First Report of Powdery Mildew Caused by Golovinomyces monardae on Catnip ( Nepeta cataria ) in New Jersey

Nepeta cataria (catnip; Lamiaceae) is a perennial herb naturalized in Europe, Asia, and North America. It is cultivated as a medicinal and ornamental plant and is valued for attracting pollinators, repelling certain insect pests, and its attractiveness to cats (Gomes et al., 2020). During the fall of 2019 and 2020, powdery mildew was observed on multiple catnip cultivars at the Rutgers University Snyder Research Farm, Pittstown, NJ. The disease was again observed from January 2025 onward on greenhouse-grown catnip at Rutgers University, New Brunswick, NJ. Symptoms consisted of white powdery colonies on adaxial and abaxial leaf surfaces and petioles that coalesced into large patches. Severely infected leaves became chlorotic, necrotic, papery, and prematurely defoliated. Disease incidence was 100% on catnip plants with an average disease severity of 60% leaf area affected. Morphological characteristics of isolate CR-1 isolated from catnip 'CS75', were consistent with Golovinomyces spp. (Braun and Cook, 2012). Hyphae were hyaline, 5–8 μm wide, with slightly nipple-shaped appressoria. Conidiophores measured 55–145 × 10–14 μm. Foot cells measured 50–100 μm and were mostly curved at the base. Conidia (n = 30) were catenescent, ellipsoid to doliiform, produced in chains of 3–5, and measured 25.2–40.0 × 15.6–26.2 μm. Fibrosin bodies and chasmothecia were not observed. To confirm identification, the internal transcribed spacer (ITS) and large ribosomal subunit (28S) rDNA regions were amplified using JumpStart Taq Ready Mix (Millipore Sigma) with primers ITS5/P3 and LSU1/LSU2 (Bradshaw and Tobin, 2020). ITS amplicons were purified (Monarch DNA Gel Extraction Kit, New England Biolabs), re-amplified with PM5G/PM6G (Scholler et al., 2016), and sequenced. Sequences were deposited in GenBank under the accessions PZ595194 (ITS) and PZ595195 (28S). Blast searches showed 99-100% identity with Golovinomyces spp. (GenBank: ITS: AB769437.1; GenBank:28S: LC076831.1). Concatenated ITS and 28S sequences were aligned with members of the Golovinomyces biocellatus complex (Scholler et al., 2016; Takamatsu et al., 2013). Maximum-likelihood analysis in MEGA12 (Kumar et al., 2024) placed isolate CR-1 within the Golovinomyces monardae clade with 95% bootstrap support. A voucher specimen was deposited in the U.S. National Fungus Collections, USDA-ARS, Beltsville, MD (accession no. BPI 937340). To fulfill Koch's postulates, infected catnip 'CS75' leaves were pressed onto healthy 'CS75' leaves. Ten plants were inoculated, and ten non-inoculated plants served as controls. Plants were maintained in an indoor grow room at 23°C, 50% relative humidity, and a 14-h photoperiod. Powdery mildew developed on inoculated plants within 6–8 days, whereas control plants remained symptomless. The reisolated powdery mildew was sequence and morphologically identical to the original isolate. The pathogenicity assay was repeated twice with similar results. G. monardae has previously been reported on mountain mint and spearmint (Lamiaceae) in the United States (Klingeman et al., 2018; Rajmohan et al., 2019). Catnip may serve as an inoculum reservoir for economically important mint crops, making accurate species identification important for disease management. To our knowledge, this is the first report of G. monardae causing powdery mildew on catnip in the United States.