First report of Peronospora cf. mesembryanthemi on ice plants in the United States.

In April and December 2015 and the following April and May, the Oregon State University (OSU) Plant Clinic received for diagnosis Delosperma ‘Ruby Jewel’ from a Washington nursery; dying Aptenia ‘Red Apple’ plants from multiple landscape plantings in San Diego Co., CA, where symptoms were widespread; Delosperma ‘Ruby Jewel’ from a nursery in Santa Cruz Co., CA; ‘Jewel of the Desert Topaz’ from a nursery in Washington State; and an unknown cultivar of Delosperma from a nursery in Monterey Co., CA. Delosperma and Aptenia species are succulent ground covers previously classified as Mesembryanthemum. All plants were showing similar symptoms: rapidly decaying water-soaked foliage that desiccated under dry conditions, or that decomposed rapidly while green under moist conditions. All material examined had downy mildew sporulating on the foliage. Measurements from Aptenia and Delosperma were similar; the former are given here. Conidiophores were aseptate, multiply bifurcated, recurved at the tips, and varied in length from 100 to 725 μm (mean 358 μm; n = 32) with a width of 6 to 15 μm (mean 11.5 μm, n = 20). Distance from base to the first branch was 32 to 552 μm (mean 177 μm, n = 20), and branch length from conidiophore to first branching tip was 37 to 147 μm (mean 95, n = 20). Conidia were ovoid to oval, pale brown, 22.5 to 45 × 15 to 26 μm (mean 33.7 × 20 μm; n = 58), with a length to breadth ratio of 1.68. Oospores were rare. Samples were deposited in the OSU Herbarium (OSC162004, 162005). We sequenced the internal transcribed spacer (ITS) region of an isolate each from Delosperma ‘Ruby Jewel’, Delosperma sp., and Aptenia ‘Red Apple’ (GenBank KX118438, KX118439, and MH059526, respectively), using primers DC6 and ITS4 (Cooke et al. 2000). We obtained 96 to 97% sequence identity to multiple species of Peronospora. There are no publicly available gene sequences for P. mesembryanthemi, nor did we find any that matched our Peronospora species. We examined the holotype specimen from the South African National Collection of Fungi (PREM: 46070), which was in poor condition, precluding measurements. We were unable to amplify the downy mildew present, even after concentrating the DNA and enriching the reaction. Based on morphology and host, our isolates most closely matched Peronospora mesembryanthemi Verwoerd, but there are some differences. Our isolates had longer and wider conidiophores and larger, slightly pigmented conidia; documentation with the holotype included a note that the conidia were not hyaline as described by Verwoerd (1924). Morphology in Peronospora can be influenced by humidity, temperature, and light (Dudka et al. 2007). Pathogenicity of P. mesembryanthemi isolate 16-480 from Delosperma sp. was demonstrated by rub inoculating two infected leaves per plant on four Aptenia plants and five leaves on one Delosperma plant. Inoculated plants were sprayed with deionized water, bagged and stored at 4°C. Uninoculated plants subject to the same conditions served as controls. Within 1 week, inoculated tissue on both the Aptenia and Delosperma plants exhibited water-soaked foliage that rapidly decayed, whereas the controls remained healthy. P. mesembryanthemi was confirmed by ITS sequencing of adjacent symptomatic tissue (100% identity match). This organism was causing extensive damage to plants in both nurseries and landscapes in California, and it continues to be problematic in the latter, especially in San Diego, Santa Barbara, and Los Angeles Counties. Previously reported in South Africa (Verwoerd 1924), Wales (Francis and Waterhouse 1988), and New Zealand (Mckenzie and Dingley 1996), this appears to be the first report of this organism in the United States.