First Report of <i>Hyaloperonospora</i> sp. Causing Downy Mildew of Stock (<i>Matthiola incana</i>) in Ohio

<i>Matthiola incana</i>, commonly known as stock, is a flowering plant species in the Brassicaceae popularly used as a cut flower due to its fragrant, long-lasting blooms. In September 2023, stock 'Iron White' plants displaying symptoms and signs of downy mildew were observed within a high tunnel in a cut flower farm in Franklin Co., OH. Disease incidence was approximately 50%, covering an area of 140 m². Water-soaked lesions, chlorosis, necrosis, and white sporulation on the abaxial side of the leaves were observed. Microscopic observation of diseased specimens (BPI 911250-51) revealed dichotomously branched sporangiophores with slender, curled tips bearing a single hyaline, ellipsoid to globose sporangium. The width of the sporangiophore trunks and ultimate branch lengths were on average 7.9 ± 1.2 µm (n=4) and 12.9 ± 4.39 µm (n=22), respectively. Sporangia measured on average 22.2 ± 2.4 µm × 19.6 ± 1.8 µm (n=100). Morphological observations indicate that this species belongs to the genus <i>Hyaloperonospora</i> (Constantinescu and Fatehi 2002). PCR amplification and bidirectional sequencing of the rDNA ITS1-ITS2, LSU D1-D3, and mtDNA cox1 and cox2 regions were performed from DNA extracted from spores and mycelium. Using GenBank BLASTn, the ITS sequences had 822/823 and 830/832 basepairs matching to <i>H. parasitica</i> from <i>M. incana</i> in Italy (MT275635; Bertetti et al. 2021) and <i>M. sinuata</i> in Spain (EU049268; Göker et al. 2009); cox1 and LSU had 649/681 and 755/764 basepairs matching to <i>H. mayli</i>, respectively (KC494943, KC495022), and cox2 had 553/577 to <i>H. arabidopsidis</i> (NC_063789). To fulfill Koch's postulates, four 7-week-old 'Iron White' plants were inoculated by tapping one heavily sporulating leaf from the originally diseased plants against the adaxial surface of 3-4 true leaves of each healthy plant. Two plants were tapped by hand with a clean set of gloves to serve as non-inoculated controls. Plants were incubated under a dark plastic tent with a humidifier set to run continuously for 48 h, then uncovered and maintained in the greenhouse at 21-26°C. Chlorosis, wilting, along with white sporulation on the abaxial side of leaves were observed on the inoculated plants 5 and 16 DPI, respectively, while the non-inoculated controls remained healthy. Inoculations were repeated on 'Iron Marine' plants with identical results. Morphological and molecular characterization confirmed the original identification (PP541422-3 [ITS]; PQ134094-5 [LSU]; PQ139659-60 [cox1]; PP750813-4 [cox2]. Downy mildew of stock was previously reported as caused by <i>Hyaloperonosopora parasitica</i> in California (Koike, 2000) and Italy (Bertetti et al. 2021). <i>Peronospora matthiolae</i> has also been found associated with <i>Matthiola</i> sp. in Europe (Farr et al. 2024). Given that the taxonomy of downy mildew species affecting Brassicaceae is under revision, we have strong morphological evidence that our isolate belongs to the genus <i>Hyaloperonospora</i>, and that <i>H. parasitica</i> is limited to the host <i>Capsella bursa-pastoris</i> (Göker et al. 2009), we identified this downy mildew as <i>Hyaloperonospora</i> sp., which is a first report for Ohio. Due to its suitability for cold season production, stock is a popular cut flower in Ohio. Downy mildew poses a significant threat to production, as once present in an environment may be persistent and pervasive, especially in intensive farming systems with limited rotations such as high tunnel cut flower production.