spanning the 18S gene region of rDNA was amplified with Rust18S-R and NS1 primers (fragment 2) (Aime 2006). PCR products were sequenced with primers LR3 and LR0R (28S gene region) and NS5, NS6, and NS4 (18S gene region), described in Aime (2006). Fragment 1 shared 99% identity with a K. uredinis isolate (HMUT100497) from China (GenBank accession KU059177). Fragment 2 shared 98% identity with a Phragmidium sp. isolate (MB) from the United States (GenBank accession EF014363). The sequence match to the Phragmidium sp. isolate (MB) was likely owing to the absence of the 18S sequence of K. uredinis in the GenBank database. Sequences were deposited for fragment 1 and fragment 2 (GenBank accessions MF158087 and MF158088, respectively). To confirm the Rubus spp., genomic DNA was extracted from leaf tissue, and PCR was performed using ITS1 and ITS4 primers found in White et al. (1990). The product was sequenced with the amplification primers and shared 100% identity with R. ursinus internal transcribed spacer 1 and 2 (GenBank accession AF055794). R. ursinus is known as a parent of the loganberry (Rubus × loganobaccus L.H. Bailey), which is commercially grown and cultivated. K. uredinis has been reported on R. ursinus in Australia, New Zealand, and South Africa, and within the United States in Washington (Shaw 1973). K. uredinis has been reported in California on Santa Catalina Island on R. vitifolius Cham & Schldl., which is often a synonym of R. ursinus, but throughout the literature these are often described as distinct species (Millspaugh and Nuttall 1923). To our knowledge K. uredinis has never been reported in mainland California on R. ursinus or R. vitifolius. This is of particular concern given that (i) urediniospores disperse long distances by wind and (ii) several Rubus spp. (e.g., blackberry, loganberry, and raspberry) are susceptible in California and are economically important crops within 20 km of this reported observation.