First Report of Bacterial Leaf Streak of Corn Caused by Xanthomonas vasicola pv. vasculorum in Illinois

Bacterial leaf streak symptoms, including long and discontinuous lesions with chlorotic margins bound by major veins, were observed on corn (Zea mays L.) in Dekalb County in September 2016. Of 67 counties surveyed in 2016, DeKalb County was the only county in Illinois with observed symptoms. Bacteria were isolated from symptomatic leaf tissue by surface disinfesting tissue with 10% bleach, cutting rinsed leaf samples into smaller sections, incubating for 1 h in sterile distilled water, and streaking the suspension onto nutrient agar. Three isolates were cultured, and colonies were yellow, mucoid, and convex on nutrient agar. DNA was extracted from bacterial isolates with an E.Z.N.A. bacterial DNA extraction kit (Omega Bio-tek, Norcross, GA). To confirm that the isolates were X. vasicola pv. vasculorum (Xvv), the diagnostic primer sets Xvv3, which amplifies a putative membrane protein, and Xvv5, which amplifies a putative exported protein, were used in polymerase chain reaction assays (Lang et al. 2017). In addition, the primer set for XopAF was used to differentiate corn and sugarcane Xvv (Lang et al. 2017). A band of 207 bp was observed with the Xvv3 primer set and 200 bp with the Xvv5 primer set, indicating that these isolates are Xvv. No bands were observed with the XopAF primer set, indicating these isolates were distinct from those that cause disease on sugarcane. Thus, isolates IL-1-1, IL-4-1, and IL-4-2 were confirmed to be Xvv. Pathogenicity was confirmed by spraying V5 (five visible leaf collars) corn plants with a bacterial suspension at a concentration of 1 × 10⁸ CFU/ml. Plants were covered in plastic bags overnight to maintain high humidity. Typical long, discontinuous lesions were observed by 7 days postinoculation, and the pathogen was reisolated from diseased leaf tissue to complete Koch’s postulates. No symptoms were observed on mock-inoculated plants. Based on our results, this is the first report of X. vasicola pv. vasculorum on corn in Illinois, and this report expands the known range of this pathogen in the United States (Damicone et al. 2018; Korus et al. 2017).