In May 2022, a heavily diseased patch of the spring ephemeral, Cardamine concatenata, was observed in Gambrill State Park (39.46425° N, 77.49221° W) in Frederick County, Maryland, USA. Symptoms included necrotic patches delimited by veins, with variable chlorosis (Fig. 1). Sparse grey sporulation was visible on the abaxial leaf surface. Disease incidence in the 10 × 20 m patch was 100% and severity was 20 – 100% on individual plants. Morphological examination found hyaline, ovate sporangia (average 18.3 ×15.6 μm, length:width 1.18) borne singly on curved to slightly curved sterigmata, typically arising from paired branchlets (Figs. 2, 3). Branchlets (mean 7.1 μm wide) extended from sporangiophores with a mean primary width of 11.7 μm. Oospores (mean diameter 35.9 μm) were prominent in cleared foliar tissue (Fig. 4). Based on these characteristics, geography, and host, Hyaloperonospora cardamines-laciniatae was hypothesised (Voglmayr et al., 2014). A voucher specimen was deposited in the U.S. National Fungus Collection (BPI 911234). To confirm the pathogen identity, DNA was extracted from 10–20 sporangiophores (FastDNA kit; MP Biomedicals, USA). Three DNA regions (ITS, LSU and cox2) were amplified using DC6-ITS4 followed by ITS4-ITS6 (Cooke et al., 2000) in a semi-nested reaction; LR0R and LR5 (Vilgalys & Hester, 1990); and Cox2-F and Cox2-R (Hudspeth et al., 2000), respectively. Amplicons were cleaned (ExoSAP-IT; Applied Biosystems, USA) and bidirectionally sequenced (Eurofins Inc., USA). Blastn analysis determined that the obtained sequences had the closest identity to H. cardamines-laciniatae (Table 1). Although downy mildew species are increasingly described using a narrow, one pathogen-one host concept, our data and the available literature indicate H. cardamines-laciniatae has at least three hosts, Cardamine concatenata, C. diphylla, and C. laciniata (Goker et al., 2004; Voglmayr et al., 2014). Cardamine concatenata is a native understory plant in eastern U.S. forests in which the above-ground plant parts are only present for two to three months of the year. Native butterflies (Pieris spp.) depend on Cardamine spp. foliage to complete their lifecycle (Augustine & Kingsolver, 2018). It is unknown whether the downy mildew severity observed at this site would influence C. concatenata foliar longevity or suitability for Pieris spp. oviposition. The earliest reports of H. cardamines-laciniatae are from Iowa (1903) and Ohio, USA (1908; Constantinescu, 1992). More recent reports are from C. diphylla and C. laciniata in 2003 (Voglmayr et al., 2014; Voglmayr, pers. comm.). This is the first report of H. cardamines-laciniatae-induced disease incidence and severity on C. concatenata, a critical host for Pieris spp. butterflies. This research was supported by the U.S. Department of Agriculture, Agricultural Research Service. USDA is an equal opportunity provider and employer.