Downy mildew disease of New England aster (Symphyotrichum novae-angliae) caused by Basidiophora simplex in New York

Authors: WALLACE, EMMA - Orise Fellow; Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2018
Publication Date: 5/17/2018
Citation: Wallace, E.C., Crouch, J.A. 2018. Downy mildew disease of New England aster (Symphyotrichum novae-angliae) caused by Basidiophora simplex in New York. Plant Disease. 102(11):2382. https://doi.org/10.1094/PDIS-04-18-0555-PDN.
DOI: https://doi.org/10.1094/PDIS-04-18-0555-PDN

Interpretive Summary: Death, deformation, and defoliation frequently occur when plants are infected by fungal-like water molds called downy mildews. This research describes a novel outbreak of downy mildew disease affecting New England aster in New York. These widely grown native plants are an important source of nectar for bees, migrating monarch butterflies and other pollinators, and are widely cultivated as a showy fall-blooming garden plant across North America. This research is significant because it is the first time this downy mildew disease has been seen on New England aster in New York for over 140 years. Knowledge of this disease re-occurrence will be useful to plant regulatory officials working to control the spread of downy mildew diseases in the United States, and provides information to gardeners, landscapers, and commercial growers.

Technical Abstract: The native perennial New England aster (Symphyotrichum novae-angliae; syn.=Aster novae-anglicae) is ubiquitous throughout most of the United States, as they self-seed and are well-adapted to many environments. New England asters are valued for their prominent dense clusters of purple flowers that attract monarch butterflies (Brouillet et al. 2006). In July of 2017, 100% of the New England asters grown in an estate’s public gardens in Nassau County, New York exhibited symptoms of downy mildew disease. Symptoms included small rectangular chlorotic lesions on the upper leaf surface. Many small lesions occurred close together, forming large chlorotic areas on the leaf, eventually leading to patches of necrosis. Abaxial lesions displayed abundant white mycelial growth. Microscopic inspection revealed abundant hyaline, unbranched sporangiophores, slightly curved and club-like, 192.9 to 261.1 µm long (avg. 227.7) and 6.6 to 13.6 µm wide (avg. 10.1), having a wide, rounded apex 11.7 to 28.1 µm wide (avg. 22.5) that held 8 to 14 straight projections, 7.0 to 12.2 µm long (avg. 9.8) and 1.5 to 2.9 µm wide (avg. 2.2). Smooth, colorless sporangia were 22.3 to 31.7 x 20.3 to 30.0 µm (avg. 25.4 x 26.7 µm) ranging from globose to broadly ellipsoidal. Zoospores and oospores were not observed. The morphology is characteristic of the genus Basidiophora (Barreto & Dick 1991). All previous reports of downy mildew on S. novae-angliae are attributed to B. entospora sensu lato (Farr & Rossman 2017) or B. simplex (Sökücü & Thines 2014). DNA was extracted and PCR products amplified with cox2 primers (Choi et al. 2015), then bidirectionally sequenced. NCBI BLAST searches showed the sequence had 100% identity to B. simplex (e.g. KM087770) and 93% identity with B. enterospora (e.g. DQ365699). Phylogenetic affinities also supported identification as B. simplex. Following the usage of Sökücü & Thines (2014), and based on host association, cox2 sequence and morphology, we identify this organism as a Basidiophora, most likely B. simplex. However, the original 1877 description of B. simplex deviates from our sample by large differences in sporangiophore length: 104-156 µm (Barreto & Dick 1991) vs. our average of 227.7 µm. It is unknown whether this difference is significant. Although recent taxonomic work with B. simplex did not connect molecular data with morphology, and excluded the type specimen (Sökücü & Thines 2014), the identification provided in that study and herein provide a solid working definition for the pathogen, until such time that the utility of morphological differences and/or host preference for species delineation in this genus is resolved. To our knowledge, this is the first report of B. simplex in the state of New York since the species was originally described over 140 years ago. As New England asters are a common wild flower and a popular ornamental plant, it may be important to understand whether downy mildew disease poses a potential threat to the health and production of these plants.