First Report of White Mold Caused by Sclerotinia sclerotiorum on Cabbage in Mexico

Abstract

The state of Puebla is the main producer of cabbage (Brassica oleracea var. capitata) in Mexico, with an area of approximately 1,858 ha (SIAP 2023). In April 2023, a field sampling was conducted in the San Luis Ajajalpan, Tecali de Herrera (18°55.57’N, 97°55.607’W), Puebla, Mexico. The average temperature was 24°C and the relative humidity was 95% for five consecutive days. Cabbage plants cv. ‘American Taki San Juan’ close to harvest, with head rot symptoms were found in a commercial area of approximately 3 ha, at an estimated incidence of 35 to 45%. More than 70% of the leaves were symptomatic on severely affected plants. Typical symptoms included chlorosis of older foliage, soft rot with abundant white to gray mycelium, and abundant production of large and irregularly-shaped sclerotia. The fungus was isolated from 30 symptomatic plants. Sclerotia were collected from symptomatic heads, surface sterilized in 3% NaOCl, rinsed twice with sterile distilled water, and plated on Potato Dextrose Agar (PDA) with sterile forceps. Subsequently, a dissecting needle was used to place fragments of mycelium directly on PDA. Plates were placed in an incubator at 25°C in the dark. A total of 30 representative isolates were obtained by the hyphal-tip method, one from each diseased plant (15 isolates from sclerotia and 15 from mycelial fragments). After 8 days, colonies had fast-growing, dense, cottony-white aerial mycelium forming irregular sclerotia of 3.75 ± 0.8 mm (mean ± standard deviation, n=100). Each Petri dish produced 14-25 sclerotia (mean = 18, n = 50), after 10 days. The sclerotia were initially white and gradually turned black. The isolates were identified as Sclerotinia sclerotiorum based on morphological characteristics (Saharan and Mehta 2008). Two representative isolates were chosen for molecular identification, and genomic DNA was extracted by a CTAB protocol. The ITS region and the glyceraldehyde 3-phosphate dehydrogenase (G3PDH) gene were sequenced for two isolates (White et al. 1990; Staats et al. 2005). The ITS and G3PDH sequences of a representative isolate (SsC.1) were deposited in the GenBank (ITS- OR286628; G3PDH- OR333495). BLAST analysis of the partial sequences ITS (509 bp) and G3PDH (915 bp) showed 100% similarity to S. sclerotiorum isolates (GenBank: MT436756.1 and OQ790148). Pathogenicity was confirmed by inoculating 10 detached cabbage heads of ‘American Taki San Juan’, using the SsC.1 isolate, according to Sanogo et al. (2015). Heads were placed on the rim of a plastic container and inserted in a moisture box with 2 cm of water on its bottom. The box was covered with a plastic sheet to maintain humidity. The control plants were inoculated with a plug of noncolonized PDA. The inoculated cabbages were covered with white to gray mycelia and abundant sclerotia within 10 days, whereas no symptoms were observed on non-inoculated controls. The fungus was re-isolated from the inoculated cabbages as described above, fulfilling Koch’s postulates. The pathogenicity tests were repeated three times. White mold caused by S. sclerotiorum on Brussels sprouts was recently reported in Mexico (Ayvar-Serna et al. 2023). In 2015, S. sclerotiorum was reported on cabbage in New Mexico, causing head rot (Sanogo et al. 2015). To our knowledge, this is the first report of S. sclerotiorum causing white mold on cabbage in Mexico. This research is essential for designing management strategies and preventing spread to other production areas.