First Report of Bipolaris sorokiniana Causing Spot Blotch on Wheat (Triticum aestivum L.) from the Kashmir Valley of Western Himalayas, India.

Abstract

The Kashmir Valley, in the Pir Panjal range, is bordered by the Himalayas to the east and the Karakoram to the north, creating a unique geographical setting in the Western Himalayas. Its temperate climate with high rainfall, temperature fluctuations, warm summers, and cool winters fosters various wheat diseases. Wheat, a vital cereal crop supporting the livelihoods and food security of Kashmir's population, is significantly impacted by diseases, particularly spot blotch caused by Bipolaris sorokiniana. Although B. sorokiniana has been reported in warmer, humid regions including Bihar, Gujarat, Rajasthan, and Uttar Pradesh (Acharya et al. 2011), its presence in the cooler climate of Kashmir has not been documented. In April 2023 and 2024, spot blotch symptoms were consistently detected in the wheat research fields of SKUAST- Kashmir, with an incidence ranging from 40-50%. Initial symptoms included light brown, oval to elliptical necrotic spots that enlarged, leading to chlorosis and leaf death. To identify the causal pathogen, symptomatic leaf tissue was excised from the diseased-healthy tissue interface, surface disinfected in 1% NaOCl for 1 min, rinsed thrice with sterile water, dried, and plated onto PDA amended with 100 ppm streptomycin sulfate. After five days at 25°C, the putative causal agent was isolated from 90% of symptomatic samples and purified via single hyphal tip technique. The culture initially appeared velvety and olive brown with a loose cottony mass of white mycelium, turning black with profuse sporulation after 8 days. The conidia were dark olivaceous brown and obclavate to cylindrical or broadly ellipsoidal with tapered ends, featuring three to eight distosepta and measuring 50-100 × 10-17.5 µm. Based on symptoms and morphological characteristics described by Manamgoda et al. (2014), the fungus was tentatively identified as B. sorokiniana. For molecular identification, eight representative isolates were amplified and Sanger sequenced using primer pairs for the internal transcribed spacer (ITS) region (ITS1/ITS4) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene (GPD1/GPD2) (White et al. 1990; Berbee et al. 1999). Sequences were deposited in GenBank (ITS:PV162803-PV162810; GAPDH:PV295261-PV295268). BLAST searches of the obtained sequences revealed 98-100% for ITS and 97-100% for GAPDH homology with B. sorokiniana sequences in GenBank, OR575723 (isolate HUBS-54) and OR260701 (isolate LB-22), respectively. To confirm pathogenicity, 4 replicates of 21-day old plants of the susceptible variety Shalimar Wheat-2 were sprayed with a conidial suspension (1 × 10⁵ conidia/ml) and incubated in plastic bags for 24 h in a greenhouse (temperature: ~25±2°C). Control plants were sprayed with sterile water. Inoculated plants developed symptoms similar to those observed in the field within 7 days, while controls remained healthy. The pathogen was reisolated from lesions and confirmed as B. sorokiniana based on morphology and resequencing, fulfilling Koch's postulates. Pathogenicity tests were conducted twice. To our knowledge, this is the first report confirming B. sorokiniana causing spot blotch on wheat in the Kashmir Valley, India. The expanded range of this pathogen, likely due to elevated temperature and relative humidity associated with climate change, poses a significant threat to wheat production in the region. Our study provides important reference information for controlling this disease.