First Report of Downy Mildew Caused by Peronospora chenopodii-ambrosioidis on Epazote (Dysphania ambrosioides) in North Carolina

Dysphania ambrosioides (L.) Mosyakin & Clemants (previously Chenopodium ambrosioides [L.]) commonly known as epazote, Mexican tea, or wormseed, is an herb used in Mexican cuisine and is also known for its medicinal properties in different parts of the world. In September 2020, stunted, chlorotic leaves with spotty brown discoloration were observed in a homegrown epazote plant in North Carolina, U.S.A. Profuse dark gray sporulation was present on the underside of the leaves (Fig. 1A and B) with hyaline sporangiophores and olivaceous sporangia (Fig. 1C). Dichotomously branched sporangiophores (Fig. 2A) measured 257 to 497 µm (n = 50) in length and were 6.04 to 10.32 µm wide at the first branch. Ellipsoidal sporangia (Fig. 2B) measured 15 to 23 × 22 to 27 µm (n = 50). Morphological characters were consistent with Peronospora sp. on epazote reported in Mexico (Ortega-Acosta et al. 2018) and Peronospora chenopodii-ambrosioidis Golenia reported in Korea (Lee et al. 2020). Genomic DNA was extracted from sporulating leaf tissue using the E.Z.N.A Plant DNA kit (Omega Bio-tek, Norcross, GA). The internal transcribed spacer rDNA region (ITS) and the mitochondrial cytochrome c oxidase subunit I (cox1) gene were amplified using Phusion High-Fidelity DNA Polymerase (Thermo Fisher Scientific, Waltham, MA) and sequenced in forward and reverse directions at the North Carolina State University Genomic Science Laboratory. Sequences were verified and submitted to NCBI GenBank with the accession numbers MW361335 (ITS) and MW385292 (cox1). These were used as queries to search for similar sequences in the NCBI database using BLASTN. The cox1 gene showed 97.37% identity to portions of the mitochondrial genomes of Peronospora tabacina isolates 968-S-26 (KT893456.1) and 968-J2 (KT893455.1). Pairwise sequence alignment of the cox1 gene (MW385292) with that of the epazote downy mildew isolate Per9.Tex (MG737302.1) isolated from epazote and reported in Mexico (Ortega-Acosta et al. 2018) showed 100% identity, although it did not appear in the BLASTN search. The ITS sequence showed 99.91% identity to Peronospora sp. DML84 (KJ002561.1) and 99.75% to Peronospora sp. isolate Per9.Tex (MG733040.1), both isolated from epazote, the former in California, U.S.A., and the latter in Mexico. ITS sequences also showed 99.62% identity to P. chenopodii-ambrosioidis voucher KUS-F20063 (MT734673.1) reported from South Korea. Although several Peronospora species have been reported on epazote (Farr and Rossman 2021), recent literature (Choi and Thines 2014; Choi et al. 2008; Lee et al. 2020) suggests that downy mildew on this host has often been misidentified as Peronospora farinosa or Peronospora chenopodii. We generated a maximum likelihood tree (Fig. 3) using the program IQ-TREE (Minh et al. 2020) in the PhyloSuite platform (Zhang et al. 2020) with ITS sequences of various Peronospora species isolated from different Chenopodiaceae hosts (Choi et al. 2008), including one isolated from epazote (EF614958). The tree also incorporated MW361335 (P. chenopodii-ambrosioidis NCEDM1), MG733040 (Peronospora sp., host C. ambrosioides, sequenced by Ortega-Acosta et al. 2018), MT734673 (P. chenopodii-ambrosioidis KUS-F20063, host C. ambrosioides, sequenced by Lee et al. 2020), and KJ002561 (Peronospora sp., host C. ambrosioides, sequenced by Klosterman et al. 2014). Peronospora species collected from epazote formed a different cluster, confirming the results of Choi et al. (2008). Molecular evidence coupled with morphological data confirmed the identity of the pathogen as P. chenopodii-ambrosioidis. Peronospora sp. on D. ambrosioides has been reported in Mexico, Australia, Brazil, Korea, Poland, and California (Farr and Rossman 2021; Klosterman et al. 2014; Lee et al. 2020; Ortega-Acosta et al. 2018). To our knowledge, P. chenopodii-ambrosioidis has not been reported previously in North Carolina. The purpose of this report is to provide information on the existence of the disease in the state to current and future epazote growers for timely application of management practices.

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