Sinapis alba (Brassicaceae), white mustard, is broadly cultivated for its seed used as component of table mustard (4). In June 2013, a group of diseased S. alba were observed in a vegetable garden on the campus of the Universidade Federal de Viçosa (municipality of Viçosa, state of Minas Gerais, Brazil). Foliage of diseased plants showed numerous chlorotic areas that developed into severe leaf blight with abundant downy mildew growth abaxially. A dried representative specimen has been deposited in the herbarium at the Universidade Federal de Viçosa (accession no. VIC 39743). The fungus had the following morphology: Sporangiophores arborescent, dichotomously branched, 540 to 840 × 8 to 10 μm hyaline, smooth, branches 105 to 210 μm long; esterigmata subacutate and curved, in pairs, 15 to 42 μm long; sporangia globose, 18 to 24 × 15 to 18 μm, hyaline, smooth. DNA was extracted using a Wizard Promega purification kit. The cytochrome oxidase subunit II (COX2) region was amplified with COX2f and COX2r primers (3). The sequence has been deposited in GenBank (Accession No. KJ396953). DNA sequences representing morphologically similar taxa were downloaded from GenBank nucleotide database, aligned in MEGA 5, and analyzed using Bayesian inference and Markov chain Monte Carlo simulation implemented in MrBayes 3.0 with five repetitions. A sequence of Albugo candida was used as outgroup in the analysis. The morphological characteristics places the fungus on S. alba in the complex of species of Pernosporaceae that attack the Brassicaceae. These are notoriously difficult to discriminate by morphology but our COX2-based phylogenetic analysis places it in Hyaloperonospora lunariae (1). This species was previously only known to cause downy mildew on other species of Brassicaceae (Lunaria annua and Erucastrum nasturtiifolium) in Europe (2). To our knowledge, this is the first report of this pathogen-host association in the world.
References: (1) O. Constantinescu and J. Fatehi. Nova Hediwigia 74:291, 2002 (2) D. F. Farr and A. Y. Rossman. Fungal Databases, Systematic Mycology and Microbiology Laboratory. Online publication. ARS, USDA, 2013. (3) D. S. S. Hudspeth et al. Mycologia 92:674, 2000. (4) B. B. Simpson and M. C. Ogorzaly. Econonic Botany. McGraw Hill, San Diego, CA, 2001.
