Butterflybush (common name in Brazil, verbasco), Buddleja stachyoides Cham. & Schltdl. (Buddlejaceae), is an erect herb or small shrub, native to Brazil, that is listed both as a folk medicinal plant and as a pasture weed (4). In March 2012, a group of B. stachyoides plants growing in a pasture in Viçosa (state of Minas Gerais, Brazil), were found bearing typical downy mildew symptoms. The only pathogen reported associated with this plant species is Podosphaera xanthii (1) and because there is no record of downy mildew on members of Buddleja in Brazil, an investigation was carried out to clarify the pathogen identity. Diseased plants had lesions on living leaves that were vein-delimited, chlorotic, coalescing, and becoming necrotic adaxially and bearing downy mildew-like colonies over diseased tissues abaxially. The samples were dried in a plant press and a representative specimen was deposited in the local herbarium at the Universidade Federal de Viçosa (Accession No. VIC 31836). Reproductive structures were scraped from leaves with a scalpel and mounted in lactophenol. Preliminary observations indicated the pathogen as belonging to Peronospora (Peronosporaceae). The pathogen had the following morphology: Sporangiophores are 288 to 641 μm long, dichotomously branching up to seven times, hyaline, smooth, 5 to 16 μm wide at the trunk, branches 63 to 202 μm long; tips subacute, in pairs or rarely single, 5 to 19 μm long; sporangia subglobose to ellipsoidal, 12 to 22 × 11 to 17 μm, pale yellowish brown, non-papillate. Only one species of Peronospora is known to infect members of Buddleja, namely Peronospora hariotii Gäum. (1). Nevertheless, the pathogen on B. stachyoides has smaller sporangia as compared to those of P. hariotii (20 to 26 × 16 to 21 μm) (2) and it was closer to P. sordida (3). DNA of the pathogen from B. stachyoides was extracted and the cytochrome c oxidase subunit II (COX2) and internal transcribed spacer (ITS) regions were sequenced. Sequences were deposited in GenBank (COX2 as JX982637; ITS as JX982638). A BLAST search yielded 99% and 100% of maximum identity with P. sordida for COX2 and ITS, respectively. A more detailed phylogenetic study is necessary to clarify the relationship between P. sordida, P. hariotii, and related species causing downy mildew on closely related hosts. To our knowledge, this is the first report of P. sordida occurring on a member of the genus Buddleja. This is also the first time that P. sordida is reported from South America.
References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, 19 October 2012. (2) S. M. Francis. C.M.I. Descr. Pathog. Fungi Bact. 767:1, 1983. (3) G. Hall. I.M.I. Descr. Fungi Bact. 1062:1, 1991. (4) K. G. Kissmann and D. Groth. Plantas Infestantes e Nocivas. São Paulo, BASF, 1997.