First Report of Erysiphe betae Causing Powdery Mildew on Chard (Beta vulgaris var. cicla) in Mexico.

Abstract

Chard (Beta vulgaris var. cicla; Chenopodiaceae) is a vegetable native to the Mediterranean, widely cultivated for its nutritional properties. In June 2020, an outbreak of powdery mildew was detected in a commercial crop of chard in San Martín Texmelucan, Puebla (19°14'37.1"N; 98°27'12.5"W), Mexico. The disease was present in 86% of the plants (n=400) and the pathogen was found to cover up to 95% of the surface of the leaves. Initially, small whitish patches were observed on both sides of the leaves. Subsequently, the patches grew rapidly to cover most of the leaf surface and premature senescence of infected leaves was observed. The signs of the pathogen were observed as abundant whitish masses of conidia. Microscopic analysis of the fungus showed amphigenous mycelia with lobed hyphal appressoria. Conidiophores (n=30) were simple and erect, 93133 × 7.58.5 μm. Foot cells (n=30) were cylindrical, predominately straight, and rarely somewhat curved at the base, 30.036.5 μm, followed by a longer cell and two shorter cells, and the conidium. Conidia (n=100) were hyaline, ellipsoid-ovoid, 3745 × 1416 μm. Germ tubes (n =30) were terminal, short (0.52.0 times the conidial width) and stout. Conidial appressoria (n=30) were mostly lobed, showing from 2-6 lobes. Chasmothecia were not found. The morphological characteristics observed correspond to previous descriptions of Erysiphe betae by Braun and Cook et al. (2012). A voucher specimen (accession no. UACH450) was deposited in the Department of Agricultural Parasitology Herbarium at the Chapingo Autonomous University. To confirm identification, DNA was extracted from the fungus, and the internal transcribed spacer (ITS) and the 28S gene region of rDNA from one sample were amplified by PCR, using the primers ITS1/ITS4 (White et al. 1990) and PM3 (Takamatsu and Kano 2001)/TW14 (Mori et al. 2000). The sequences obtained from our specimen were registered to the GenBank under the accession numbers ON157053 and ON157047 for ITS and LSU, respectively. Our sequences shared 100% identity for ITS (KX574674) and 99.8% for LSU (OM033348 and OM368494) with sequences of E. betae in BLAST'n search. Based on phylogenetic analysis using the Maximum Likelihood method including a published ITS + 28S dataset for Erysiphe species, the isolate UACH450 was grouped into a clade with E. betae. Takamatsu et al. (2015) found that E. betae, E. malvae and E. heraclei are phylogenetically indistinguishable (they form the E. heraclei species complex), nevertheless, E. malavae infects Lavatera and Malva (Malvaceae), E. heraclei predominately forms on hosts of Apiaceae and E. betae is commonly found on Beta and Chenopodium (Chenopodiaceae) (Braun and Cook 2012). Pathogenicity was verified by spraying a suspension of conidia (1107 conidia/ml) onto the leaves of six healthy chard plants and six plants were sprayed with sterile distilled water to serve as controls. All plants were maintained at temperatures from 28 2 °C and relative humidity of 802 %. All inoculated leaves developed powdery mildew symptoms after 14 days, whereas the control plants remained symptomless. The pathogenicity test was performed twice, observing the same results. The recovered pathogen showed the same morphological characteristics as the inoculated pathogen, thus fulfilling Koch's postulates. To our knowledge, this is the first report of Erysiphe betae causing powdery mildew on Beta vulgaris var. cicla in Mexico. This pathogen has been previously reported in Iraq (Amano, 1986) and Greece (Vakalounakis and Kavroulakis, 2017) on Beta vulgaris var. cicla. Also, Erysiphe betae has been reported in Mexico on Chenopodium and throughout the world on sugar beet (Farr and Rossman, 2022). This pathogen is a major issue as it can completely cover the leaves of the diseased plants, making them difficult to market.