First Report of Puccinia menthae Causing Leaf Rust on Oregano (Origanum vulgare) in Mexico

Oregano (Origanum vulgare; Lamiaceae) is an herb native to the Mediterranean Region, recognized for its medicinal and culinary properties. In July 2024, symptoms and signs of leaf rust were observed in two commercial oregano fields in San Martín Texmelucan, Puebla, Mexico (19°14'48.9"N, 98°27'39.1"W). The disease incidence was 45%, and the severity was 25%. The initial symptoms of the disease appeared as small, chlorotic spots on the upper surface of the leaf, which later turned necrotic and gradually spread over the leaflets. At this stage, light brown, globose pustules became visible on the underside of the leaves. In the final stage, necrotic spots were observed on the upper surface of infected leaves, coalescing and surrounded by chlorotic halos. Infected leaves exhibited numerous subepidermal, erumpent uredinia, accompanied by hyaline and cylindrical paraphyses. Urediniospores (n = 100) were individually supported on pedicels; they appeared obovoid, echinulate, and hyaline to light brown in color, featuring two germinative pores, measuring 18.5–24.5 × 11.5–23.5 μm, and possessing a wall thickness of 0.5 μm. The morphology of the spores was closely aligned with the description of Puccinia menthae by Kabaktepe et al. (2017). Two voucher specimens were deposited under accession no. IPN 10.0219 and IPN 10.0220, in the Herbarium of the Department of Plant-Insect Interactions at the Biotic Products Development Center of the National Polytechnic Institute CEPROBI-IPN, located in Yautepec, Morelos (Mexico). For molecular identification, the 28S gene region of rDNA was amplified by a nested PCR using the primer sets Rust2inv (Aime, 2006) and LR6 (Vilgalys and Hester, 1990) for the first reaction, and Rust28SF (Aime et al., 2018) and LR5 (Vilgalys and Hester, 1990) for the second reaction. Subsequently, a phylogenetic analysis using the Bayesian Inference and Maximum Likelihood Method (Model GTR+I+G), including a published 28S dataset for Puccinia species was conducted. Our specimens were grouped within a clade of the type specimen of P. menthae, with a posterior probability/bootstrap of 1/100. The consensus sequences were deposited in GenBank under the accession No. PV731274 and PV731275, which showed 100% sequence homology with the type-specimen sequence of P. menthae (DQ354513) from Cunila origanoides in the USA (Aime, 2006). The number of base pairs matching the type-specimen sequence was 891 and 898 out of 1304 bp for the isolates IPN 10.0219 and IPN 10.0220, respectively. Pathogenicity was demonstrated by spraying a suspension of urediniospores (1×105 spores/ml) from the specimens onto five healthy 30-day-old origanum plants, while five other plants were sprayed with sterile distilled water. All plants were kept in a wet chamber at 25±2°C for 72 h and then transferred to the greenhouse for an additional 12 days. All inoculated plants exhibited disease symptoms after 15 days, while the control plants remained healthy. The pathogenicity assay was performed twice, yielding similar results. The morphology of the fungi recovered was identical to that collected initially, thereby fulfilling Koch’s postulates. This pathogen has been reported in Mexico, affecting both peppermint (Marquez-Licona et al., 2023) and spearmint (Solano-Báez et al., 2022). To our knowledge, this is the first report of Puccinia menthae causing leaf rust on Origanum vulgare in Mexico. The decline in the quality of foliage used in traditional Mexican cuisine renders their commercialization impossible. Therefore, efforts should be directed towards managing the disease.