Lettuce (Lactuca sativa var. longifolia) is an important food crop and source of vitamins, minerals, and bioactive compounds that help prevent cardiovascular disease and cancer (Shi et al. 2022). In Mexico, the state of Puebla is the fourth largest producer of lettuce (SIAP 2025). In June 2025, a random sampling was carried out on 1,000 plants in a one hectare area at the Center for Horticultural and Native Plants Research of UPAEP (18°55′54″N, 98°23′59.9″W). Lettuce plants 65 days old close to harvest showed leaf blight and dieback, with disease incidence and severity of 50% and 60%, respectively. The symptoms included chlorosis of older foliage, necrosis, and soft rot, with abundant white and gray mycelia, and irregular sclerotia. The organism was isolated from 40 symptomatic plants. This was done by collecting sclerotia, disinfesting with 3% NaOCl, rinsing with sterile distilled water, its drying, and plating on potato dextrose agar media. The plates were incubated at 25 °C for 10 days in the dark. Forty isolates were obtained using hyphal tip culture, one from each symptomatic plant. After 10 days the colonies formed dense aerial mycelia, cottony white and fast growing, forming irregular sclerotia with diameters of 2.96 ± 0.61 mm (mean ± SD, n = 100). Colonies in each Petri plate produced 22.78 ± 3.32 (mean ± SD, n = 40) sclerotia after 14 days. The sclerotia were initially white and became black over time. Based on these morphological characteristics, the isolates were tentatively identified as Sclerotinia sclerotiorum (Saharan White et al. 1990), and deposited in GenBank (ITS, PX674584 and PX676276; G3PDH, PX678019 and PX678020). BLAST analysis of the partial ITS (462 bp) and G3PDH (887 bp) sequences of both isolates showed 100% shared identity with isolates of Sclerotinia sclerotiorum (GenBank: MT378216 and PV730054) (Hahn et al. 2025; Marin et al. 2020). Pathogenicity was confirmed by inoculated 30 healthy, 65 day-old, plants of Lactuca sativa var. longifolia. They were growing in pots with sterile soil and had 10 sclerotia from isolate SsL-5 placed at the stem base, 10 mm below the soil surface. Ten plants were inoculated with sterile distilled water and served as negative controls. The plants were kept in a greenhouse at 25 °C and 90% relative humidity. After 15 days, all the inoculated plants showed necrosis and soft rot on the leaves with abundant white and grey mycelia, as well as sclerotia, consistent with the initial symptoms observed in the field. No symptoms developed on the negative controls. The fungus was reisolated (isolate SsL-7) from inoculated plants and was confirmed as S. sclerotiorum following morphological and molecular characterization as described above, thus completing Koch’s postulates. The pathogenicity assays were carried out three times. Although S. sclerotiorum has been reported causing white mold on cabbage (Brassica oleracea var. capitata) in Puebla, Mexico (Terrones et al. 2024), to the best of our knowledge, this is the first report of it causing white mold on lettuce in Mexico. This research is crucial for developing integrated management strategies for this disease and preventing spread to other production areas.
Terrones-Salgado et al. (Thu,) studied this question.