First Report of Phytophthora cactorum Causing Crown Rot of Shepherdia × utahensis in the United States

Shepherdia argentea is a shrub native to the intermountain west, often found in riparian areas. S. rotundifolia is also native but is limited in distribution to xeric environments in Arizona and southern Utah. The interspecific hybrid S. rotundifolia × argentea, named Shepherdia × utahensis, was made in an attempt to create an attractive, locally adapted, low water use landscape shrub. In fall of 2015, one of several failing young S. × utahensis plants from an ornamental native plant production and evaluation garden in Utah was submitted to the Oregon State University Plant Clinic for diagnosis. Additional plants from the site had already been destroyed. Symptoms included sparse foliage and extensive water-soaked necrosis in the root crown and lower branches. Tissue samples from the necrotic margins were surface disinfected (3 min, 10% household bleach [v/v]), rinsed, plated to 5% V8 modified PARP (0.1 g/liter of dichloran substituted for PCNB [PARD]), and incubated at 20°C. Canker margins were positive for Phytophthora spp. using a Phytophthora ELISA kit (Agdia, Elkhart, IN), and a Phytophthora sp. was recovered in culture. Sporangia were subglobose to ovoid (30 × 42 μm, n = 25), sympodially arranged, caducous, with short pedicels and single papillae. Antheridia were subspherical and paragynous. Oospores formed in single culture were hyaline, smooth-walled, and plerotic (23 to 30 μm, n = 25). DNA from a hyphal tipped culture was used for internal transcribed spacer (ITS) and cox2 gene amplification and sequencing (Bonants et al. 1997; Martin et al. 2004). An 1,107-nucleotide BLAST of the ITS sequence (GenBank accession no. KX906603) shared 99% identity with 53 different P. cactorum isolates including Q-bank’s representative strain, CBS 108.09 (Q-bank 2017). A 413-nucleotide BLAST of the cox2 sequence (GenBank accession no. MF595111) shared 100% identity with eight P. cactorum isolates. Using the curated Phytophthora-ID, both sequences shared 99% identity with P. cactorum isolates (Grünwald et al. 2011). The pathogen was identified as P. cactorum (Lebert & Cohn) J. Schröt., based on morphological features and sequence identity. Pathogenicity was confirmed by placing a 3-day-old PARD plug (4 mm in diameter) onto a shallow wound directly above the root crown of 1-year-old S. × utahensis plants and wrapping the wounds in Parafilm. Plants were grown at 21 to 23°C in a greenhouse. Two trials were completed, each with three P. cactorum-inoculated plants and three plants treated with uncolonized PARD plugs. Inoculated plants began developing symptoms 2 weeks postinoculation (wpi), initially exhibiting brown to black water-soaked lesions around the inoculation site; all inoculated plants developed symptoms similar to those on the original plant. Control plants lacked any symptoms. Reisolations were made at 2 or 5 wpi, and a combination of sequencing and morphological comparisons confirmed recovery of P. cactorum from the canker margins of all inoculated plants. We were unable to recover Phytophthora from the symptomless control plants. Previously, it was found that inoculated Shepherdia argentea is resistant to P. parasitica (Kratsch and Rupp 2010), but we found S. × utahensis is susceptible to P. cactorum. This susceptibility may curtail the utility of the hybrid plant in Utah’s native plant breeding and water conservation landscape programs. This report appears to be the first of a naturally occurring Phytophthora infection of any Shepherdia species in the United States.