First Report of the Occurrence of Bacterial Leaf Streak of Corn Caused by Xanthomonas vasicola pv. vasculorum in Brazil

Abstract

Bacterial leaf streak of corn, caused by Xanthomonas vasicola pv. vasculorum, was recently reported in the United States and Argentina (Korus et al. 2017; Lang et al. 2017; Plazas et al. 2018). Further, the disease has reached epidemic levels in some corn-growing states of the United States (Korus et al. 2017; Lang et al. 2017). In April 2018, diseased corn leaf samples were collected in commercial fields in the western region of the state of Parana, Brazil. The leaves with initial symptoms of the disease had translucent yellow to brown narrow linear lesions with irregular margins, confined to the interveinal spaces. Older symptoms on highly susceptible corn hybrids had coalescing long lesions of dark coloration, reaching large extension along the leaf blade. Preliminary examinations did not show any structure of fungi but revealed the presence of bacterial ooze. Bacterial isolates established on nutrient agar medium produced yellow mucoid colonies typical of bacteria of the genus Xanthomonas. The pathogenicity of the bacterial isolates was confirmed by spray inoculation of a 10⁸ CFU/ml suspension in plants at the V4 stage of the Zea mays hybrid JM 2M77, kept under greenhouse conditions. Lesions similar to those observed in the samples from commercial corn fields started to show up 9 days after inoculation. We reisolated consistently from the affected leaf tissues bacteria identical to those originally inoculated. The bacterium obtained had a rod-shaped cell, was gram negative, nonfluorescent and nonfermentative. Based on the analysis of polymerase chain reaction amplified sequences of the 16S rDNA region, using the universal primers fD1 and rD1 (Weisburg et al. 1991), the bacterial isolates showed genetic similarity of 99% with X. vasicola pv. vasculorum strain SAM119 (NCCPB no. 4614), isolated from corn in South Africa, the pathotype strain of the pathovar (Lang et al. 2017). Further, a neighbor-joining phylogenetic tree reconstructed based on partial 16S rDNA sequences alignment using MEGA7 (Kumar et al. 2016) confirmed that our bacterial strains clustered with X. vasicola pv. vasculorum strain SAM119. The sequences of two bacterial strains established in our studies were deposited in GenBank as accessions MH497007 and MH497008. The disease was present in commercial fields in at least eight municipalities of corn-producing regions of the state of Parana, Brazil. In addition, bacterial leaf streak of corn was detected under natural conditions on Z. mays plants of at least 30 different commercial hybrids. Although the occurrence of X. vasicola pv. vasculorum causing bacterial leaf streak of corn has been suspected in Brazil for previous years, this is the first confirmed report.