First Report of Xanthomonas prunicola causing Leaf Streak Disease on Wheat (Triticum aestivum) in the United States

Wheat (Triticum aestivum) suffers an average annual yield loss of 21.5% due to pests and diseases (Savary et al. 2019). Among these, bacterial leaf streak (BLS) has emerged as a serious concern, resulting in an estimated $78.5 million in losses across the U.S. wheat (Crop Protection Network 2023). In July 2023, leaf samples from winter wheat exhibiting typical BLS-like symptoms were collected from a field in Volga, South Dakota (44.30°N, 96.92°W). Approximately 40% of disease incidence was observed across 100 randomly sampled plants. Symptoms included necrotic and chlorotic streak lesions along the leaves, strikingly similar to bacterial leaf streak caused by Xanthomonas translucens. To isolate the causal organism, symptomatic leaf tissues were cut into 1-cm sections, surface-sterilized using 10% NaOCl (3 min) followed by 70% ethanol (3 min), rinsed with sterile distilled water, and soaked in 100 μL of sterile water for 5 minutes. The resulting suspension was streaked onto nutrient agar (NA) and incubated at 28°C in the dark for 48 hours (Duveiller et al. 1997). A single colony was subcultured three times to obtain a pure culture, designated as strain AL-XP-24. The isolate formed smooth, raised, yellow colonies and was identified as Gram-negative. Genomic DNA was extracted using the Wizard HMW DNA Extraction Kit (Promega, Madison, WI), and sequencing was conducted using the Oxford Nanopore MinION R9.4 platform. Annotation via the RAST pipeline (Grant et al. 2023) facilitated extraction of key taxonomic and phylogenetic marker genes, including 16S rRNA, gyrA, infB, ropB, dnaK, topA, and recA. These sequences were deposited in GenBank under accession numbers: 16S rRNA (PV504626), infB (PV504627), gyrA (PV504625), topA (PV504628), ropB (PV504629), dnaK (PV504630), and recA (PV504631). BLASTn analysis revealed high sequence similarity to Xanthomonas prunicola strain Arg-1 (CP094827), including 100% identity for 16S rRNA, topA, dnaK, and recA, and 99% identity for infB, gyrA, and ropB. To verify pathogenicity, 15-day-old seedlings of the spring wheat line SD4892 were grown in 30 × 30 cm pots under controlled greenhouse conditions (16-hour photoperiod, 23–25°C). The inoculum was prepared from a 48-hour NA culture, adjusted to OD₆₀₀ = 0.5 (~1 × 10⁸ CFU/mL) in 1X phosphate-buffered saline (PBS) with 0.01% Tween 20. Plants were inoculated via foliar spray and point inoculation (15 replicates), while control plants received PBS with Tween 20 only. Following a 48-hour incubation at 95% relative humidity, inoculated plants developed BLS-like symptoms, including water-soaked and necrotic lesions with chlorosis. No symptoms were observed in control plants. The pathogen was successfully re-isolated and confirmed by sequencing to be genetically identical to the original isolate, thereby fulfilling Koch’s postulates. The experiment was independently replicated with consistent results. Although X. prunicola has previously been reported as a pathogen of nectarine in Europe (López et al. 2018) and more recently associated with wheat in Argentina (Martino et al. 2023) and Uruguay (Clavijo et al. 2022), this is the first confirmed report of its pathogenicity on wheat in the United States. These findings underscore the urgent need for continued surveillance, pathogenicity profiling, and epidemiological assessment of X. prunicola in North American wheat-growing regions to inform effective management and resistance breeding strategies.