First Report of Pseudomonas syringae causing bacterial leaf blight on wheat ( Triticum aestivum ) in Michigan

Wheat (Triticum spp.) is a cereal crop grown as a pivotal food source globally. The state of Michigan is a producer of soft winter wheat (Triticum aestivum), with a record high state average yield of 90 bushels per acre in 2025 (USDA-NASS 2025). On June 23, 2025, soft red winter wheat at Feekes 11.1 growth stage in Genesee County, Michigan was observed with discolored streak-like yellow lesions on the foliage reminiscent of bacterial leaf blight (Large 1954). A field area of 490 m² was surveyed, where symptoms were observed in approximately 10% of plants, and severity reached 10% on the most severely infected leaves. Symptomatic leaf tissue was collected and transported back to the lab on ice. To isolate bacteria, the leaves were surface disinfected with 70% ethanol. Lesions were sampled using a sterile 0.5 cm diameter cork borer and homogenized in 1 mL of 10 mM MgCl₂ before being plated on King’s B Agar (KBA) with cycloheximide (25 ug/mL). Single colonies were confirmed to be Pseudomonas syringae phylogroup 2 via PCR for the known P. syringae toxin syringomycin (syrB) (Sorensen 1998). One isolate called MI_Wheat_4 was selected and made rifampicin-resistant for further pathogenicity assays. Three replicates of one-month-old wheat of cultivar Ambassador grown in 9 cm square pots were syringe infiltrated at the third newest leaf on the primary tiller with a 1 mL blunt-end syringe with 2x10⁷ CFU/mL of bacteria in 10 mM MgCl₂. Three replicate controls were infiltrated with 10 mM MgCl₂. After leaving inoculation sites to dry, wheat was incubated at 20°C with 16 hours of light daily before assessment. After 7 days, symptoms consisted of yellowing and slight browning around the inoculation site. Leaf discs were excised using a sterile 0.5 cm cork borer and homogenized in 10 mM MgCl₂. Population counts were performed 48 hours after dilution plating the resulting leaf extract on KBA with rifampicin (50 ug/mL). Population counts of MI_Wheat_4 showed that the strain reached high population counts above 2x106 CFU/mL after 7 days. For spray inoculations, three replicate pots of one-month-old wheat plants were sprayed to drip point with 2x10⁹ CFU/mL of bacteria in 10 mM MgCl₂ with 0.01% Silwet L-77. Control plants were sprayed with 10 mM MgCl₂ with 0.01% Silwet L-77. 7 days after inoculation plants exhibited yellowing. MI_Wheat_4 was sent for Oxford Nanopore genome sequencing via Plasmidsaurus and was submitted to NCBI (PRJNA1468252). For phylogenetic analysis, a variety of P. syringae phylogroup 2 genomes were obtained from GenBank. Genomes were annotated using Bakta v. 1.11.4 (Schwengers et al. 2020). Core genomes were aligned using Panaroo v. 1.5.2 (Tonkin-Hill et al. 2020). IQTREE was used for phylogeny construction with automatic model selection using 1000 ultrafast bootstraps in safe mode (Minh et al. 2020). This categorized MI_Wheat_4 as a P. syringae phylogroup 2d strain (Figure 2). Phylogroup 2d strains use phytotoxins and type III effectors to cause disease on a diverse range of hosts such as cherry (Prunus spp.) and bean (Phaseolus vulgaris) (Vadillo-Dieguez et al. 2024). Previous reports of P. syringae on spring wheat show that it was a weak foliar pathogen, illustrating a possible evolution in virulence (Shane & Baumer 1987). To the best of our knowledge, this is the first report of Pseudomonas syringae on wheat in Michigan, highlighting it as a potential threat to wheat production and the importance of state-wide disease monitoring.