First Report of Ceratobasidium sp. AG-K Causing Rhizome Rot on Turmeric ( Curcuma longa ) in Mexico

Turmeric (Curcuma longa) is cultivated in tropical and subtropical regions worldwide and is valued for its health benefits. Cultivation is increasing in Mexico, with the greatest production in Xicotepec municipality of Puebla state (SIAP 2026). In this municipality, turmeric plants (200 days old) were observed with basal stem, root and rhizome rot. In July 2025, at 5 hectare area in Las Pilas (20°19'54.5"N, 97°56'35.1"W) a random field sampling (1000 plants) showed 50% incidence and 45% severity in affected rhizomes. Diseased plants showed foliar wilting, soft rot, reduced stem base diameter, and root rot. Rhizomes of affected plants had necrosis and soft rot, with dark brown mycelia visible on the surface. Fifty symptomatic rhizomes were sampled and 5mm subsamples were submerged in 3% sodium hypochlorite (1 min) to surface-disinfest, rinsed with sterile distilled water, then plated on potato dextrose agar to isolate the causal organism. Plates were kept in the dark at 28°C and monitored for nine days. Morphologically, 100% of colonies resembled Rhizoctonia. One pure isolate per diseased rhizome was obtained using hyphal tip isolation. Abundant aerial mycelium darkened over time. Microscopic examination showed septate hyphae (n=50) with diameters of 3.7 - 6.1 µm. Hyphae branched at 90° angles and showed constrictions at the base. Staining with Safranin-O revealed two nuclei per cell. These morphological characteristics matched those of Ceratobasidium (Sneh et al. 1991). To identify, DNA was extracted from two representative isolates using the CTAB protocol followed by amplification and sequencing of the internal transcriber spacer (ITS) region and β-tubulin (TUB2) gene (White et al. 1990; Li et al. 2024). ITS and TUB2 gene sequences of isolates CCl4 (GenBank PX487807, PX548793) and CCl8 (GenBank PX474826, PX548791) were deposited in GenBank. BLAST analysis of the partial ITS and TUB2 sequences (CCl4: 653, 288 bp; CCl8: 656, 291 bp) showed that both isolates exhibited 100% sequence identity with Ceratobasidium sp. AG-K isolates (GenBank MN160668; MZ945444) (Kodati et al. 2022). Phylogenetic analysis grouped them within the AG-K clade of Ceratobasidium sp. Pathogenicity was confirmed by inoculating 25 turmeric plants (100-days-old) grown in pots with sterile soil. Mycelium from an eight-day old culture of isolate CCl4 was inserted into the rhizome of each plant at a depth of 5 mm using a colonized wooden toothpick. For controls, a sterile wooden toothpick was inserted into the rhizomes of 10 plants. The plants were kept in a greenhouse at 25 °C and 90% relative humidity. After 14 days, all inoculated plants showed wilting of foliage, soft rot, and constriction at the base of the stem; the rhizomes showed necrosis and soft rot with abundant brown mycelium, while controls developed no symptoms. The fungus was re-isolated from inoculated plants and characterized morphologically and molecularly, yielding identical results as previously described, and was identified as Ceratobasidium sp. AG-K, fulfilling Koch’s postulates. Pathogenicity tests were repeated three times. Ceratobasidium sp. AG-K has been reported to cause root rot in Lilium longiflorum in Oregon, USA (Lakshman et al. 2019), however it has not been previously reported in Mexico. This is the first known report of turmeric root and rhizome rot caused by Ceratobasidium sp. AG-K in Mexico. Growers should consider using prophylactic treatments to manage this soilborne pathogen in affected areas.