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First Report of Downy Mildew Caused by Plasmopara halstedii on Gerbera jamesonii in Brazil

African daisy (Gerbera jamesonii Bolus ex Hook. f.) is an important species for both the cut flower and potted plant industries worldwide (4). Since the winter of 2009, plants showing severe downy mildew symptoms have been observed in a greenhouse located in an experimental area of the Universidade Federal de Viçosa (state of Minas Gerais, Brazil). The disease appeared as ill-delimited adaxial chlorosis of lamina; tissues became yellow and then brown with age with intense blighting of leaves of entire plants, leading to their death, when untreated. Dense, whitish sporulation was observed on the lower surfaces since early stages. A representative sample was dried in a plant press and deposited in the local herbarium under accession number VIC 32070. Slides were prepared with fungal structures mounted in lactofuchsin and observed under a light microscope (Olympus BX 51). Fungus morphology: Sporangiophores hypophyllous, emerging through stomata, cylindrical, up to 650 μm long and 5 to 10 μm wide, with slightly swollen base from 6.5 to 13 μm, hyaline, aseptate, straight, with up to 6 monopodial ramifications occurring mainly at right angles, the final branch ending in 3 or 4 ultimate branchlets; sporangia globose to ovoid, from 20 to 28 μm long and 13 to 18 μm wide, hyaline, smooth. Oospores were not observed. In order to further clarify the identity of the fungus on G. jamesonii, genomic DNA was extracted directly from the plant tissue and part of cytochrome c oxidase subunit 2 was amplified with the primers COX2 (3). The generated sequence was submitted to GenBank (Accession No. KC690148) and when compared with other entries revealed a high sequence similarity (99%) with Plasmopara halstedii (Farl.) Berl. & De Toni (EU743813) from Helianthus annuus L. This was also supported by the morphological data as compared with published descriptions (2) and it was then concluded that the chromistan fungus involved in downy mildew of African daisy was P. halstedii. Two different downy mildew genera, Bremia and Plasmopara, cause downy mildew disease on G. jamesoni. Bremia lactucae has been recorded in Argentina, Brazil, Germany, and Poland (4). There is only one record of a Plasmopara on this host in the United States (1), but this is an obscure report with no identification at the species level. Although P. halstedii has been commonly recorded on numerous hosts belonging to the Asteraceae worldwide, it has never been reported on G. jamesoni. To our knowledge, this is the first report of P. halstedii on G. jamesoni in Brazil. This disease has the potential to become important and cause significant losses because of a combination of the high severity to untreated plants and the increasing importance of African daisy in the flower market in Brazil.

References: (1) S. A. Alfieri, Jr. et al. Bull. 11. Index of Plant Diseases in Florida (Revised). Florida Dep. Agric. Consumer Serv., Div. Plant Ind., 1984. (2) G. Hall. Plasmopara halstedii. CMI Descriptions of Pathogenic Fungi and Bacteria No 979. Mycopathologia 106:205, 1989. (3) D. S. S. Hudspeth et al. Mycologia 92:674, 2000. (4) S. M. Wolcan, Australas. Plant Dis. Notes 5:98, 2010.