LAUSR

The native perennial New England aster (Symphyotrichum novae-angliae; syn. = Aster novae-angliae) is ubiquitous throughout most of the United States, because it self-seeds and is 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 2017, all (n = 25) New England asters grown in a public garden in Nassau County, NY, exhibited symptoms resembling 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 white mycelial growth. Microscopic inspection revealed abundant hyaline, unbranched sporangiophores, slightly curved and club-like, 192.9 to 261.1 µm long (x¯ = 227.7 µm) and 6.6 to 13.6 µm wide (x¯ = 10.1 µm), having a wide, rounded apex 11.7 to 28.1 µm wide (x¯ = 22.5 µm) that held 8 to 14 straight projections, 7.0 to 12.2 µm long (x¯ = 9.8 µm), and 1.5 to 2.9 µm wide (x¯ = 2.2 µm). Smooth, colorless sporangia 22.3 to 31.7 × 20.3 to 30.0 µm (x¯ = 25.4 × 26.7 µm) ranged from globose to broadly ellipsoidal. Zoospores and oospores were not observed. The morphology is characteristic of the genus Basidiophora (Barreto and Dick 1991). All previous reports of downy mildew on Symphyotrichum are attributed to B. entospora sensu lato (Farr and Rossman 2017) or B. simplex (Sokucu and Thines 2014). DNA was extracted (BPI 910619), and polymerase chain reaction products were amplified with cox2 primers (Choi et al. 2015) and then bidirectionally sequenced. NCBI BLAST searches showed the sequence (MH187901) 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 Sokucu and Thines (2014), and based on host association, cox2 sequence, and morphology, we identified 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 to 156 µm (Barreto and Dick 1991) versus 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 (Sokucu and Thines 2014), the identification in that study and herein provides a solid working definition for the pathogen that will be helpful until the significance of morphological differences and/or host preference for species discrimination 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. This pathogen poses a potential threat to the health and production of New England aster, a common wildflower and popular ornamental. It will be important to learn whether it can affect a wider group of plants in the Asteraceae.