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First Report of Downy Mildew on Dysphania ambrosioides Caused by Peronospora sp. in Mexico

Ortega-Acosta, S. Á., Palemón-Alberto, F., Cruz-Lagunas, B., Toribio-Jiménez, J., Damián-Nava, A.
Plant disease 2018 v.102 no.11 pp. 2383
Dysphania, necrosis, herbal tea, cytochrome-c oxidase, genes, greenhouses, orchards, sporangia, ribosomal DNA, Peronospora tabacina, downy mildew, internal transcribed spacers, Nicotiana tabacum, mitochondria, chlorosis, mycelium, pathogens, relative humidity, polymerase chain reaction, pathogenicity, branches, sporulation, leaf blade, condiments, stomata, Dysphania ambrosioides, disease incidence, Mexico
The epazote or Mexican tea (Dysphania ambrosioides) is an important species for Mexico, used in the preparation of tea, as a condiment in the kitchen, and it also has some medicinal uses. In October 2017, the presence of downy mildew was detected in various epazote orchards in Texcoco, Edo. of Mexico. The average disease incidence was 25%. The symptoms on the underside of the leaves were characterized by the presence of dense masses of mycelium, from gray to dark, which subsequently led to tissue necrosis. The leaves with severe infections presented chlorosis and distortion of the leaf blade. The sporangiophores (n = 50) that emerged from stomata measured from 251 to 450 μm (standard error [SE] = 9 μm), hyaline, and with curved branches and dichotomous terminations. Olivaceous and ellipsoid sporangia (n = 50) measured 22 to 27 (SE = 0.2) × 14 to 20 (SE = 0.19) μm. The morphological characteristics were consistent with the genus Peronospora (Spencer 1981). Total DNA was extracted from mycelia and sporangia that were removed from leaves. The internal transcribed spacer (ITS) rDNA (Cooke et al. 2000) and the mitochondrial cytochrome c oxidase subunit I (COI) gene (Robideau et al. 2011) were amplified by polymerase chain reaction. Sequences were deposited in GenBank (ITS as MG733040; COI as MG737302) and were compared with those available in GenBank using BLASTn. The ITS nucleotide sequence showed 99% identity with a Peronospora sp. from Korea (GenBank EF614958) (host Dysphania ambrosioides, synonym Chenopodium ambrosioides), and the nucleotide sequence COI revealed 97% identity with Peronospora tabacina (GenBank KJ654154) (host Nicotiana tabacum). Morphologic and molecular analyses indicated that this pathogen is an unknown species of Peronospora. To perform the pathogenicity tests, 15 healthy epazote plants were used, which were spray inoculated with a water suspension containing 1 × 10⁵ sporangia/ml. As a control treatment, 10 healthy epazote plants were sprayed with sterile distilled water. Inoculated plants were placed in a greenhouse at 24 ± 2°C and relative humidity of 90 to 95%. After 5 days, typical downy mildew symptoms were observed in the inoculated plants as well as sporulation typical of a Peronospora sp. Control plants showed no symptoms. The experiment was repeated a second time, and the same results were obtained. A Peronospora sp. has been reported in epazote plants in the United States, Australia, Brazil, and Korea (Farr and Rossman 2018; Klosterman et al. 2014). To our knowledge, this is the first report of a Peronospora sp. causing downy mildew on epazote in Mexico (Farr and Rossman 2018). This information is important to implement management strategies for this disease and for producers to improve the quality and quantity of harvested epazote.