First Report of Sclerotinia minor Causing White Rot of Lettuce ( Lactuca sativa ) in Mexico

Abstract

Lettuce (Lactuca sativa; Asteraceae) is an annual leafy vegetable widely cultivated worldwide. In August 2023, severe soft-rot symptoms accompanied by white mycelial growth and black sclerotia were observed on lettuce plants cv. Iceberg in a commercial field in Toluca (19°06'14.5"N, 99°35'16.6"W), State of Mexico, Mexico. Disease incidence was 40%. For fungal isolation, sclerotia from 10 plants were surface-disinfested in 70% ethanol for 10 s and rinsed three times in sterile distilled water. The sclerotia were dried for 1 h and transferred to Petri plates containing potato dextrose agar (PDA) medium (Difco, USA) and incubated at 18 °C in the dark for 5 days to induce sclerotial germination. Ten colonies obtained from the sclerotia were purified using the hyphal-tip technique on water agar and subsequently transferred to Petri plates containing PDA medium. On PDA, fungal colonies were white with cottony aerial mycelium (mycelia growth rate 2.4 ± 0.23 cm) and produced 26 sclerotia cm2 (n =50), which were black, irregular in shape, and measured 2.3 ± 1.6 mm. Morphological characters matched those of Sclerotinia minor (Saharan and Mehta 2008). A representative isolate was deposited in the Culture Collection of Phytopathogenic Fungi at the Research Centre for Food and Development (Sinaloa, Mexico) under accession number CCLF772. For molecular identification, genomic DNA was extracted (DNeasy Plant Mini Kit, Qiagen, Germany), and and the rDNA ITS region, calmodulin (CaM), and DNA replication licensing factor (Mcm7) loci were amplified and sequenced using primers ITS1/ITS4 (White et al. 1990), CAL-228F/CAL-737R (Carbone and Kohn 1999), and Mcm7-709for/Mcm7-1348rev (Schmitt et al. 2009), respectively. BLASTn analyses revealed 99–100% identity with the ex-type isolate of Sclerotinia minor (CBS 112.17). ITS phylogeny analysis placed the isolate CCLF772 within the S. minor clade. The resulting sequences were deposited in GenBank under accession numbers PX317470 (ITS), PZ576806(Mcm7), and PZ576805(CaM). Pathogenicity tests were performed on 10 healthy 30-day-old lettuce plants (cv. Iceberg) grown in pots containing sterile substrate. The inoculum suspension was prepared by blending actively growing fungal mycelium, and the concentration was adjusted to 1 × 10⁴ colony-forming mycelial fragments mL⁻¹. Lettuce plants were inoculated with 100 mL of the mycelial suspension per plant and covered with plastic bags for 5 days at 20°C. Control plants were treated with sterile distilled water. At 13 days after inoculation, typical symptoms of soft rot, including white mycelial growth and sclerotia production, were observed on all inoculated plants, whereas control plants remained asymptomatic. The pathogenicity test was performed twice. The fungus was reisolated from the artificially inoculated plants, thus fulfilling Koch’s postulates. Sclerotinia minor is a widespread pathogen affecting a wide range of crops (Farr et al. 2025). In lettuce, this fungus has been reported in Canada (Jarvis 1985), Korea (Kim and Cho 2002), and Serbia (Mihajlović et al. 2022). Based on our literature review, this is the first molecular report of S. minor infecting lettuce in Mexico. The occurrence of this pathogen represents a potential risk to lettuce production in Mexico and underscores the need for early detection and implementation of integrated disease management strategies.