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

Following reports of a new bacterial disease of corn (Zea mays) caused by Xanthomonas vasicola in the nearby states of Colorado, Kansas, and Nebraska (Korus et al. 2017), irrigated corn fields in Caddo, Garvin, Kingfisher, and Texas Counties of Oklahoma were surveyed for symptoms of bacterial leaf streak in August 2016. Symptoms consisting of elongated necrotic lesions mostly confined within leaf veins, with irregular yellow to orange colored margins, were observed in several fields in Texas County, although disease severity was low (less than 5%). Bacterial streaming was observed from excised leaf sections, and yellow mucoid bacterial colonies were present on nutrient agar cultures obtained by streaking aqueous bacterial suspensions from lesion sections. Isolates were purified and grown on nutrient agar for 3 days, and DNA was extracted using a quick method. The quick method consisted of heating an aqueous bacterial suspension (one colony picked with sterile toothpick in 100 μl) for 15 min at 100°C and retaining supernatant following centrifugation at 1,200 rpm for 5 min. The primers Xvv3 that targets membrane protein and Xvv5 that targets putative exported protein of X. vasicola pv. vasculorum (Lang et al. 2017) were used in polymerase chain reaction (PCR) assays to identify the pathogen. Isolates OK-6-1, OK-6-3-1, OK-6-3-2, and OK-8-1 were PCR positive for the expected 207-bp product for Xvv3 and for the 200-bp product for Xvv5 primer pairs, confirming the pathogen in 2 of the 19 fields surveyed. Other bacteria including nonpathogens isolated from the corn leaves and X. euvesicatoria were PCR negative. Pathogenicity tests were performed on 21-day-old plants of corn hybrid 208-71VT2PRIB (two plants per isolate and inoculation method) grown in the greenhouse at 22 to 29°C. Bacterial suspensions (10⁷ CFU/ml of sterile tap water) were sprayed to run-off on plants incubated for 48 h pre- and postinoculation in humidity chambers in the greenhouse at 100% relative humidity (RH), or were infiltrated into leaves of plants left on the greenhouse bench at 50 to 60% RH. Sterile tap water served as the noninoculated control. Typical bacterial leaf streak lesions developed within 7 days postinoculation with the four PCR-positive isolates. Symptoms on spray-inoculated plants were more severe, and the pathogen often killed symptomatic leaves compared with infiltrated leaves, which remained intact 14 days postinoculation. The other bacteria described above and the water control did not cause symptoms. The pathogen was reisolated as described above from a symptomatic leaf representing each isolate and inoculation method. Reisolated bacteria were identified by colony characteristics and the two PCR primers. This is the first report of bacterial leaf streak of corn caused by X. vasicola pv. vasculorum in Oklahoma and expands the current reported geographic range of this new disease in the United States (Korus et al. 2017). The pathogen is synonymous with X. vasicola (Korus et al. 2017) and X. campestris pv. zeae (Qhobela et al. 1990).