First Report of Powdery Mildew Caused by <i>Golovinomyces ambrosiae</i> on <i>Verbena</i> × <i>hybrida</i> in the United States

<i>Verbena</i> × <i>hybrida</i>, also known as common garden verbena, has an important ornamental value for their wide range of flower colors and for attracting hummingbirds and butterflies. During the winter of 2021-2022 (December through February), more than 50% pot-grown verbena plants showed symptoms of powdery mildew in a field trial at a Syngenta Crop Protection research facility in Vero Beach, FL. Symptoms were characterized by the development of white, superficial mycelium on the adaxial side of leaves which, eventually, progressed to covering the whole surface of leaves, causing leaf discoloration, shoot distortion, and eventual plant death. Morphological characterization was carried out by observing powdery mildew colonies under the microscope. This powdery mildew forms dense patches of white mycelia, mainly on the adaxial leaf surfaces. The mycelium was a mat of hyphae with septa. Conidiophores were erect. The foot cells were straight, followed by one to three short cells bearing short chains of up to four conidia. The conidia were hyaline and ellipsoidal to doliiform in shape. Conidial germination is of the <i>Eudoidium</i> type. The conidia ranged from 25 to 32 μm long by 12 to 16 μm wide. The length to width ratio ranged between 1.6 and 2.3, but most were between 2.0 and 2.2. This is further verification of its identity as <i>Golovinomyces</i> <i>ambrosiae</i> and not <i>Golovinomyces</i> <i>latisporus</i>, because the length to width ratio of the latter species is consistently less than 2.0 (Qiu et al. 2020). Chasmothecia were not observed. Additionally, the ITS, <i>GAPDH</i>, and IGS regions were sequenced using the primer pairs ITS4/ITS5 (White et al. 1990), PMGAPDH1/PMGAPDH3R (Bradshaw et al. 2022a), and IGS-12a/NS1R (Carbone and Kohn 1999), respectively. The ITS region (GenBank number=PP924119) cannot distinguish between <i>G.</i> <i>latisporus</i> and <i>G. ambrosiae</i> and as such aligned 100% with both species on GenBank. However, the <i>GAPDH</i> and IGS regions can be used to distinguish <i>G. ambrosiae</i> from <i>G. latisporus</i> (Bradshaw et al. 2022b). The <i>GAPDH</i> (GenBank number=PP931995) and IGS (GenBank number=PP931996) regions aligned 100% with multiple <i>G. ambrosiae</i> sequences from GenBank including ON360708 and MK452567, respectively. The specimen was deposited in the Larry F. Grand Mycological Herbarium (NCSLG 24479). To confirm pathogenicity, 'Tuscany® Pink Picotee' and 'Quartz XP Violet with Eye' plugs were transplanted to 10-cm diameter pots containing ProMix potting mix and maintained in a greenhouse (± 26 °C). Inoculation was carried out 21 days after transplanting by touching infected leaves onto healthy leaves of 15 disease-free plants of each variety. Fifteen non-inoculated plants of each variety were used as controls. Typical powdery mildew symptoms and signs were first observed ten days after inoculation and the pathogen was more aggressive on 'Tuscany® Pink Picotee'. Symptoms were not observed on non-inoculated plants. The fungus was morphologically identical to the one originally recovered from infected plants in the field. There have been many reports of <i>Golovinomyces</i> spp. affecting <i>Verbena</i> spp. worldwide; however, this is the first report of <i>G. ambrosiae</i> causing powdery mildew on <i>Verbena</i> × <i>hybrida</i> in the U.S. (Braun and Cook, 2012, Choi et al., 2021; Bradshaw et al. 2024). Powdery mildews reduce plant quality and decreases the aesthetics value of infected plants, causing great losses to the ornamental industry. Correct identification of the causal agent is crucial to recommend appropriate control methods, as they may differ according to the pathogen species.