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

Matthiola incana, 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 Hyaloperonospora (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 H. parasitica from M. incana in Italy (MT275635; Bertetti et al. 2021) and M. sinuata in Spain (EU049268; Göker et al. 2009); cox1 and LSU had 649/681 and 755/764 basepairs matching to H. mayli, respectively (KC494943, KC495022), and cox2 had 553/577 to H. arabidopsidis (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 Hyaloperonosopora parasitica in California (Koike, 2000) and Italy (Bertetti et al. 2021). Peronospora matthiolae has also been found associated with Matthiola 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 Hyaloperonospora, and that H. parasitica is limited to the host Capsella bursa-pastoris (Göker et al. 2009), we identified this downy mildew as Hyaloperonospora 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.