First report of Hyaloperonospora sp. causing downy mildew of stock (Matthiola incana) in Ohio

Authors: HAVAN, AMBROSIA - The Ohio State University; LAIRD, ALEACIA - The Ohio State University; Salgado-Salazar, Catalina; MARTING, DANA - The Ohio State University; PEDUTO HAND, FRANCESCA - The Ohio State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/24/2024
Publication Date: 3/10/2024
Citation: Havan, A.E., Laird, A.E., Salgado-Salazar, C., Marting, D.E., Peduto Hand, F. 2024. First report of Hyaloperonospora sp. causing downy mildew of stock (Matthiola incana) in Ohio. Plant Disease. 109(3). https://doi.org/10.1094/PDIS-08-24-1708-PDN.
DOI: https://doi.org/10.1094/PDIS-08-24-1708-PDN

Interpretive Summary: Fungi and fungus-like organisms cause a great many diseases on economically important plants, resulting in yield losses or outright death of the host. Downy mildews cause some of the most severe and destructive diseases of ornamental plants, impacting both the yield and the quality of the final product. This study uses DNA sequences and microscopy to describe the first observation of a Hyaloperonospora sp. on ornamental stock plants (Matthiola incana) grown in greenhouses in the state of Ohio. Although the potential impact this finding has on Ohio's ornamental plant industry is still unknown, the results of this study have important implications for production and disease management. The information from this study will be useful to plant pathologists, regulatory officials and others involved in ornamental stock plant industry.

Technical Abstract: Matthiola incana, commonly known as stock, is a species of flowering plants 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, where disease incidence was approximately 50%. Water-soaked lesions, chlorosis, necrosis, and white mycelial growth 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, elliptic 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 and host distribution indicates 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. Using GenBank BLASTn, the ITS sequences showed 99.7% similarity to Hyaloperonospora parasitica from Matthiola incana in Italy (MT275635; Bertetti et al. 2021) and M. sinuata in Spain (EU049268; Göker et al. 2009); cox1 and LSU showed 96% and 98.8% similarity to H. mayli, respectively (KC494943, KC495022), and cox2 showed 96% similarity to H. arabidopsidis (NC_063789). DNA sequences were deposited in GenBank under Accession No. PP541422-3 (ITS), PQ134094-5 (LSU), PQ139659-60 (cox1), and PP750813-4 (cox2). To fulfill Koch’s postulates, two 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. After 5 days, chlorosis, wilting, along with white sporulation on the abaxial side of leaves were observed on the inoculated plants, while the non-inoculated controls remained healthy. Additional inoculations on the cultivar Iron Marine produced identical results. Morphological and molecular identification of the pathogen confirmed the original identification. Downy mildew of stock was previously reported as caused by Peronosopora parasitica in California (Koike, 2000) and Italy (Bertetti et al. 2021). Peronospora matthiolae, a species likely to belong to Hyaloperonospora, has also been found associated with Matthiola sp. in Europe (Farr et al. 2024), however no taxonomic resources for this species are currently available to provide accurate species comparison. As H. parasitica is limited to the host Capsella bursa-pastoris (Göker et al. 2009), we identified the downy mildew species on stock as Hyaloperonospora sp. This is the first report of this species causing downy mildew of stock in 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 it is persistent and pervasive, especially in intensive farming systems with limited rotations such as high tunnel cut flower production.