First Report of Puccinia menthae Causing Leaf Rust on Peppermint (Mentha x piperita) in Mexico.

Abstract

Peppermint (Lamiaceae) is an aromatic herb with culinary, medicinal, and industrial properties. In June 2022, symptoms and signs of foliar rust were observed in four commercial fields of peppermint (Mentha × piperita) in San Buenaventura Tecalzingo, San Martín Texmelucan, Puebla, Mexico (19°14'34.0"N 98°27'25.4"W; 19°14'16.7"N 98°27'21.4"W; 19°14'37.0"N 98°27'07.7"W; 19°15'00.6"N 98°26'54.7"W). Two diseased plants were collected at each site. The disease was present in 50% of the plants and the damaged foliar tissue was under 17%. Initial symptoms included small chlorotic spots on the adaxial surface of the leaves, which later spread to form a necrotic area surrounded by a broad chlorotic halo. Necrosis developed only in the presence of abundant reddish-brown pustules on the abaxial surface of the leaf, while smaller pustules were observed on the adaxial surface. The signs were detected as numerous reddish-brown pustules on the abaxial surface of the leaves. The infected leaves of all samples showed subepidermal uredinia, erumpent, with hyaline and cylindrical paraphyses. Urediniospores (n = 50) were hyaline to light brown, echinulate, with two germinative pores, obovoid (16.5-26.5 × 11.5-25.5 μm, mean ± SD = 22 ± 1.6 × 19 ± 0.4 μm and 0.6 μm of wall thickness), individually supported on pedicels. Morphological characteristics aligned most closely with the description of Puccinia menthae by Kabaktepe et al. (2017) and Solano-Báez et al. (2022). A voucher specimen was deposited in the Herbarium of the Department of Plant-Insect Interactions at the Biotic Products Development Center of the National Polytechnic Institute under accession no. IPN 10.0115. From one sample, genomic DNA was extracted, and 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), and Rust28SF (Aime et al., 2018), and LR5 (Vilgalys and Hester, 1990) for the first and second reactions, respectively. The obtained sequence (GenBank accession No. OQ552847) showed 100% homology (902/1304bp) with the type-specimen sequence of P. menthae (DQ354513) from Cunila origanoides from USA (Aime, 2006). A phylogenetic analysis using Maximum Likelihood including a published 28S dataset for Puccinia species was executed and the isolate IPN 10.0115 was grouped into a clade of P. menthae with bootstrap support value of 100%. Pathogenicity was assessed by spraying a suspension of urediniospores (1×104 spores/ml) of the isolate IPN 10.0115 onto six healthy peppermint plants (Mentha × piperita) that were 30 days old, while six other plants were sprayed with sterile distilled water. All plants were kept in a wet chamber for 48 h at temperatures from 28±2°C and relative humidity of 95%, after which the plastic bag was removed. All inoculated plants developed disease symptoms after 15 days, whereas the control plants remained symptomless. The pathogenicity assay was conducted twice with similar results. The morphology of the pathogen recovered from the pustules of the inoculated plants was identical to that originally recollected, thus fulfilling Koch'postulates. To our knowledge, this is the first report of Puccinia menthae causing leaf rust on Mentha × piperita in Mexico. This species has been previously identified using morphological characteristics in Brazil, Canada, Poland, and USA on Mentha × piperita (Farr and Rossman, 2023). Since the disease defoliates peppermint plants reducing yield, further information on disease management is needed.