First Report of Alternaria alternata causing leaf spot on sorghum bicolor in Texas

Abstract

In October 2024, a foliar disease was observed in a grain sorghum hybrid, Pioneer 37-07, planted in a 2-ha sorghum field in Potter County, Texas, USA, with an average disease incidence of approximately 50%. Symptoms included tan to gray, elongated lesions with distinct dark brownish-red borders, measuring 25–100 mm long and 2–4 mm wide, which frequently coalesced into larger necrotic areas. To identify the pathogen, ten symptomatic leaves were cut into 2 × 2 mm segments, surface-sterilized in 1% sodium hypochlorite for 1 min, rinsed thrice in sterile distilled water, blotted dry on paper towels, and plated on potato dextrose agar (PDA). Plates were incubated at 25 ± 2°C with a 12-h photoperiod for 3 days and single-spore isolates were obtained by transferring 4 mm2 agar blocks onto fresh sporulation media plates (Shahin and Shepard 1979) to produce spores and then transferring single spores onto fresh PDA plates. Conidia averaged 44.8 × 27 μm, with 2 to 4 transverse septa and occasionally 1 to 2 longitudinal septa. Based on morphological characteristics (Simmons 2007), all isolates were identified as Alternaria alternata. Genomic DNA was extracted, and the internal transcribed spacer (ITS) region, partial elongation factor 1-alpha (tef-1α), and histone 3 (H3) gene regions were amplified and sequenced using primers ITS1/ITS4 (White et al. 1990), EF1-728F/EF1-986R (Carbone and Kohn 1999), and H3-1a/H3-1b (Glass and Donaldson 1995), respectively. BLASTn searches revealed 100% sequence identity with A. alternata reference sequences in GenBank: ITS (OP581421; 515/515 bp), tef-1α (KJ638247; 389/389 bp), and H3 (MF172975; 404/404 bp). Sequences generated in the present study were deposited in GenBank under accessions PX106441 (ITS), PX108893 (tef-1α), and PX108892 (H3). A maximum-likelihood tree was constructed in IQ-TREE 3, which placed the isolate obtained in this study (Bushlandround3) in the same clade as A. alternata strain OP581421.1, with 100% bootstrap support. Pathogenicity was tested on GW-18072 cultivar using the isolate Bushlandround3. Prior to planting, seeds were surface sterilized with 50% Clorox for one minute and rinsed thrice with water. Twelve sorghum plants at the 3–4 leaf stage were sprayed with a conidial suspension (1.6 × 10⁴ conidia/mL), and four plants were sprayed with sterile distilled water as a negative control. Plants were maintained at 25 ± 2°C and 100% relative humidity for 48 h. Tan to gray lesions developed on inoculated plants within 2-4 days, measuring 2.1–2.5 mm wide and 6 mm long, that later developed dark-brownish red borders. Alternaria alternata was reisolated only from symptomatic leaves, and its identity was confirmed using both morphological and sequence-based identification methods. Alternaria alternata has been reported as part of the sorghum grain mold complex, along with several other fungal genera, in Texas (Prom and Cuevas 2023), and as a causal agent of severe leaf spot on sorghum in China (Zhao et al. 2016). To our knowledge, this is the first report of leaf spot on sorghum caused by A. alternata in Texas. This report alerts extension agents, crop consultants, and growers to an emerging threat that may warrant additional disease management strategies should the disease continue to spread.