First Report of <i>Macrophomina tecta</i> Causing Charcoal Rot in <i>Sesamum indicum</i> in India

Sesamum indicum L. (sesame) is a significant oilseed crop known for its high-quality oil rich antioxidants and essential fatty acids, with considerable nutritional, medicinal, and economic value. In June 2023, sesame plants at the Agricultural Experimental Farm, Calcutta University, Baruipur, West Bengal, India (22°22′ N, 88°26′ E), exhibited chlorosis, wilting, and root rot, affecting approximately 56% of the plants, distributed randomly across the field. Initial symptoms included drooping leaves, chlorosis, and wilting, leading to plant death while foliage remained intact. Cross-sectional analysis of the collar region revealed necrotic tissues and dark brown discoloration in vascular and cortical tissues, with dark brown microsclerotia on stem bases and blackened roots. Pathogen isolation was performed using root and crown tissues from five symptomatic plants. Tissue sections (0.5–1 cm) were surface-sterilized with 2% NaOCl for 2 minutes, rinsed with sterile water, and plated onto potato dextrose agar (PDA). The isolates displayed rapid mycelial growth at 28°C under a 12-hour photoperiod. Initially, the mycelia were hyaline (average 4.13 μm width), later turning grey to black within 7 days. Spherical to oblong microsclerotia (75 μm × 134 μm on average) developed within 5 days. For molecular identification, genomic DNA was extracted from a representative isolate (VAS10). Amplification of ITS, TEF-1α, CAL, ACT and β-TUB regions was conducted using primers ITS1/ITS4 (White et al., 1990), EF-728F/EF-986R, CAL-228F/CAL-737R and ACT-512F/ACT-783R from Carbone and Kohn, 1999 and T1/T22 (O'Donnell and Cigelnik, 1997), respectively. The resulted amplicons were sequenced at Barcode Biosciences, Bangalore, India and deposited in the NCBI GenBank database (ITS: PQ368303, TEF-1α: PQ383497, CAL: PQ383499, β-TUB: PQ415078, and ACT: PQ383498. Sequences were aligned with several isolates of Macrophomina tecta (MK968306, MW592218, MW592136) previously reported (Poudel et al., 2021) using ClustalW. The pathogenicity of M. tecta was tested on 6-week-old sesame genotypes (var. Rama and VRI-1) planted in 8-inch pots filled with autoclaved soil and maintained in a greenhouse at 28±3°C with 75% RH. The inoculum was prepared by culturing M. tecta in potato dextrose broth for 7 days at 28°C. Mycelial mats were blended in 250 mL of sterile distilled water, filtered through four layers of cheesecloth, and adjusted to a concentration of 10⁵ microsclerotia/mL using a hemocytometer. Fifteen plants per genotype were inoculated via soil drenching with 50 mL of the suspension; while five control plants received sterile water. After 3 weeks, inoculated plants exhibited lower stem lesions and microsclerotia formation, whereas control plants remained healthy. The pathogen was reisolated from infected plants and identified as M. tecta based on morphological and molecular analysis. Koch’s postulates were confirmed through two independent repetitions with consistent results. Previously, M. tecta was reported on sorghum and mungbean in Australia in 2019 by Poudel et al. This study represents probably the first report of M. tecta infecting sesame, both in India and globally. The pathogen thrives in high humidity and elevated soil temperatures, posing a significant threat to sesame production in India. The emergence of this new species underscores the need for further research and management strategies to mitigate its impact on sesame cultivation.